//
BULLETIN No. 33.
PRICE: 6d.
UNION OF SOUTH AFRICA
DEPARTMENT OF FORESTRY
Forestry and Water Supplies
IN
South Africa
BY
c. L. WICHT, Dr.-Ing., F. R. S. (S. A.)
Chief Forest Research Officer, Department of Forestry
PRINTED IN THE UNION OF SOUTH .AFnICA ny THE GOVERNlIEXT PRIXTER, PRETORIA
19.19
G .P.-S.1385 7-1948-9-3, 000
BULLETIN No. 33.
PRICE: 6d.
UNION OF SOUTH AFRICA
DEPARTMENT OF FORESTRY
Forestry and Water Supplies
IN
South Africa
BY
C. L. WICHT, Dr.-Ing., F. R. S. (S. A.)
Chief Forest Re search Officer, Department of Forestr)
PR[:-;TElJ rC" THF: rC"wC"
"r
S o rTil AFltI C \ BY TH E GO\-ERC"ME" T PRI:"T ER, PHET OH H
194~j
~ . ~} ,- -.: _!
33 57 - 19 43-9 ·,- 3. 000
CONTENTS.
PAGE.
PREFACE
I.--HYDROLOGY OF SOUTH AfRICAN FORESTRY REGIONS ..... . .... . . . . . . . . .
Rainfall. ... '" ....... . ........ .
Frequency of Dry Periods ........ .. .
Temperature ........... .
Streamflow .. .
Ground-water ...... . . . ....... . . ........................ .
H.-MoDIfICATION OF THE WATER CYCLE THROUGH LAND MANAGEMENT.
Influence
Influence
Influence
Influence
of Land
of Land
of Land
of Land
Management on
Management on
Management on
Management on
Precipitation... .. . . . . . . . . . . . .
Evaporation and Transpiration
Infiltration. . . . . . . . . . . . . . . . . .
Streamflow. . . . .
1I1. ---SPECIFlC EVIDENCE Of EffECTS OF FORESTS ON WATER SUPPLIES IN SOUTH
AFRICA....... . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . .......
Cape Peninsula ....
Bredasdorp ....
Eerste Ri veL ... . ...... . .................. .
Simondiurn ...
Knysna ......... . ... .. ................ .. . . ...... . .. .. . .. . . .
Grahamstown . . ..... . ................... . ........ . ...... . .. .
Buffalo River Catchment. .... . . . . .... . .... . .. . .. . . .
Duivelskloof-Tzaneen ........ . ... . . . .... . .................. . .
Soutpansberg .......... . .. . . . ..... . . . ........ . ............. .
Natal Midlands ...... . .. ... ... . ... . ....... . .. . . . . .. . ....... .
Zululand ...
I V. - CONCLUSION . . . . . . . . . . . . . . . . . . .
Acknowledgements ..... .
References .............. . ,
2
:.'
II
16
22
26
27
29
30
34
34
38
39
40
40
41
43
43
44
45
52
53
53
53
55
56
PREFACE.
The Department of Forestry has for many years been actively
concerned with the conservation of mountain catchment areas under its
control. It has purchased large areas of mountain land solely for the
purpose of protecting the natural plant growth and animal life and conserving water supplies. It is also alive to the possible effects which
extensive forests of exotic trees may have on water supplies and this
question is being intensively investigated at research stations situated at
lonkershoek, near Stellenbosch, in the Cape Province and at Cathedral
Peak on the slopes of the Drakensberg in Natal. Government-owned forests,
established by afforestation. at present cover about 625 square miles
and it is expected that 1,500 square miles will eventually be planted
up. This is equal to about one fifth of the total area of land controlled by
the Department. Another fifth is clothed with indigenous forest which
will be preserved as such and the remaining three-fifths, mostly mountain
land, is held solely to preserve the natural vegetation and protect the soil
and the water supplies. The area afforested by bodies and persons other
than the State is about 1,200 square miles. The total area afforested in
the Union at present is about 1,825 square miles, which is considerably
less than one half of one per cent. of the land area of the Union (472,550
square miles). Many members of the public have nevertheless shown
considerable concern about the effects of afforestation on water supplies,
and it was felt that there was a need for an authoritative statement on this
subject. Dr. C. L. Wicht. who was in charge of investigations at lonkershoek for 12 years. was therefore delegated to undertake a reconnaissance
of catchment areas in various parts of the Union. and investigate instances
where afforestation is alleged to have had a desiccating effect on water
supplies. This report is the outcome of the investigations undertaken by
him.
The report places the problem of the influence which forests of exotic
.trees have on water supplies in its proper perspective. It is by no means
final but it provides as complete an answer as it is possible to give at
this stage to the questions which have been put by various members of
the public and should serve as a guide to those who undertake afforestation.
Further results from investigations at the research stations at lonkershoek and Cathedral Peak will be published from time to time.
W. E. WATT.
Director of Forestry.
Pretoria. 5th March. 1949.
Forestry and Water Supplies In South Africa.
By C. L. Wicht.
(Chief Forest Research Officer, Pretoria.)
more than fifty years the manner in which water supplies in
F ORAfrica
are affected when forests are destroyM or plantatIons
~uth
established, has been the subject of controversy. It is likely to continue
to be so until investigations over long periods have yielded complete
evidence. In the meantime there is an insistent demand for information
from farmers. municipalities, and other users of water, as to whether and
how water supplies will be influenced by forestry. To meet this demand
it will be attempted in this report to synthesize the meagre and sometimes
problematic data available into a coherent statement, from which practical
recommendations .can be deduced. These recommendations may have
to be revised, when planned in.vestigations have yielded conclusive evidence.
Much of the evidence available is circumstantial, and can therefore often
be interpreted in various ways; and, where evidence is inadequate, gaps
have at present to be bridged by theories, which will no doubt have to
be modified in time.
The effects of plantations of exotic trees on water supplies have been
particularly vigorously disputed. When these were first established it
was generally accepted that their effects would be beneficial aAd similar to
those usually attributed to the natural forests. In recent y(,'US it has,
however, been alleged that plantations dry up water supplies, exhaust the
soil and promote erosion. Extravagant claims have been made in support
of afforestation and against it, but the actual influence of afforestation on
the water cycle has not yet been satisfactorily observed and explained.
As a result of recommendations made at the Empire Forestry Conference in South Africa in 1935, a research station was established at Jonkershoek near Stellenbosch, Cape Province, with the object of determining the
effects of afforestation and other forms of veld management on water
supplies. A second research station is now being rapidly developed at
Cathedral Peak on the Drakensberg, Natal. This station was contemplated
in 1936 and work on the area was begun in 1938-1939, but the development
was postponed for the duration of the war. Research being undertaken
at these centres has been described elsewhere (Wicht, 1947). The final
solution of the problem of the effects of tree-planting on water supplies
will depend on the outcome of investigations at these research centres.
and the statement presented here should be looked upon merely as a
temporary working hypothesis correlating the information available at
this stage. In drawing conclusions and making recommendations care has
been exercised to maintain a guarded and conservative attitude.
The beneficial effects of indigenous forests have never been doubted.
Their destruction has repeatedly been deprecated; their protection
commended. The forests constitute great national ass~ts. They must be
preserved for their beauty and scientific interest, and, it is generally
agreed, to conserve the water supplies in the moist soils on which they
grow. The relative importance of the influence of these forests on water
supplies of the country is, however, slight, because they cover only a
small area-less than a quarter of one per cent. of the Union.
2
The traditional conception of the influence of forests on natural water
supplies was set forth by Braine (1908) in a paper read before the Grahamstown Meeting of the South African AssociatIOn for the Advancement of
Science in 1908. This report presents a modern review of the subject in
which the results of subsequent research have been incorporated.
1. HYDROLOGY OF SOUTH AFRICAN FORESTRY REGIONS.
To understand the extent and significance of the effects ' of forests
on water supplies, the hydrographic environment in which they exercise
their effects must be studied. In South Africa eight forestry regions, may
be recognised, which have distinctive hydrographic features:1. North-eastern Transvaal.
7 Eastern Transvaal.
3. Zululand.
4. Natal Midlands.
5. Natal Drakensberg.
6. Eastern Province.
7. Cape Midlands.
8. South Western Cape.
In order to provide a back-ground for the analysis of the hydrological
influences of forests, it is proposed to examine the degree and nature of
the variation of hydrological variables with reference to these regions.
RAINFALL.
Most striking and significant are variation in the volume and distribution of rainfall.
In figure I, relative rainfall distributions over the year are represented.
These graphs show the obvious differences between the three main zones.
Winter-rainfall is reflected by the Jonkersboek graph; constant-rainfall by
the Deepwalls: and summer-rainfall by the Evelyn Valley and Wood bush
graphs.
Closer study of more compreliensive· data than are presented here
will show that, although these .three zones are clearly defined, there is a
tendency towards a transition in the degree of concentration of rain.
from one type of rainfall distribution to the next. Thus it is found in the
nortbern portion of the winter rainfall zone near Clanwilliam. that the
rainfall is strongly concentrated in the winter months (Wicht, 1945).
Further south, towards Cape Town, the concentration is somewhat less
marked. From Swellendam to the east constant or all-the-year-round
rainfall is found. In this zone there is a tendency to pass over to
summer rainfall, as is shown by the Deepwalls graph in Figure 1. In the
Eastern Province region, for example at Evelyn Valley, the rainfall is
distinctly concentrated in the summer months. Through Natal towards
the Eastern Transvaal the concentration of rain in summer becomes more
and more marked-shown by graphs for Evelyn Valley and Wood bush
.i n Figure 1.
Such considerable variations in the supply of water in the form of
rain will strongly influence the volume consumed by vegetation. The
uneven distribution in the South Western Cape, the Eastern Province,
Natal and Transvaal regions, is much less favourable for the conservation
of water than the even -distribution in the Cape Midlands. At the Cape,
moreover. the critical dry period is tbe hot summer, when water losses
through transpiration and evaporation are severe. Tn the summer-rainfall
3
EVELYN VALLEY .
WOODBUSH.
JONKERSHOEK.
DEEPWALLS.
18
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Relative Rainfall Distribution over Year.
zone the critical dry period is the cool winter which mitigates the loss of
unduly large volumes of water. In this zone much of the precipitation is.
however, evaporated and transpired soon after it falls, because the hot
weather is favourable for these processes. The rainfall is also much less
effective because it frequently occurs in strong downpours associated
with thunder storms. None of these features of rainfall may be ignored
when the effects of forests on water supplies are being considered.
4
Within the zones very considerable vanatlOns occur in the volumes
of rain deposited annually at different centres. Forests and plantations
are restricted to the areas of high rainfall. Annual volumes of rainfall
at typical centres are:Jonkershoek, South Western Cape: 42· 5 inches.
Deepwalls, Cape Midlands: 48· 4 inches.
Evelyn Valley, Eastern Province: 70·0 inches.
Graskop, Eastern Transvaal: 68· 7 inches.
Woodbush, North Eastern Transvaal: 69· 3 inches.
Entabeni, Northern Transvaal: 73· 6 inches.
The minimum rainfall for economic afforestation varies from about
25 to 30 inches in the constant- and winter-rainfall zones, and from 30
to 40 inches in the summer-rainfall zone. True forestry land in the
summer-rainfall zone is usually to be found in the "mist belt", where
transpiration is greatly reduced by high humidity of the atmosphere.
If trees are planted away from the wet, true forestry sites such as
those that have been mentioned, less water is available for use, and the
conditions for the squandering of water through transpiration are aggravated. Hours of sunshine are longer, lengths of dry periods between rains are
greater, and the humidity of the atmosphere is less. The effects of plantations should therefore not be judged in these drier areas, where they
should not be established on a large scale for timber production. Treeplanting must not be condemned because trees have in places been planted
where it is, in fact, too dry for forestry, and where the land has consequently been somewhat desiccated; no more than wheat-growing must be
condemned because many farmers have. through subsidies, been persuaded
to grow wheat on lands that should never have been used for this purpose.
and which have consequently been eroded.
The total anp.ual rainfall received at a particular centre varies
in different years. This variation is much greater in the
summer-rainfall zone than in the constant- or winter-rainfall zones. Dry
years and very wet years are therefore more rare at the Cape and in the
Midlands than elsewhere. At Jonkershoek the range recorded is from
35·1 to 60· 3 inches per year; at Deepwalls it is from 37· 9 to 63· 9 inches
per year; whereas at Evelyn Valley it is from 40·0 to 117·7 inches per
year; at Woodbusb from 34·6 to 115·0 inches per year and at Entabeni
from 36·0 to 115'1 inches per year. In the dry years conditions favourable
for the excessive loss of water through transpiration and evaporation are
prevalent, in the wet years they are restricted. The circumstances are. in
fact, analogous to those described for wet and dry areas.
~ansiderably
The variations in total annual rainfall show distinct secular trends,
which are of very great significance in judging the effects of forestry on
water supplies. Such trends were demonstrated by Schumann and Thompson (1934), who analysed data by special rainfall districts constituted for
the purpose. Similar graphs to those used by these two authors have been
drawn for single representative centres, and these are depicted in Figures
2 to 7. Each graph shows the successive annual totals in the form of
a line graph, the means for the period of observation as a horizontal
line, and the trend as a smooth curve. The curve was derived by
employing a method also used by Schumann and Thompson. Points on
the curve were computed by weighting observed totals on either side,
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The effects of tree-planting are frequently confused with those due to
secular variations such l}S are shown in these graphs, and they need to be
carefully examin.ed before the subject can be elucidated further.
The oldest records available are from the Royal Observatory near
Cape Town. They go back for over one hunderd years. The trend in
Figure 2 shows clearly a general rise in rainfall up to a peak period
culminating in 1890; from that time there is a general drop to a minimum
in 1929. The drop in trend is from 30· 0 inches to 18· 9 inches, which is
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Summer Rainfall.
equivalent to about 45 per cent. of the mean for the periQd of observation.
This trend was demonstrated by Schumann and Tbompson (1934). Figure
2 also shows that in recent years the trend has been upward and has even
exceeded the mean for the observation period.
The Royal Observatory is not a true forestry centre, but long records
are not available for such centres. The short graph for lonkershoek
starting in 1925. shows. however, a trend similar to the end portion of
the Royal Observatory graph. There appears to be some justification for
9
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Natal and Eastern Transvaal.
Summer Rainfall.
10
assuming therefore that these two centres in the same region have
experienced analogous secular variations in rainfall. Probably 10nkershoek also had a very wet period about 1890. This may be accepted more
readily in view of the finding by Schumann and Thompson that: "the
greater part of the Union enjoyed a period of plentiful rains around the
year 1890."
It so happens that this was about the time when tree-planting was
being expanded at the Cape, and many instances of alleged desiccation by
trees are explained by the falling off in rainfall coincident with the
expansion of afforestation.
Rainfall trends in other regions besides the South Western Cape, are
equally suggestive. The long record from George (see Figure 3), selected
as representative of the Cape Midlands region in the zone of constant
rainfall, shows a peak around 1905. The decrease from this peak has been
maintained-except for a minor peak around 1932-up to the present
time. The short curve for the more typical forest centre, Deepwalls,
shows approximately the same form as the later portion of the curve for
George, and it is likely that this and also other centres in this region
received higher rainfall in the past. Schumann and Thompson's data (1934),
support this conclusion.. Here again the decrease in rainfall coincided
with marked extension of plantations of exotic trees. To accept a casual
relationship between these phenomena would, however, not be justified.
Records from Grahamstown, and from Evelyn Valley on the Amatola
mountains, have been selected as representative of the Eastern Province
region (see Figure 4). At Grahamstown the rainfall is somewhat too low
to provide good conditions for forestry, but the record is a long one, and
this is also a centre where it has been alleged that afforestation and the
natural spread of Pinus pinaster Soland, and other exotics since about
1890, have caused streams to become depleted. The graph shows clearly
that a high rainfall period was experienced, which culminated about 1887.
Since that time the general trend has been downwards, so that the present
level is approximately ten inches lower than the level in 1887. This is a
very striking decrease indeed, which must be given. full weight when
jUdging the effects of trees on the water supplies. At Evelyn Valley, the
trend rises sharply to 19lO, and drops sharply to the present time. Climatic
conditions at these two centres are rather different, but the two trend curves
nevertheless show notable agreement in. maximum and minimum turning
points. Data from this area presented by Schumann and Thompson support
the hypothesis that the rainfall was higher from 1890 to 1895.
No records of sufficient length to warrant analysis are available from
the Natal Drakensberg. From the Natal Midlands only the rather short
record from Weza (Figure 5), situated near the Cape border, is presented.
This shows no clear up or downwards trend. Data presented by Schumann
and Thompson indicate. however. that this area probably also received
higher rainfall from 1890 to 1895.
No graph has been prepared for a centre from the Zululand. semitropical area, but the records for Empangeni have been analysed. and
these do not show a decrease in the annual rainfall.
Records from Graskop (Figure 5). Eastern Transvaal, show an even
trend. except for a sharp peak caused mainly by the exceptionally wet year,
1939.
The Eastern Transvaal area where the effects of tree-plating on
water supplies have been strongly debated, is in the Tzaneen-Duiwelskloof
11
s(!.;tion. Extensive plantations of Eucalyptus saligna have been established
here for the production of mining timber. Three graphs are presented
(Figures 6 and 7): trom Duiwelskloof, Wood bush and Westfalia. The
graph from the town Duiwelskloof is somewhat irregular, but there is a
distinct drop in the rainfall after 1920. The graph from the forest Woodbush shows even greater fluctuatioins, but the smoothed curve is, nevertheless. almost 20 inches lower in 1945 than it was in 1906. The graph
for Westfalia was constructed from data kindly supplied by Dr. H.
Merenskv. The drop in rainfall is remarkable. These three graphs from
the same region show considerable agreement in the maximum and minimum turning points, and there can be little doubt that the general tendency
has been a progressive decrease in rainfall.
At the Northern Transvaal centres, Louis Trichardt and Entabeni
(Figure 8). the decrease in rainfall is extraordinary and the influence on the
water supplies of this occurrence, is likely to be more effective than that
of any other.
The analysis of rainfall trends presented in this paper is not exhaustive.
Considered in conjunction with the data presented by Schumann and
Thompson (1934) in a more comprehensive study; it does, however,
corroborate the general conclusion that rainfall has in recent years shown
a downward tendency, and it indicates that this tendency has been maintained up to 1945, also at forestry centres.
Water supplies, except for infinitesimal volumes, whether taken from
the ground or streams, are ultimately derived from the rainfall. It is
entirely logical to accept, therefore, that the fluctuations in rainfall, which
have been demonstrated. must have had marked effects on the water
supplies.
FREQU ENCIES OF DRY PERIODS.
The depletion of water resources in a forestry area, will depend, not
so much on the volumes of rain received from time to time, as on the
distribution of the rainfall and the frequencies and lengths of dry periods.
The study of dry periods, rather than rainfall, appears to offer a new
positive approach to the problem of drought, and suggests a promising
manner of interpreting the influence of environment on the volumes of
water used by forest vegetation.
Table No. 1 shows the frequency distributions of dry periods of
different lengths at various forest centres during the period 1925 to 1945 .
The frequency distributions deviate widely from the normal distribution,
which is symmetrical and bell-shaped. They are I-shaped, that is, strongly
asymmetrical and leptokurtic. Dry periods were recorded in numbers of
days, and all days on which less than . 5 inch of rain fell were ignored.
The class interval used in the table is five days, and, to facilitate general
comparison of distributions from various centres, the frequencies were
again grouped into those that did not exceed fifteen days, those that
were more than 15 and not more than 60 days, and those that were more
than 60 days (or two months) long. The total number of dry periods
experienced at various centres and the arithmetic mean length of dry
periods are other statistics that are of interest. The statistical analysis
of such skew distributions is more involved than that of normal distributions, and has not been so fully worked out. A great deal can however be
learnt even from a casual examination of Table No.1.
12
t
•
II
110
11
100
11
"
II
"
9+
w(/)
~~
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I
WOODBUSH
,
f 11
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DUIWELSKLOOF.
20
IOL-____
1900
~
'10
____
_ J_ _ _ _ _ _~_ _ _ _ _ _L __ _~
'20
'30
JAAR-YEAR.
'40
FIG. 6.
Rainfall Trends. Eastern Transvaal. Summer Rainfall.
'50
13
90
WESTFALIA _
80
.ct)
lJ.IW
~:r:
-u
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•
20
IOL---__~--~~----~~--~
1910
'20
'30
~O
YEAR.
JAAR.
FIG. 7.
Rainfall Trends.
Eastern Transvaal.
Summer Rainfall.
'50
14
r
I
110
II
II
il
II
100
I I
~
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II
I i
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11
1900
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__-L_______
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JAAR - YEAR.
FIG. 8.
Rainfall Trends.
Northern Transvaal.
Slimmer Rainfall.
'50
15
TABLE No 1.-Frequency Distributions of Dry Periods, 1925-45.
(Daily rains of less than . 5 inches ignored.)
No. of Davs.
(
I JonkersI
hoek.
~f-5
":'
6-10
11-15
rl6-20
21-25
26-30
31-35
~ 36-40
..tJ 41-45
46-50
51-55
56-60
-
f61-65
66-70
71-75
76-80
81-85
86-90
91-95
96-100
101-105
106-110
111-115
116-120
0
\C)
121-125
cu 126-130
;>
131-135
0
136-140 :
141-145
146-150
151-155
156-160
161-165
166-170
..
l17l-m
176-180
181-185
186-190
191-195
162}
101 323
' 60
165}
•
1
~J
16
30
22
12
8
3
3
4
6
0
146
1
0
0
0
0
0
0
0
2
?J
-
-
-
-
-
-
-
-
-
-
-
-
51
32
31
12
11
2
5
2
0
-
-
-
I
-
Weza
l
.
i
4
3
2
2
2
0
0
1
1
1
0
1
4
1
1
1
0
0
0
0
0
0
0
0
0
0
1
I
Evelyn
Valley.
263}
139 474
72
139}
92 278
47
1~ 384
25
14
38
13
~ 116
3
3
0
2
0
1
0
2
0
3
1
0
1
I
Deepwalls.
38
17
12
4
6
4
5
4
4
88
,
94
0
0
1
0
1
0
0
1
0
0
0
0
0
0
0
0
0
1
25
-
-
-
I
4
Woodbush.
270}
112 445
63
24
14
7
15
5
3
4
6
1
IEntabeni.
235}
102 389
52
79
34
20
11
9
9
6
4
2
2
97
15
6
2
1
0
2
1
1
0
1
1
0
1
0
0
0
0
1
1
0
0
0
0
19
2
1
0
1
3
1
0
1
0
0
1
0
0
0
0
0
1
2
0
0
1
0
-
-
-
-
-
-
-
-
TOTAL. . . . . .
6,710
6,620
6,549
5,762
6,247
6,440
MEAN..... . .
14'8
12'4
16'7
10'1
11'6
12'8
The distribution at Jonkershoek shows that lengthy dry periods are
fairly frequent there. In the twenty years studied, 16 dry periods of
more than two months. in length occurred. It is generally true that the
longer the dry periods are, the more severe is the depletion. of water
supplies through' transpiration, evaporation, deep percolation, and run-off.
During these long dry periods the water resources in the Jonkershoek
region, from which the town of Stellenbosch and riparian land owners
along the Eerste River derive their supplies, were severely strained.
16
At Deepwalls, in the constant-rainfall zone, only ty.'o periods exceeding
two months occurred in the 20 years, and here- provided all other factors
are equal- the danger of extreme depletion of water supplies must have
been much less.
At Weza, 25 dry periods of over two months each were recorded in
the 20 years. One was more than 190 days long. It appears to be
characteristic of the summer-rainfall centres that exceptionally long dry
periods occur occasionally. At all four centres studied{ periods of 5
months and more occurred.
•
The summer-rainfall centre, where conditions are most favourable
for water conservation, is Evelyn Valley on the Amatola mountains, where
only 4 periods exceeded two months. Conditions at this centre are generally condusive to low usage of water supplies. Table No. 1 indicates
also that the total number of dry periods at this centre was considerably
less than elsewhere, and the mean length of dry periods the lowest of any
of the centres.
It is a feature of the three mountain centres from the summer-rainfall
zone-Evelyn Valley, Wood bush and Entabeni-that ~he frequencies of
the shortest dry periods ar~ very high. These three distributions are
consequently very strongly leptokurtic. Compared with the winter- and
constant-rainfall centres, the frequencies of dry periods of average length.
are relatively low. High frequencies of short periods, low frequencies of
average periods and high frequencies of long periods, reOect the almost
daily occurrence of rain (mists or thunder storms) in summer, and the
almost rainless winters.
These very striking differences in the frequencies and lengths of dry
periods at various centres must have a considerable bearing on the volume
of water transpired by vegetation and evaporated, and must strongly affect
water supplies available for use. The danger of generalisation without due
regard to specific local conditions is once more demonstrated by these
data.
TEMPERATURE .
Equally important in judging the effects of forests on water supplies
are variations in temperature, because these directly affect the evapotranspiration losses of moisture.
Temperature conditions are reflected by the graphs reproduced in
Figures 9 to 14. This type of graph was developed by G. F. Deasy (1941).
Absolute maximum. absolute minimum. mean maximum and mean minimum temperatures for the months have been plotted for the ten-year
period 1936 to 1945.
The annual temperature range is shown by the sweep of the four
curves. The shaded space between the mean maximum and mean minimum
curyes repr~sents the average or "normal" diurnal temperature range
dun~g.t~e different months of the ye.ar. Extreme temperatures, which may
be h.mltmg factors for plan.t ~nd anImal life, are reflected by the absolute
maximum and absolute mmlmum curves. The distance of the extreme
wrves from the mean curves indicates the daily temperature irregularity
for different months.
.Wh~n the features of these graphs have become familiar they are
readdy mterpreted at a glance, and very satisfactory mental pictures of
general temperature conditions at the various centres are presented.
17
HOT
HEET
Abs.rna.. .
Ab~.mdI<S .
WARM
o
WARM
0
lLlL
a:w
::>0::
::>:>
';(1;{
0::0::
ww
a.. a..
L~
ww 50
- -
-~ ---~ ~ -- -~-r-;;t"
~I-
COOL
KOEl
32
- Ab!>.m;" .
A b!>.min .
- -- ---------- -- - ~ -~ -.- - - . --- .- ---~-.-- ---- -----____l
COLD
KOUD
l~_L_ ~-.J
o
J
f
M
__ l_~ __ J ~ ____L_~_ . ~L-.l
AM)
FIG. 9.
Temperature Chart.
JONKERSHOEK.
J
A
SON
0
18
HOT
HEET
80r-------------------------------------~·
WARM
WARM
COOL
KOEl
32r-------------~-=~-==-~~------------4
COLD
KOUD
J
F
M A M
J
J
FIG. 10.
Temperature Chart.
Deepwalls.
A
SON
0
19
The effect of the hot dry summer on the JoI,lkershoek graph (Figure 9)
can readily be deduced. This graph shows considerable 'annual range,
chiefly due to the hot days experienced in summer. Conditions are said
to be "summer-sun controlled". Days range, on the average, from hot
in summer to mild in winter. Hot days (over 80 degrees) are experienced
throughout the year, and, in summer, temperatures of over a hundred
degrees are occasionally recorded. Diurnal fluctuation-shown by width
of shaded band- are on the average greater in summer than in winter.
Conditions for eva po-transpiration loss of moisture are therefore unfavourable throughout the year, and are serious in summer.
Nights range on the average from mild to cool, and frosts do not
occur. Cold does not induce a marked rest period in the growth of
vegetation. Tree growth is practically continuous throughout the year. In
the sclerophyll scrub vegetation of this region most plants have, however,
become adapted to a 'period of rest during the critical dry hot summer
months. Many are dormant during that time, for example, therophytes,
geophytes and deciduous phanerophytes.
The Deepwalls graph (Figure 10), shows the maximum and minimum
curves in more or less the same positions as those for lonkershoek. Hot
days of over 100 degrees occur here too, chiefly on days when the" bergwind", a dry, hot wind, coming over the mountain from the dry, hot
interior and blowing down to the sea (a "fOhn" type), is blowing. Average
conditions are, however, milder; diurnal fluctuations are less throughout
the year, and the seasonal range is less. Days range on the average from
warm to mild. Nights are mainly mild. Temperatures therefore appear
to be such, that, other factors being equal, somewhat lower evapo-transpiration losses might be expected here than in the winter-rainfall region.
On the other hand it is difficult to judge the significance of the more continuous transpiration due to the mild summer conditions which makes it
unnecessary for most plants to enter a dormant stage. This is true for all
the forest species and the mild temperature conditions are at the same
time one of the two important factors permitting the development of
forests; the second being the constant availability of soil moisture, due to
constant rainfall and the liberal volumes of seepage over shallow rock
formations.
The true forest centre at Evelyn Valley also shows mild temperature
conditions (Figure 11). The average diurnal variation (the width of the
shaded band) is somewhat greater than at Deepwalls, but this is due to
cooler nights, not to hot days.
Weza experiences more severe conditions (Figure 12). Diurnal fluctuations are generally considerable. Days are warm even in winter. Nights
are mild in summer and cool in winter. Cold nights with frost occur
from April to October. Under these conditions very high evapo-transpiralion losses of moisture may be expected.
The wet true forestry centre, Woodbush, again demonstrates the
typically mild temperature conditions (Figure 13). Days are warm to
mild and no temperatures of over 100 degrees have been recorded. Nights
are mild to cool but frosts are rare.
Conditions most likely to promote the loss of moisture through
evaporation and transpiration are reflected by Louis Trichardt, which is
not a true forestry centre (Figure 14). Days are hot to warm on the average,
and very high temperatures are recorded. The seasonal fluctuation is
considerable, due to the hot summer. Only for a brief period in winter
are the ni~hts cool.
20
HOT
HEET
Abs. ma)f .
A bS'.maks.
80~---------------4----~--------------~
,
WARM
WARM
0
~ ~ 68~~~~~~------------------------~~~~
COOL
KOEL
32~------------~--------------~~~~~~
COLD
KOUO
F
M
A
M
FIG.
J
J
11.
Temperature Chart.
Evelyn Valley.
A
SON
0
21
HOT
HEET
COOL
KOEl
COLD
KOUD
FIG. 12.
Temperature Chart.
Weza.
22
These temperature graphs are not intended to provide a complete
representation of tempreature conditions in the regions discussed. They
are merely random samples to show that considerable variations do occur.
These are important in judging the effects of forests on water conservation.
The graphs indicate also that mild conditions are generally associated with
true forestry sites.
STREAMFLOW.
The discharge of water by a stream depends on the nature of the
rainfall; the topography of the catchment; the depth, texture and composition of the soil; the atmospheric temperature and other climatic
conditions; the geological structure of the area; and the vegetal cover.
The site factors together with the plants and animals, form interrelated
systems which are the basic units in nature and have been called
., ecosystems" by Tansley (1935). A knowledge of ecosystems as wholes
is necessary in order to understand fluctuations in the discharge of
streams.
For a particular area there is primarily, the climate, associated with the
geographic position of the site; and. the geology, characteristic of that
locality. Soils. topography and vegetation are resultants of the reaction
of climatic factors on the rock formations that are present. As topography
and soils develop, and plant succession advances, these become interacting site factors. The depth and texture of soil will be affected by topography; the character of the soil formed. for example. whether it erodes
easily or not. will react on the topography. Soil composition will strongly
affect the nature of the vegetation; vegetation will react and modify the
nature of the soils. Parallel to these counter-balanced processes a system
of streams. adapted to the site and indeed determined by the characteristics
of the site, will develop.
These considerations indicate that characteristic streamflow will be
associated with each part of the coun.try.
Stream discharge is the balance of water derived from , precipitation,
which has not been held in the catchment, returned to the atmosphere,
or lost through deep underground seepage. The portion of water returned
to the atmosphere is decisive in determining the water· cycle or water
economy within the catchment. The volumes held and disappearing as
seepage are often relatively small and fairly constant from year to year.
provided the conditions in the catchment are not fundamentally altered.
The portion returned to the atmosphere is termed "consumptive use" by
American hydrologists; it is approximately equivalent to "evapo-transpiration loss" of water.
Investigations in recent years tend to show that the volume of moisture
returned to the atmosphere through transpiration is greater than the volume
evaporated, that is if two identical areas are available-one covered with
vegetation and one bare-the loss from the vegetation will be greater than
from the bare ground. Results of experiments chiefly by Professor Viehmyer are summarised by Adams, Ewing and Huberty (1947). Whether a
catchment is covered with vegetation or not will therefore have a significant
effect on the water discharged by streams. Ground cannot. however,
be preserved if it is not covered by vegetation; its conservation depends.
in fact, on maintaining entire ecosystems. The problem is thus one of
determinin.g the relative effects on stream discharge of various forms of
vegetation.
23
HOT
HEET
80
WARM
WARM
o
Gem maks
0
LLLL
68
a::lLJ
~a::
~::::>
t.;(~
ww
a:: a::
Q.Q.
~~
wlLJ 50
1-1-
COOL
KOEl
32~________________~____~-=~==~~
COLD
KOUD
M A
M
FIG. 13.
Temperature Chart.
Woodbush.
Abs. min.
__~A~b=s~.~m~in~._
24
HOT
HEET
32~--------------~--~----------------~
COLD
KOUO
FIG. 14.
Temperature Chart.
Louis Trichardt.
25
The role played by vegetation in water conservation, induding its
influence on stream discharge has generally been underestimated. It is the
vital process by which plants pump water out of the soil into the air
(transpiraton) and not the purely physcal process of evaporat?n, that
accounts for the major loss of water, and thus largely determtnes the
volumes available for use in the ground and in streams.
In this general consideration of streamflow, evapo-transpiration losses
of moisture must, therefore, constantly be taken into account.
It has been estimated that the total run-off of water in streams and
rivers from the Union of South Africa is about six per cent. of the total
volume of water deposited as rainfall. Within the country the variation is
considerable. In. very dry areas, it may be practically nothing. In precipitous.
high-rainfall areas it may be 80 per cent. or more.
If the absolute run-off (approximately given on Map number XIV
of the Drought Investigation Commi&sion Report, 1923) and evapo-transpiration losses are considered, we find that the losses in high-rainfall parts
are greater than in dry parts (Wicht, 1945). In these areas the evaporation may even be lower than in drier ones and the higher losses of
moisture must largely be ascribed to higher transpiration of the more
luxuriant vegetation.
In the drier parts of the sderophyll scrub in the Cape vegetational
region, for example, north of Piquetberg, where the annual. rainfall is
about 10 inches. more than 95 per cent., or 9 · 5 inches of rain are
returned to the atmosphere. Along the south coast about 90 per cent.
of rainfall, or between 14 and 18 inches, are lost. In a mountain catchment
area in Ionkershoek-:-Bosboukloof-where the average rainfall is about
43 inches, 28· 5 inches-three times more than the volume consumed north
of Piquetberg-are returned to the atmosphere through evaporation and
transpiration. It was also observed in this catchment, that, although
the annual run-off was very much affected by variations in rainfall, the
annual evapo-transpiration losses were much more constant.
There are probably catchments in which the losses are greater than
in Bosboukloof. The differences between the areas mentioned are.
however, sufficiently striking to show that the effects of the natural vegetation on the water cycle varies considerably, and that it is probably
relatively less strong in high rainfall areas, but that the absolute volume
of water transpired is probably greater in wetter parts. It is reasonable
to accept that the effects of forests and plantations will vary in a like
mann.er in different regions. Most probably the eva po-transpiration losses
from an area covered by a certain type of vegetation-grass, scrub, plantation, forest-will tend to remain constant, wherever it grows. This is
suggested by the relatively constant figure obtained from Bosboukloof in
dry and wet years. It may therefore be deduced that an artificial plantation, for example of pines, will have a relatively much more severe effect
in dry regions than in wet ones. This implies that such plantations are
much more likely to dry up streams away from true forestry areas than
in them.
Perennial streams in the winter- and summer-rainfall zones show
extreme variations in discharge, even when the catchments are well maintained. A disturbance of conditions in the catchment may have serious
results. In the winter-rainfall zone the depletion during the long dry
summer may be very severe. In winter, prolonged heavy rains in the
mountains may lead to high concentrations of flood waters in streams and
26
rivers. Thus the Eerste River, gauged at Jonkershoek, varies in flow from
a trickle of about four cubic feet per second in very dry periods, to over
5,000 cubic feet per second in flood. In the summer-rainfall zone. flow
goes down very much in the long dry winter. In summer a rivulet may be
converted to a foaming flood for a few hours by an intense thunder
storm. In the constant-rainfall zone, perennial flow is usually more
constant, but heavy spates are associated with exceptional rains.
GROUNDWATER.
In the moist forestry regions groundwater conditions are imperfectly
known. Groundwater has been studied chiefly in areas where boreholes
are required and the slopes normally used for tree-planting have not been
considered. These slopes are, however, catchment areas not only for
streams, but also for groundwater which may move slowly down.wards,
underground, to replenish supplies in the lowlands, and it would be in
the national interest to acquire more knwledge of infiltration and movement of groundwater in such parts.
It may be accepted that there will be considerable regional differences
in groundwater. At Jonkershoek, for example, soils are deep, sandy and
porous. Conditions under a .good cover of sclerophyll scrub are excellent
for infiltration of considerable volumes of water. These soils probably
yield seepage to streams relatively rapidly, however. and consequently
dry out fairly quickly. At Deepwalls, in the indigenous forest of the
Knysna area, the high organic content of soils will tend to increase their
water-holding capacity, but here the soils are exceptionally shallow and
storage is low. Dry periods therefore cause rapid depletion of streams due
to decrease in groundwater. At a centre along the escarpment in the
Eastern Transvaal conditions for infiltration into deep soils may be
excellent and here we may expect high groundwater storage and steady
streams fed from such supplies.
In the mountain areas false or floating . water tables will usually
be encountered. Here groundwater may percolate deep down out of reach
of the roots of plants. In the forestry area in Zululand and on the peneplain of the Zitzikama in the Cape Midlands, the water table is always near
the surface of the ground and accessible to roots of trees. In these level
areas. depletion of groundwater by vegetation will probably be considerable.
There are reasons to believe that the usage of water by plantations,
forests and other plant communities will depend chiefly on the volume of
water available in the soil. Variations in groundwater on different sites
are therefore of considerable importance, and the factors determining
the groundwater conditions need to be very carefully investigated in order
to disclose the true effects of trees on water supplies.
*
*
*
*
*
The data presented so far suggest five conclusions: (1) water losses
may be dangerous in the hot summers at centres like Jonkershoek, (2)
losses may also be dangerous in dry winters at centres like Weza, (3) losses
may become dangerous away from true forest centres for example at
Louis Trichardt. (4) water losses are probably less important at true forestry
centres in mountainous areas. (5) on relatively level areas, with shallow
watertables like in Zululand and the Zitzikama, depletion of groundwater
through transpiration of vegetation may be severe.
27
II. MODIFICATION OF THE WATER CYCLE THROUGH LAND
MANAGEMENT.
The first section of this report dealt with variations in the water
cycle in different parts of the country where trees are cultivated. At a
particular centre the water cycle defined in terms of volume of water, will
depend on the volume and distribution of rainfall, frequencies and lengths
of dry periods, atmospheric temperature, cloudiness, wind, topography, soil
and the vegetation. By land management the vegetation can be modified.
transformed or removed. The manner in which various forms of land
management might influence the natural movements of moisture must now
be considered; which is equivalent to examining the role that vegetation
plays in determining the water cycle. Casual references to the influence
of vegetation have been made in the first section.; details must be added.
Available data are insufficient to explain the influence of vegetation
fully. This is probably the most important -field of research in South
Africa which awaits exploration at present. Some significant results from
investigations in the United States of America and elsewhere, may indicate
what the effects in South Africa are likely to be, but, until local investigations have been completed, conclusions must remain largely speculative
(Meinzer, 1942; Kittredge, 1948).
Vegetation influences the water cycle in many ways, directly and
indirectly, for example:1. Precipitation is intercepted by the foliage, twigs and branches
and evaporated from there. In this way as much as one
third of the annual precipitation at a particular centre may be
returned to the atmosphere.
2. The flow of water over the soil surface is impeded by roots and
stems of plants, and, indirectly, through their litter.
3. Vegetation affects the infiltration of precipitation into the soil
indirectly by modifying the structure and composition of soil
and by protecting the soil surface from puddling and erosion.
4. Vegetation also improves the water-holding capacity of soils by
increasing the humus content and thus the colloids in. the soil.
5. Vegetation depletes soil moisture by extracting a portion of it
through transpiration.
All forms of plant growth use water. If it is desired to determine how
the water cycle will be modified by a change in land management, it is
therefore not merely necessary to measure how much water the subsequent
vegetation will use, but whether it will use more or less than the original
vegetation. The five ways in which vegetation can affect the water cycle
must be evaluated in· detail, before and after the change in. managment.
in order to ascertain whether interception has become greater or less.
whether surface run-off is accelerated or impeded, wether infiltration is
facilitated or inhibited. whether soils are exhausted and eroded, or beinl!
built up, and whether transpiration. is stronger or weaker. Alternatively
the recording of the single variable: stream-discharge from a confined
catchment, can be used to demonstrate the composite effect of all changes
in the water cycle on the water yielded for use by man.
Whatever method is used to estimate the effects of a change in management, the basic problem is: the comparison of different plant communities.
In the present study, for example, plantations of exotic timber trees will
be compared with the scrub and grass communities replaced by them. and
with natural forests on comparable sites.
28
The study of such communities is the concern of plan~ ecology. It
has established basic relations of the development or successIOn of vegetation to water supplies. The effects of replacing sclerophyll scrub-fynkosin the Midlands and South Western Cape, and grass m the summer-ramfall
zones by plantations of trees should threfore also be examined in the
light of ecological findings.
The communities replaced by plantations are forthe most part" seral ",
that is, they are stages in the development of the vegetation towards a
climax vegetation, which, in the regions of high rainfall, is a closed
forest. This development is seldom completed in nature, because It IS
interrupted by recurrent fires. Without the fillip, which is given to the
succession of vegetation, when trees are planted, the climax can only be
reached when the site has been developed, under the seral communities,
so that it is fit to carry the natural forest. This process of development
entails, if we examine it carefully, a building-up of the soil, while the
humus content is being increased. The effect of the development on a
wet site-hydrosere-is to dry out the site, until it is fit to carry the forest
climax. Theeffect of the development on a dry site -xerosere- is, to
increase the water-holding capacity of the soil by increasing the humus
content, so that the hygrophilous tree species are then able to grow.
Although starting from the wet and dry extremes the developments therefore tend towards relatively stable and uniform conditions in the forests.
The forests formed by the South African indigenous trees on dry .
sites, apparently require and use more water than the fynbos and grass
communities which they succeed. There are good reasons to suppose
that the artificial forest climax, created by establishing plantations of
exotic trees on such sites, may also use more water than the fynbos or grass
but not necessarily more than the natural forest. What the magnitude
of the difference will be is difficult to ascertain. It may be supposed that
it will be significan.t in the winter rainfall parts, where the fynbos is semidormant, and uses little water during the driest periods of the summer.
It will probably be fairly marked in the summer rainfall parts, where the
grass is quite dormant during the dry winter. On the other hand if tall
tynbos, such as occurs extensively in the Zitzikama and along the edges
of most forests, is being replaced, the difference is likely to be slight.
The fact that indigenous high forest species often invade mature
plantations of gums and pines and compete successfully with these exotics
for moisture is evidence that the exotics do not necessarily make heavier
demands on water supplies (see Figures 21 and 22).
.
These statements are generalisations, which have to be modified in
accordance with specific conditions, if they are applied to particular sites.
They are to a large extent hypothetical and have to be tested by further
observations.
The plant ecological arguments, which have been briefly recorded
here, do, however, indicate that plantations of exotics constitute communties of the same order as natural forests. The development of co~munities
of a particular ecological order on a specific site, and their usage of water,
probably depends more on the volume of water ·available on that site,
than on the species of plants represented and the detailed composition
of the communities. Fully-developed evergreen plant communities use all
the available moisture on the site in the dry season. Transpiration is ·
reduced when less water is available, and increased up to a degree, when
the supply is augmented.
29
This general introduction leads to the c~msideration of the effects
of land management on particular phases of the water cycle.
INFLUENCE OF LAND MANAGEMENT ON PRECIPITATION.
Men have argued about the effects of forests on rainfall for more
than a century. The problem remains unsolved. There are reasons to
believe that complete removal of vegetation over a vast area could have
considerable effect on the precipitation on that area and on areas beyond.
Tannehill, a well-known American meteorologist, believes that rainfall on
the North American continent has been strongly affected by clearing the
land for farming, highways, pasturage and other purposes (1947). He
considers that this has occurred because of an increase in the temperature
contrast between continent and ocean brought about by the clearing. It is
almost certain, however, that the extent of destruction of indigenous forests
and establishment of exotic plantations in South Mrica could have had
no significant effect on the rainfall of the country and even the local
distribution of rainfall could have been only slightly affected. Forests
were never extensive and plantations cover only a fraction of one per cent.
of the area of the Union.
The presence or absence of forests or the maintenance or destruction
or vegetation on a particular site may, however, influence the effectiveness
of precipitation on that area very significantly. To ascertain this influence
it is necessary to determine the net-rainfall, that is the rainfall penetrating
the vegetation to reach the ground.
Experiments in Europe and the United State of America have
established that interception of rain by forests canopies may be as high
as one third of the total annual precipitation. The technique used in these
experiments has mostly been unsatisfactory. In recent years the problem
of determining net-rainfall under forest canopies has received attention
at the J onkershoek Research Station, and Dr. H. C. Wi 1m of the Rocky
Mountain Forest and Range Experiment Station, Forest Service, United
States Department of Agriculture, who worked on similar lines, has
developed technique of measurement, methods of sampling and statistical
analysis, that are Qot likely to be improved upon to any extent (1943).
Future observation of net-rainfall in South Africa will be greatly improved
in consequence of these advances. Unfortunately satisfactory methods of
estimating interception by scrub and grass vegetation has not yet been
evolved, so that no comparison is possible at present. The observation of
net-rainfall is indispensable in the study of streamflow from afforested
catchments.
At Jonkershoek net-rainfall was determined under the canopies of a
mature stand of Populus canescens, Smith, (Wicht 1941) and a young stand
of Pinus radiata Don (Beekhuis, 1943). The poplars intercepted 14·2 per
cent. of the summer rains, but only 3 per cent of the heavy winter rains
when the trees were without leaves. About 8 per cent. of the total annual
precipitation was intercepted. In the young pine stand, where the canopy
was not yet quite closed interception was slight, but much heavier interception may be expected in older pine stands.
It is doubtful whether much more rain will penetrate to the soil under
fynbos or grass vegetation than under trees, but such surmises are risky
while reliable observations are wanting.
.
It has been arlSued that forests may catch misty precipitation and
that considerable volumes of moisture may be added to the ground through
"drip "and "stemftow". The Jonkershoek investigations did not show
13857·2
30
that this was so, but in the .< mist belt " of Natal Midlands and Eastern
Transvaal circumstances may be very different. This question must also
remain in abeyance until reliable observations have been made in scrub
and grass as well as in forests and plantations.
Other hypothetical influences of forests and plan.tations on precipitation such as: increase due to cooling of atmosphere; increase due to
rising aircurrents over forests; and increase due to raising of effective
height of ground level by dense mass of trees; have not as yet been. demonstrated experimentally. It is however noteworthy that the influences of
forests and plantations on precipitation whatever they may be. are nN
regarded as adverse.
INFLUENCE OF LAND MANAGEMEl\-· ON EVAPORATION AND TRANSPJRATIOl'.
If the water economy on a particular site is considered by striking a
balance with water added to soil on the credit side and water lost to
soil on the debit side, it would immediately be apparent whether the
effects of the vegetation are favourable or not. Unfortunately the loss of
water to the atmosphere, especially the extraction of water from the soil
by plants through transpiration, is virtually impossible to estimate satisfactorily in the field.
Experiments have been carried out that indicate that transpiration
is the decisive process largely determining the character of the water cycle
on a particular area. The work of Viehmeyer (1938a, 1938b, 1947; with
Johnston, 1944; Sampson, 1944) and others in the chaparrel vegetation of
California is n.oteworthy. This vegetation is akin to the sclerophyll scrub
of the Cape and grows in a somewhat similar climate. At the end of
the dry season Viehmeyer compared soil moisture determinations from plots
that had been burned, with plots that had remained under vegetation. In
the topmost 6 to 12 inches, the differences were negligible, but at depth
of 1-!- feet to 2 feet, the denuded plots showed higher soil moisture than
those under vegetation. He. conduded that this was chiefly due to losses
through transpiration. A few inconclusive observations made under sclerophyll scrub at 10nkershoek tended to support these findings (Wicht, 1948).
Investigations of diurnal fluctuations in streamflow. and in the water
level in boreholes by Blaney, Taylor, and Young (1930) and Troxell (1936)
in the United States, and observations of diurnal streamflow fluctuatioID
at 10nkershoek (Wicht, 1941), have led to the conclusion that these are
mainly caused by transpiration of vegetation. Recently Croft (1948) in
the United State has ascribed increased discharge of streams after removal
of riparian vegetation to decrease in transpiration rather than a change in
evaporation. This demonstrated further the very considerable influence
which transpiration can exercise on the water economy of an area.
Dr. M. Henrici (1940, 1946a, b and c), has attempted to arrive al
estimates of the volumes of water transpired by various plant communities.
and to find out whether indigenous trees in South Africa transpire less than
exotic trees. Her attempts to attain these objects are open to serious
criticisms.
The technique used by Dr. ·Henrici, depends on the rapid weighing of
twigs, within a few minutes after cutting, on a sensitive Hartmann and
Braun torsion balance. Weinmann and Ie Roux (1946) have made a
critical study of the torsion balance for use in measuring transpiration.
They found that:" Agreement in the rate of water loss for three minutes before
and after cutting was poor. Only in approximately ten per cent of
31
the experiments with barley, maize, oats and fescue, and in 35 per
cent. of the experiments with wneat did the transpiration rates agree
within 20 per cent. The average discrepancies ranged from 49 per
cent. in maize to 106 per cent. in barley.
The correlation co-efficient for the actual water loss during
the three minutes before and after cutting in all (85) experin;lents
was - 0'319."
They concluded, with complete justification: "The results indicate that
in the plants investigated the determination of the water loss from the cut
plants cannot be regarded as a reliable index of the true transpiration
rate of the intact plant."
Data yielded by a technique which can be so unsatisfactory hardly
warrant the definite conclusions drawn by Dr. Henrici.
Also unsatisfactory is the manner in which Dr. Henrici's
investigations were designed. She does not indicate in her publications
how sampling was done for the estimation of the volume of water transpired by whole trees and stands or natural associations. Evidence is
available that, in trees, shaded twigs generally transpire less than those
in the sun. and Huber (1923) found in a Sequoia that leaves from lower
branches transpired six times as much as those from near the tree top.
The manner in which sample twigs are taken is therefore very important.
Dr. Henrici considers that larger, taller exotics must use more water
than smaller indigenous species, but there is no evidence to support this
theory. She states that faster growing exotics must use more moisture
than slower growing indigelious trees. Her data show no differences however in the transpiration of exotics and indigenous species per unit weight
of fresh material per unit of time. They merely tend to show that the
<!xotics use water more efficiently.
Dr. Henrici concludes from her data that exotic trees (especially pines)
do not transpire more than indigenous species. She states: "A glance
at the figures reveals immediately that there cannot be any question of a
higher transpiration of the exotics" (Part II, page 6). "The conifers
inve'Stigated always have a small transpiration, and it is quite out of the
question that they could by transpiration endanger water sources" (Patt
£II, page 32). "It has been stated that if the transpiration values for
indigenous and exotic trees respectively are plotted, two curves are obtained
which intersect at more than one point and hence it cannot be deduced
that the transpiration values of exotics exceed that of the indigenous
-trees (part IV. page 12). Transpiration figures for gums and wattles
determined by Dr. Henrici, are, however. as high as the highest determined
for indigenous species.
On the basis of the observations on pines and gums, hypothetical data
are deduced to show that exotics when grown in plantation form. will use
more water than the natural forests. From observations on a few single
tre~s grown in the open. the consumption of water by trees grown in close
plantation form is deduced. Her final conclusions are therefore based
on a theory propounded on data. which. in view of the arguments set
forward here. and the tests carried out by Weinmann and Ie Roux,
cannot be accepted as reliable. In estimating the water used by stands
uf trees on the basis of observations on cut twigs from single trees so
many factors have to be taken in.to account, that the value of the estimates
is highly problematic.
tt
32
The premise, also accepted by Dr. Henrici, that fast growing trees
must use more water than slow growing ones, needs to be further examined.
The hypothesis that variations in rate of growth could equally well be
caused by differences in the efficiency with which trees use water, does not
appear to have been considered. Alternatively it may be that different
species or varieties of trees differ merely in their inherited tendency to grow
fast or slowly, in which case the volume of water transpired will not
determine the growth. Indeed, it is a wellknown physiological fact that
the water transpired by plants is far in excess of what is needed for their
growth, and the true significance of transpiration in the life of the plant
has never been explained (Miller, 1938, pages 495-496). Dr. Henrici
herself states (Part III, 1946): "The main water consumption of trees is
for transpiration and only in the second place for the formation of wood."
Experiments to determine the effects of soil fertility on the water
requirements of plants have shown that the addition of fertilizers causes a
reduction of water requirement. " ... In poor soils the water requirement
may be reduced by half or sometimes as much as two-thirds by application
of nutrients ", Miller states (page 500). "When the supply of nutrients
in the soil approaches exhaustion the rate of growth of the plant is
greatly reduced, but no corresponding change occurs in the transpiration
rate. This fact is evidence that transpiration is not a measure of growth".
On the other hand rate of growth cannot be used as a measure of
transpiration. Dr. Henrici (1948) has stated: "As all wood formation
depends mainly on photosynthesis and, as such. demands a definite amount
of water, it is logic to state that, apart from transpiration. wood produced
in ten vears will use a certain amount of water, whilst the same amount
of wood produced in 50 years by another species will use the same amount
of water within a period five times longer." This conclusion mayor may
not be correct, but it clearly refers only to the relatively small amount of
water used in photosynthesis, and does not provide any reason to assume
that the total consumption of water by fast growing trees in a given time
will be significantly greater than that of slow growing trees.
Experiments have shown that, the faster specimens of a particular
species, or, more precisely, individuals of the same genotype, are grown.
the more water will they use, but, the salient point here is, that individuals
belonging to one genotype cannot be grown faster than. others of the same
genotype, unless they are grown on a better site, which invariably means
that more water becomes available to them. If two species or varieties
are grown on the same site, on which a specific volume of water is
available for use, it is likely that the faster growing one is merely using
the wafer more elficiently because of an inherited tendency to do so.
Some evidence is available In support of this theory. It happens.
especially with fruit trees, that a fast growing variety is grafted on a
slow growing variety, or the reverse. Then the diameter of the tree bole
above or below the graft, according to whether the stock or the scion is
the faster growing. becomes greater or less than below. because of the
differential rates of growth. Yet the two varieties are united in a single
stem and the same sapstream passes through both (see Figure 15).
There is a ten.dency to personify trees when their usage of water
is being discussed. They are then treated as if they were organisms that
drink water when they are thirsty.· This conception is misleading. Man
seeks and drinks water because his systems needs it, and he stops when
his thirst is quenched. The tree is different. It wllI absorb and transpire
continuously while conditions for evaporation are favourable and water
33
B.- Grafted Pear Tree. Scion grown more slowl)
than stock.
A.-Grafted Apple tree. Scion
grown more rapidly than stock .
(After:
BUsgen, MUnch and
Thompson, 1929.)
FIG. 15 .
Diverse rates of growth in grafted tree boles.
is available. Only when the water supply becomes so depleted that
wilting sets in, that is, the cells begin to collapse because too much water
is withdrawn from them and turgidity cannot be maintained, is the loss
of water to the atmosphere checked. Transpiration thus constitutes a
danger to the tree, and the tree can survive only if it has efficient expedients
for checking the loss of water when wilting starts. The ability of a tree
to survive depends chiefly on how effectively it can stotJ using water by
rapidly closing its stomata. Drought resisting trees and other plants are,
generally speaking. the most efficient in checking the loss of moisture.
Gums are probably more efficient than pines. and both these more efficiel,.
than indigenous high forest species. They can therefore live on less wall."
on drier sites. That is one of the reasons why they cltn be grown sucessfully away from the natural forest sites in South Africa. Maximow
has shown that such drought resisting species may even use more water
than so-called moisture loving ones, if given sufficient supplies of water.
The Consumption would therefore seem to depend on the volume of water
available more than on other factors.
.
34
It is apparently not the species used. the size of individuals. the
composition of communities, or even the conditions regulating evaporation.
as much as the volume of water available in the soil. that will determine
how much is consumed. That is why trees will dry up vleis and swamps
where the water is stagnant. or nearly so. and therefore constantly available.
From the discussion of the question: "How does treeplanting affect
~vater suplies?". it would appear that the answer is dependent on:
(1) the volume of water available in the soil; (2) the climatic conditions
affecting rate of transpiration; and (3) the inherited tendency of trees to
use more or less water. Variations in these three factors. which determine
transpiration losses of moisture, make generalisations impossible. .Much
of the controversy which exists is no doubt due to differences in observations under different conditions.
LAND MANAGEMENT ON INFILTRATION.
The precipitation reaching the ground surface is separated into that
which infiltrates into the soil and that which runs off over the surface.
The soil can take up a certain volume of moisture and acts as a reservoir
which holds this moisture. To insure the most efficient use of this
reset voir. the maximum infiltration of moisture is desirable. The phenomena
on the soil surface during rain. whereby the water is separated into infiltration, run-off and evaporation. are therefore of the very greatest importance.
and their elucidation will most likely' provide significant evidence towards
the solution of the problem of how land management will influence water
conservation. Unfortunately experimental data are almost entirely lacking
in South Africa.
Investigations in the United States. Europe and elsewhere have indicated that infiltration is high where there are undisturbed natural forest
canopy and floor (Kittredge. 1948). and it is almost certain that this will
also apply to conditions in well-preserved indigenous forests in South
Africa. On the other hand run-off will be strong and infiltration slight
where grass and shrub veld are burnt and grazed (Rycroft. 1947; Thompson. 1936). The question that remains to be answered is. what will be
the effects on infiltration. of establishing plantations of exotic trees?
In most true forestry regions. the floors of well-managed plantations
are covered with leaves or needles and litter. and are well protected from
erosion. even while the plantations are relatively young. There is apparently
no reason why the infiltration of moisture. after it has penetrated through
the canopy and ground-cover to the soil. should not be good. As the
plantations grow older conditions are further improved and on suitable
sites. tend more and more towards those prevailing in indigenous forests.
If it can be shown that conditions for infiltration in plantations are
favourable. so that there will be good replenishment of groundwater this
will counter-balance evidence which might be found. for example. that
plantation trees transpire more water than the communities they replace.
On the other hand it may be found that the leaves. twigs and other litter
in forests absorb IJIuch moisture which is evaporated before penetrating
to the soil.
INFLUENCE OF
LAND MANAGEMENT ON STREAMFWW.
The water discharged by a stream is the residue. or surplus water not
utilized on the catchment area or returned to the atmosphere through
transpiration. The streamflow represents water in form in which it is
INFLUENCE OF
I.
Z
1.0
·8
VERDAMPING -TRANSPIR.ASIE
EVAPO -
tl~
~'4I
~~
'V~
~
-5
w
Vl
~ ~
~
~;;j
\)C)
~~
Q
2
o
M
N
f
M
N
M
V
M
2
DAYS
FIG. 16.
Schematic Hydrograph .
N
4-
3
DAE
M
N
5
All
36
immediately accessible, and can be used for irrigation or municipal purposes
either directly or after impounding in a reservoir. The only other form
in which precipitation on mountain catchments can become available for
use. is as groundwater. which may feed resources at lower levels where it
can be pumped out of boreholes or wells.
The effects of tree-planting on streamflow from mountain catchments
are of special interest. In order to understand these. the fluctuations in
discharge, typical of mountain streams, must be clearly understood.
The discharge of streams may be divided into: storm discharge and
groundwater discharge. depending on whether it is derived from precipitation which has run off over the ground-surface or penetrated only very
slightly, or whether it is derived from infiltrated water that has subsequently seeped out into the stream. By careful study of hydrographs
(which show the fluctuations in stream discharge over time) the proportions
of storm and groundwater discharge can be approximately separated.
Figure 16 is a hypothetical hydrograph in which the separation of
discharge into its components is indicated. Other hydrographs have been
described elsewhere (Wicht, 1941, 1942, 1943, 1947).
Storm discharge is delivered when the stream is in spate. It is
associated with a rapid 'rise in discharge to a peak and then a slower
depletion until the discharge curve impinges upon the groundwater discharge
curve. Irregularities in distribution and occurrence of precipitation. cause
many deViations from the basic form of spate illustrated.
It is generally considered that storm discharge is reduced and smoothed
out where heavy forest vegetation is present in the catchment. Such
, vegetation will retard the precipitation on its way to the stream, by:
interception on stems, leaves and twigs; and by impeding the run-off by
stems, roots and litter on the soil surface. It is also considered that
storm discharge is reduced in forested areas because infiltration is facilitated. During catastrophic storms it has however been observed that the
influence of vegetation becomes reduced to insignificance.
There is no satisfactory evidence of the effects of forests on. spates
in South Africa, though such evidence should be yielded by investigations
that are now proceeding. Rycroft (1947) has demonstrated significant
effects on storm discharge. caused by removing sclerophyll scrub vegetation
through burning late in the autumn.
Groundwater discharge constitutes the basic flow of the stream, and
is derived from water that has infiltrated into the soil and seeped out into
the stream. It maintains the streams during dry periods and is the chief
source of irrigat.ion and domestic water supplies where storage reservoirs
are not available. Groundwater supplies are replenished during and
immediately after rain by infiltration. and increased groundwater flow in
streams results. The flow depletes during dry periods. at first rapidly,
and then more and more slowly in a characteristic manner (Wicht 1942).
To improve stream discharge it is generally desirable to augment groundwater flow and if possible, also retard its depletion so that the discharge
will be well maintained during lengthy dry periods.
It is no simple matter to decide how vegetation will affect groundwater
flow. If depletion is accelerated or flow is much reduced, the stream will
tend to be dried up: if depletion is retarded and flow increased the position
will be improved. The most favourable time to observe the effects is at
the end of a long dry period when conditions will be most critical. If
37
improved infiltration and soil moisture storage conditions outweigh th.e
draught on soil moisture through transpiration, the discharge will -be
improved.
Groundwater discharge shows a second type of fluctuation, referred
to previously (page 30). which is apparently strongly influenced by streambank vegetation. It is a daily drop in discharge caused by evapo-transpiration losses from the stream. The extent of the drop is related to
variations in factors influencing evaporation and transpiration. By studying the diurnal fluctuation in streams from expermental catchments it will
be possible to determine whether trees planted to replace fynbos or grass
draw more or less water from the stream and its moist banks than the
original vegetation (Wicht, 1941).
Some striking evidence has become available chiefly from the United
States of America, of the manner in which the removal of forest vegetation
from a catchment will affect the discharge of a stream (Kittredge, 1948;
Adams, Ewing and Huberty, 1947).
An experiment carried out at the Coweeta Experimental Forest in
the Southern Appalachian Mountains, which has been described by
Hoover (1944), consisted of determining the regression of discharge from
one catchment on discharge from another very similar one, while both
were covered with forest. On one catchment the vegetation was then
cut down. and stream discharge records further analysed. The author
states: "The results to date indicate that in this locality a forest-stand
with an understory of dense shrub annually uses from 17 to 22 inches in
transpiration. Removal of this vegetation has been shown to increase
streamflow by an amount equal to the estimated transpirations. if sproutgrowth is prevented." In this experiment there was a minimum of disturbance to the forest-floor during cutting. The cut trees were left where
they fell. The results therefore apply to a catchment with a forest soil
but no vegetation. Haig (1946) gives similar results from the same region.
Dunford and Fletcher (1947), also working in this region, showed that
cutting of riparian. vegetation has a very significant effect on the discharge
of streams. They found that diurnal fluctuations were almost smoothed
out and the ground-water flow was increased. Croft (1948) has reported
similar results from Farmington Creek in northern Utah. These effects
were all determined for periods of one or two years after removal of
vegtation, when the forest must still have had a strong after effect on the
site. It should be noted that it was the native vegetation which was
experimentally removed. There is no reason to expect dissimilar effects
if vegetation were to be removed from South African stream catchments.
Instances have in f!lct been reported where discharge has been increased
after removal of vegetation by burning or cutting. Evidence of the effects
of replacing one type of vegetation by another is as yet inadequate.
The draught of riparian vegetation on streamflow reflected in the
diurnal fluctuations of small streams, must also by implication have a
marked effect on the low groundwater discharge recorded during long dry
periods, and the rate of depletion of groundwater flow. Particular attention should therefore be given to the role played by riparian vegetation
(Croft. 1948), and the policy of the Forestry Deparment. to which it has
adhered since 1932. of not afforesting streambanks and the sources of
streams, appears to be a wise one. The water in streams when flow is low
is drawn chiefly from the moist streambanks, where the moisture is concentrated by gravitation from higher portions of the ca.tchments, and here
it is constantly accessible and available for use by the vegetation, in the
same way as the mOIsture remains always available in swamps and vleis.
*
*
*
*
The ways in which vegetation can affect various phases of the water
cycle have been very briefly described. When judging the total effect ot
a change in vegetation, such as the planting of trees in grass or scrub veld,
it is of the utmost importance to consider all phases of the water cycle
in conjunction, and not to select the influence on one phase and stress
that alone. To do this is equivalent to quoting portion of a coherent
passage out of its context.
The evidence and arguments which have been adduced, leave no doubt
that vegetation can ha ve ~ a highly significant influence on the constitution
of the water cycle on a particular site. The chief object of this investigation
is to discover irrefutable instances where a change in vegetation, not merely
the complete removal of vegetation, has caused a real change in the water
cycle. It will also be the task of future hydrological research to provide
through well-planned controlled experiments, further unequivocal evidence
of changes in stream discharge as a result of modifications of vegetation.
HI. SPECfFIC EVIDENCE OF EFFECTS OF FORESTS ON WATER
SUPPLIES IN SOUTH AFRICA.
An attempt has been made in the foregoing pages to place the problem
of the effects of forestry on water supplies in its true perspective. The
hydrological environments have been outlined in order that the effects
of changes in environmental factors would. not be confused with those of
the establishment of forests. The possible extent and nature of the effects
of forests on water supplies were examined, so that some criterion could
be provided on which to judge whether the claims of the effects of forests
put forward , were reasonable or not. With the background of the problem
provided in this way, it remains now to study specific evidence, or alleged
evidence, of the effects of forests on water supplies on particular sites.
Many instances where it has been alleged that trees have dried up water.
have been investigated, and the results of these investigations are set forth
here.
Unequivocal proof of the effects of forests can not be obtained by
investigations such as these. Proof can be provided by planned and
controlled experiments only. Circumstantial evidence will however be
obtained, from which it may be possible to infer the effects which forests
exercise on water supplies with a reasonable measure of assurance. To
a large extent use will have to be made of qualitative reasoning until more
complete data make quantitative reasoning possible.
Statements that forests have affected water supplies have appeared:
[n the popular press; in correspondence adressed to the Forestry Department since as far back as fifty years ago; and in special communications to
the Director of Forestry elicitea by a circular letter sent out by him in
1947. The Director's letter was sent to about 60 prominent persons and
institutions, and 36 replies \vere received. The most definite statements
have almost always come from laymen, while informed opinion has usually
been more reserved . Communications from trained scientists are for the
most part restricted to discussions of available evidence and to suggestions
that certain effects may be possible.
39
THE CAPE PENINSULA.
Extensive afforestation on the Cape Peninsula was commenced during
the high rainfall period, shown in Figure 2, which occurred shortly before
1900. Elderly people thinking back, have remembered a peninsula on
which fewer plantations were to be found. They have also remembered
that streams used to be stronger, and have attnbuted decreased flow to
the growth of trees, chiefly because the decrease in rainfall was less
evident to them.
Mr. J. D. Keet (1940), formerly Director of Forestry, has shown that
fluctuations in streamflow on the peninsula may be largely attributed to
rainfall trends. He has personal knowledge of the Tokai-Constantia area
since afforestation began in 1884, and he has effectively countered the
claims of farmers that afforestation has diminished the flow of streams in
the Constantia Valley.
The owner of a farm below Cecilia plantation has claimed that a
stream was greatly diminished as the result of the development of a stand
of Eucalyptus trees in the catchment. These trees were clear-felled in
1935, and it was claimed that the flow of the stream was subsequently
improved. It is significant that the rainfall trend has also been upwards
since 1935 (see Figure 2), and further grounds to accept the rainfall fluctuations, and not the tree growth, as the dominant influence, have been derived
from recent enquiries. These yielded the information that the flow had
remained fairly strong. An examination of the catchment showed, however, that the coppice of Eucalyptus had grown in the 12 years since
clearfelling so that a full vigorous stand occupied the entire area. The
farmer who claimed that the flow had been satisfactory since the clearfelling had not realised that this regrowth had taken place.
It has been suggested that the clearfelling of pines around the Rhodes
memorial on the slopes of Devil's Peak has caused water to reappear
in a spring which was dry while the pines were standing. This area was
examined with the Manager of the Groot Schuur Estate, and it was agreed,
that the evidence was inadequate. Many years ago the spring had been
led into a water trough, but this had been disused for years. The flow
had, however, as far as could be ascertained always been intermittent. The
presence of the trough was the only evidence that the flow may formerly
have been more regular and stronger. There had apparently been no
significant improvement in flow after the felling of the pines.
No more definite evidence than that which has been mentioned could
be disclosed on the Cape Peninsula. Professor R. H. Compton, Director
of the National Botanic Gardens at Kirstenbosch, who has an intimate
knowledge of the Cape Peninsula, has summarised the position clearly in
his reply to the Director of Forestry's circular minute:"I know of course that one is frequently told that plantations
of trees on mountain slopes have resulted in drying up the streams
and springs below them. This is said quite dogmatically by people
in the Tokai and Constantia valleys, for instance, and it seems
to be a rather general opinion.
When ,one co~siders the .evidence, however, one cannot accept
the theory as bemg conclUSively established. Other factors are
involved, e.g. veld burning, generalisations from occasional years
of sub-normal rainfall etc., and these may be responsible for some
or all of the blame ascribed to the trees.
40
The theory may of course be true, but this can only be settled
satisfactorily by means of experiment and observations which have
not been made in the past: And this is where the work proceeding
at lonkershoek and initiated at Cathedral Peak is likely to be of
such importance. In the absence of such data one can only say
that there is a possibility that plantations have a reducing effect
on streamflow which may be of a serious nature in cases where
the streams or springs are naturally low in the dry season: but that
this possibility is at present unproven."
BRlDASDORP.
The town of Bredasdorp obtains its water supplies from a kloof
known as Langfontein. A marshy area along the stream course carried
large Eucalyptus trees which were removed in 1937. After that time the
discharge of the stream improved considerably, and it is possible that this
improvement is due to the removal of the trees. The area lies outside
the true forestry region and is one where high rates of transpiration must
be expected, especially on such a site where there was a constant supply of
groundwater.
THE EERSTE RIVER.
In .the press and in communications addressed to the Director of
Forestry by the Lower Eerste River River Board. it has been claimed that
afforestation in lonkershoek, near the source of the Eerste River. is
adversely affecting the summer low-flow of the river. The lonkershoek
Forest Influences Research Station is situated in the catchment and full
data of streamflow and rainfall are available. There is however no
evidence that the flow of the Eerste River or of any other stream in
lpnkershoek has been affected by tree-planting.
The upper catchment of the Eerste River which is controlled by the
Forestry Department is 15,680 acres in extent. Of this some 3.200 acres
are to be planted. The planted area will be on lower slopes which receive
less rain than higher parts. Within the 10nkershoek vaHey there is also
an area of private property eqaul to 4,160 acres, on which the rainfall
is still lower. The volume of rain water deposited in the catchment is
distributed in such a way that about 4/ "ths fall on the mountain areas
which are never to be afforested. About one tenth falls on the slopes
which will eventually be planted up. Of these less than half have as yet
been planted up. and it is quite unlikely that this degree of afforestation
could noticeably affect the flow of the river. A small tributory catchment
of the Eerste River- Bosboukloof- has been afforested as completely as
possible. and 80 per cent. af the area is under plantation. Complete
records of rainfall in this catchment and of stream discharge are available
since before afforestation. This intensive afforestation has not yet had any
effect on the streamflow.
.
Figure 17 shows the total annual rainfall plotted against the total
annual discharge of the stream. The values used are for streamflow years.
th::!t is April to March. Strong streamflow is obviously closely correlated
with high rainfall years in this small catchment. If the dates of the
various plotted points are examined no sign of a trend indicating either
th3t tIle discharge has decreased or increased relative to the rainfall is
found. If this ;ere so, recent values would be grouped either above or
nelow the regression line which has been fitted to the data.
41
70.--------------·- -- -
44-45.
45-46 •
40-41.
..43-44
,
,
38-39
40
• '39 -40
MILJO£N£ G£I..LINt;J".
MILLIONS OF GALLONS
100
200
.300
400
TOTAL ANNUAL DISCHAREE -BOS80UKLOOF
TOTALE JAAR.LIKSE VLO£/ .
FIG. 17.
Regression of Discharge on Rainfall.
Figure 18 shows a number of regressions of monthly discharge from
the un-afforested Biesievlei catchment on the afforested Bosboukloof
catchment. If afforestation had affected the discharge this would have
changed the relationship of discharge in Biesievlei to that of Bosboukloof.
and there would have appeared significant differences in the regression
coefficients. These did not appear. Examination of the regressions
determined for s!Jccessive years also shows that they do not fall into any
order. Their positions are jumbled. If the oldest plantation concentrated
in Bosboukloof catchment has not influenced discharge form this stream,
it is extremely unlikely that younger plantations and lower degree of
afforestation could have affected the Eerste River in the manner alleged.
SJMONDlUM.
In a letter addressed to the Director of Forestry dated 30th June,
1947, Mr. D. Gordon Mills of Cape Town has referred to a stream on
the farm Donkerhoek near Simondium in the Paarl division, as providing
evidence of the drying effect of a pine plantation. This stream was
examined in the field and the evidence obtained; that it became weak
during the period of low rains experienced at the Cape from about 1925
500
42
9OOor------.------'1-------r------,------.----~_,
I. 1940-41: y", 23S. 33 + -/87{X-131,J-J3)
2 . 1941-42: Y 2484·S0~·IZ7(X-37.J9-08)
8000
3. 1942 -43 : Y :>329·58 + ·13b (X -2289·/7} - - - t r - - - - j
4.1943-44: Y :2G8:00 +-/34(X -/685·4Z)
5.1944-45: Y:.386·92 +'131(X -2G65·00)
6.1945-46: Y:t:410·25 +'/48(X -2G09''''2rr---t---r---I
7_19~
I
-47: Y =258-42 +'/36(X-I7/JOOj
5000r-------~-------r------_r~~~~--------r_------1
III
~
'II
..(:
\)
.~
Q
~
4000r---------~------,---~~y_------r_------+_-----­
..(:
.....
I::
~
~ 3000r-------+-----~~~--~_r------_+------~r_L----­
C)
C)
..J
~
1
~
~ 2000r-~--~+-~~~-r------_r------_f------~r_----__c
ti
ex)
I
1000~--~~~-------r-------
o
tI,
I
o
I
800
1000
400
600
BIESIEVLE 1- MonthlJ Discharge - Maafldelikse Vloe" 10,000J'
200
FIG.
18.
Regressions of Discharge from Biesievlei Stream on Bosboukloof.
to 1935. In recent years the streamflow has again improved. During this
time there has also been an increase in rainfall. Mr. Gordon Mills claims
that fluctuations in discharge are due to tree-planting on the slopes of the
Simons berg behind the farm. The stream however rises on an. unafforested
area beyond the plantation. Some minor tributories only come from the
plantation area. The matter was discussed with Mr. Kock, who lives on
43
Donkerhoek and has known the stream well for man.y years-he gauged
the flow regularly at one time-and he agreed that the fluctuations in flow
-:auld not be ascribed to the planting of trees.
KNYSNA.
Professor J. D. Scott of Pietermaritzburg has stated in a letter to the
Director of Forestry that the Bees River at Concordia near Knysna was
dried up by planting of exotics in its catchment and that it had been
reported to him that it began to flow again after clearfelling of Eucalyptus
stands. There is no proof that the drying of this stream is due to the
afforestation. but there is reason to believe, that the trees may have had
a desiccating effect. At the time of planting, marshy or vlei areas in
this catchment were drained and these areas certainly have become quite
dry under the eucalypt stands and the stream has dried up. Officers of
the Forestry Department who know the stream well, and have inspected it
to ascertain whether the clearfelling of the trees had caused the flow to
improve, have found no conclusive evidence that this has occurred. Flow
apparently occurs only intermittantly when sufficient rain has fallen. In
part of the catchment of the stream where no tree-planting has been done,
fynbos has been burnt and pastured or cleared for cultivation. Tributaries
of the Bees River from this region have also dried up. It is probable,
that decreased rainfall has influenced the drying up of this stream, but the
drying up of the marshy ground by drainage and the transpiration of the
eucalypt stands may have been the major cause in this instance. The
relatively shallow soil over impermeable subsoil would also provide the
very conditions under which trees are most likely to have a desiccating
effect.
GRAHAMSTOWN.
It has been repeatedly suggested that the natural spread of Pinus
pinaster and other exotics from trees planted along the mountain drive
above Grahamstown-about 1880 onwards-have caused the streams
flowing into the Grey reservoir to dry up. The Grey reservoir is no longer
in use and Grahamstown derives its water supplies from other sources.
Grahamstown is not a true forestry centre because the rainfall is
somewhat too low, and it is possible that under the con.ditions prevailing
there trees will have a severe effect on water supplies. The evidence that
the growth of exotics has dried up the streams is however inadequate.
At Grahamstown there has been a striking decline in rainfall as shown
is Figure 4. This must have had a significant effect on water supplies and
has undoubtedly. caused the level of the water table to drop.
Pumping from boreholes in the town and neighbourhood must also
have affected the general level of the water table. One borehole at
Gowies's nurseries ti'sed to deliver 800 gallons per hour for 24 hours when
first put down. Now it can be pumped dry in a few hours. This general
lowering of the water table may also have had an effect on streams from
the hillsides, because it would promote the rapid seepage of rainwater to
deeper levels.
There is some evidence that springs near Grahamstown have dried
up where no trees have grown. For example on the fann "Carl's Rust '".
Highland Rail, Albany, which indicates that there are other influences
besides the transpiration by trees, at work.
44
THE BUFFALO RIVER CATCHMENT.
In the East London "Daily Dispatch" of 17th June, 1947, it was
claimed that: " Those who have known the Pirie Mountains for the last
50 years have no hesitation. in asserting that the flow of the Buffalo River
and other streams which rise there has appreciably diminished in that
time, and they put the reason down to the extensive plantation established
on the heights near the Evelyn Valley forest station."
The facts do not bear out this statement. The total area of the
catchment of the Buffalo River above Fort Grey is about 500 square miles.
Of this area only 2,433 acres or about 3· 8 square miles have been planted
up with exotic trees. It can hardly be supposed that this extent of
afforestation could have had a significant effect on the flow of the river.
FIG.
19.
Mineprop plantation of Eucalyptus saligna, Selukwe Valley. Tzaneen, Eastern Transvaal.
45
This example and some of those previously referred to, appear to
be derived from somewhat irresponsible statements based on poor observation or even on hearsay. Along the escarpment in the eastern Transvaal
more comprehensive observations have however been carried out and
these must be examined in greater detail.
FIG. 20.
Valuable stand of Eucal),pllIs saligna grown for mine-props on Rooikoppies Estate,
Eastern Transvaal.
Age: 12 years. Height: 100 feet.
DUIWELSKLOOF-TZANEEN.
The most detailed studies in this region have been carried out on
the Westfalia group of farms owned by Dr. H. Merensky, who has himself
come to the conclusion that exotic gums and pines have a desiccating
effect on water supplies. The problem has also been studied throughout
this region and reviewed by Prof. T. W. Gevers, Professor of Geology,
University of the Witwatersrand. His conclusions have been given considerable publicity, particularly in "The Star" evening newspaper published in
Johannesburg (1947, a-h), and have consequently been widely quoted.
Prof. Gevers has kindly summarised his findings in a special memorandum
on "Drying Up of Rivers in the N.E. Transvaal ", which he has prepared
for the author of this report (1947, I). The conclusion to this special
memorandum is reproduced here in extenso, particularly because Prof.
Gevers' statements have been in part misrepresented:"Needless to say, I realise that a large number of different
factors operate in the accumulation of groundwater and its subsequent release in streams and springs. The above account should
have made clear that I consider plantations of closely spaced young
gumtrees to be only a contributory cause of the obvious desiccation,
varying in importance under differing conditions in different catchments.
I wish to stress that most cases cited refer to comparatively
small accumulations of surface water, such as swampy kommetjies
and vleis, small springs, trickles and streamlets of shallow origin_
These are the very conditions under which gum plantations are
known by experience elsewhere to be active in drying up the soil.
While individually small, it must also be stressed, however, that
under the hydrologic and hydrographic conditions obtaining, in the
aggregate their contribution to streamflow is quite considerable.
My evidence is largely circumstantial and I know that to
establish the detailed facts accurate measurement of a large number
of factors is necessary over prolonged periods, such as is carried
out at 10nkershoek. But I consider that the evidence I have
collected and which is based on painstaking observations over a
number of years establishes a prima facie case for serious consideration.
The area dealt with is ideal for prolonged research on the lines
of the 10nkershoek and Cathedral Peak Forestry Research Stations.
The effects of gum and other plantations as against indigenous tree
and bush cover, and again of tree cover in general as against
grasslands could be studied with great profit."
With these general sentiments expressed by Prof. Gevers, the author
tinds himself in almost complete agreement. A full report on his findings
(Gevers, 1948) has also recently been published which agrees almost
~ntirely with the memorandum referred to above.
Specific instances of alleged desiccation in this region will, however.
be reviewed in greater detail.
The marked descending trend of rainfall- also stressed by Prof.
Gevers-illustrated in Figures 6 and 7, is probably the chief cause of
desiccation here, as in other parts of the country, and there can be no
doubt that desiccation has occurred.
A cause of increased erosion and run-off and consequently of desiccation in this region, which is generally conceded, is the destruction of natural
vegetation, particularly on ground occupied by natives. Dr. Merensky is
d9ing outstanding work in stopping such destruction and improving
damaged areas.
,
It is also generally agreed that the increased use of water by a
growing population-a factor which is often most surprisingly overlooked
when attempts are made to account for insufficient water supplies-has
probably affected the water regime adversely. The extent of boring along
the valleys has been singled out as a factor which might have a wide-spread
desiccating effect, due to the lowering of the water table.
Special attention must, however, be focussed on th,e effects of plantations of exotic trees, particularly coppice stands of Eucalyptus saligna.
grown for mine props. These are considered to be an "important contributory cause ", of desiccation by Prof. Gevers and others.
The growing of eucalypts for mine props is a very severe form of
land use. The props are cut at 5 to 7 years, so that growth is maintained
at the maximum rate, and, because the trees do not mature, stable, forest
conditions, which are essential for site improvement, are not developed
in the stands. Under this system no ground vegetation, forest shrubs or
other trees, can complete with the eucalypts. Figures 19 and 20 are
typical of E. saligna stands grown in this way. Prof. Gevers' references
to Eucalyptus stands do not include mature stands. He states: "Finally
it is agreed that the point at issue does not concern mature eucalypt
forests, in which a natural balance between growth and decay has been
established, but closely spaced (9-12 feet) young trees in plantations
in tended for pole growing and clear felled every 6 to 8 years.
47
It must be expected that where grass communities are replaced by such
densely grown stands of eucalypts, that the demands on the water
resources will be markedly increased. If, however, the stands are thinned
and grown to larger size on a longer rotation, then true forest conditions
are approached, which resemble to some extent the conditions in natural
eucalypt forests in Australia. This is often done in Government plantations. It is evident in such stands that the eucalypts are not making
excessive demands on soil moisture, because indigenous forest speciesherbs, shrubs and trees-invade the plantations and compete successfully
with the eucalypts for water (see Figure 22). This applied, naturally, to
true forestry sites only and not to dry sites that could never carry indigenous
FIG. 21.
Indigenous forest Vegetation invading plantation of Pinus pinaster, near Stutterheim.
Eastern Cape Province.
48
FIG. 22.
Jndlgenous forest vegetation invading stand of Eucalyptus, near TZaneen. Eastern
Transvaal.
for~st. On moist sites it is therefore unlikely that plantations of eucalypts
will use more water than natural forests, but they probably do use more
than the seral grass or shrub communities which they usually replace.
Mineprop stands have frequently been established where conditions
are marginal due to low rainfall. This applies especially to areas situated
low down on the escarpment. Here mineprops may still be grown with
profit, but it is possible that the consumption of water will be high and
that water supplies will be detrementally affected. Such conditions arc
found in parts, if not everywhere, of the Selukwe (or Shellock) valley near
Tzanecn. On such sites the economic value of the timber produced may
have to be weighed against the value of the water absorbed in producing
that timber.
49
The evidence that eucalypt plantations in the Selukwe valley have
illied up water supplies is. however. by no means exhaustive. Here also
the downward rainfall trend must be considered, and the most vital part
<)f the valley, the moist floor, the "sponge" area, has not been planted
up. but placed under native agricu1ture. Marshy places have been drained
and ploughed for growing maize and other crops. and pasturing is heavy
(see Figure 23). It may therefore be that the bad farming has been the
major cause of desiccation rather than the tree-planting on the slopes above.
The instance of the Brandboontjies River has been widely quoted
an authentic example of desiccation by eucalypts. and it needs to be
dosely examined .
is
The catchment extends from the high rainfall true-forest land on the
farm Christina's Rust to dry lower areas below Mojadje's location. The
lower part is devoid of plantations of exotic trees. On the ridge which
forms the watershed east of the town of Duiwelskloof fairly extensive
eucalypt stands occur. and in the upper catchment above Duiwelskloof.
smaller stands have been established.
The percentage of area of the entire Brandboontjies catchment which
has been afforested is small and could not affect the flow of the river as
a whole significantly . Even if the area considered is restricted to the upper
catchment which comprises the original farms Christina's Rust and
Korthannie. and constitutes the source of water supplies for Duiwelskloof.
<Jnly 9 per cent. of the area is under eucalypt stands. When this area is
visited by car only, the area under eucalypts appears. however. to be
larger, because the plantations lie chiefly along the main road. This is
un the drier side of the valley. The tributaries that feed the Brandboontjies
here. rise chiefly on the wet side of the valley. in remnants of indigenous
forest.
Observations on the source spring of the Brandboontjies (see Figure 24)
were previously considered to have yielded "irresistible evidence" of
desiccation by a eucalypt plantation. It was claimed that two or three
years after eucalypts were planted in 1938 the spring dried up. and further
that when the trees were felled in 1945, the spring began to flow again.
This is. however, an incomplete account of what actually occurred.
It is important to remember when considering this example. that the
spring is a small one. that might easily be influenced by a slight variation
in hydrological conditions. The catchment is only about 33 acres in extent.
Such a spring might easily dry up at certain times and flow again at others,
due to natural fluctuations i'n the water cycle. which might occur even
where the vegetal cover remains entirely stable.
Eucalypts were planted in this catchment over about a quarter of
the area (24 per cent.) above the point where the flow was observed. The
possible effect on the flow of treatments applied over the remainder of the
area must therefore also be considered. The bed of the stream, or. rather.
rivulet, was not planted up. When water flows in the upper part it COmes
from the area formerly occupied by natives, where there are now remn.ants
of scrub forest and abandoned lands which are being planted up with
Trema bracteolata. Only at one point did the course of the stream pass
through a corner of the eucalypt plantation for a very short distance. It
is quite in.correct to state that the catchment was at one time planted up
with eucalypts and subsequenly cleared.
50
An alternative explanation of the behaviour of the stream which is.
apparently quite plausible, and yet does not depend on the influence of
the eucalypts is: it dried up in about 1940 because of the decreasing rainfall (see Figure 6) but began to flow again after 1945, at the time when the
FIG. 23.
Selukwe Valley near Tzaneen, Eastern Transvaal. Slopes planted with Eucalyptus
saligna. Valley floor occupied by natives, grazed and cultivated.
eucalypts were felled, because the natives were simultaneously removed
from the major portion-three quarters-of the catchment, and the whole
area completely protected. These steps were taken by Dr. Merensky
who owns this area. He also has evidence from other catchments on
Westfalia that the removal of natives and protection may have had this
effect.
51
.
9.,
1.
\
~
~,
f-T~-f
OPSKI2IF
LEGEND.
~
L-~
---~
~ ~
~, t
q
~
~
INDI(f£HOIIS FOItEST
~ OU MIELIL-t.AND£"
OLD M£ALlE LANDS.
~ OULANOE .
~...
OLD LANDS.
~ KAPRYPi! 60MME
~
FIG. 24.
Source of Brandboontjies River on Farm Christinas-Rust.
~ INHEEMSE WOUD.
MllrUIZE GUMS.
~
~ DENNE80ME.
PINES.
(After T. W. Gevers.)
Another possible cause of the flow which began in the year after the
eucalypts were felled, is that it was a wet year~ The year July to June.
1944-45 received 32·31 inches of rain, the following year-1945-46received 53 ·20 inches. It is also significan.t that Prof. Gevers, when he
inspected the spring in November 1947 found that the flow had been
reduced to a mere trickle.
The possibility that the eucalypts did have a contributory effect on the
behaviour of this sprin~ is not entirely excluded by the arguments presented
here, but this is certainly not a clear-cut case. There is no evidence that
the eucalypts had a more severe effect than natural forest-which is the
climax vegetation on this site-or even than a relatively stable, scrub
vegetation, would have had.
Prof. Gevers states in the special memorandum which he prepared as
his contribution. to the investigation reported here and also in his subsequently published paper (1948): "Summarising the conditions of the Brand·
boontjies River, it must be said that the bulk of the desiccation must be .
ascribed to decrease in rainfall and despoliation of vegetal cover, and soil
erosion, with gum plan.tations a subordinate contributory cause in the
uppermost source area and on the Duiwelskloof ridge." This is a valid
conclusion. From the analysis set forth here it is certain that the influence
of the eucalypts must have been very slight in this area.
52
Further observations on Dr. Merensky's farm Westfalia have not
yielded more conclusive data, but there are undoubtedly indications of
the drying effect of trees on vlei or swampy areas, and that removal of
tree growth from a catchment may cause an immediate, but probably
temporary, improvement in discharge similar to that observed at Coweeta
Experimental Forest in America (see p. 37).
Within the Brandboontjies catchmentMr. A. J. C. Volschenk (1948)
has observed that gums have a drying effect on vlei areas. In the same
region Mr. W. P. Po hi (1948) has observed that the indigenous tree, the
Waterbessie, Syzygium cordatum Hochst. may have a desiccating effect
on the flow of a stream. The exchange of ideas, and the diverse opinions
on the effects of Eucalypts in the Brandboontjies catchment on water
supplies, expressed by these and other farmers (Maberley 1948) in letters
to the "Fanner's Weekly" are of considerable interest. It is possible
that Mr. Volschenk and Mr. Pohl, who supports the author of this report,
are both right in their contentions as to the effects of trees on water
supplies, and that disagreement is entirely due to differences in the degree
of the effect, and to variations in the circumstances under which specific
effects were observed.
Professor Gevers (1948) refers to two instances which he considers
have yielded conclusive evidence that the Eucalyptus, mine-prop, plantations dry up streams: The source of the Masatani. River on Middelrand
and two streams on the southern slopes of the Baccarat (not Bakkerad)
Forest Reserve in the valley of the Great Letaba. The second example
was also inspected in June, 1947, by the author of this report and a
number of other instances, not mentioned by Professor Gevers, were
described to him where such mine-prop plantations have apparently had
a desiccating effect on small streams. It should be noted that in all these
instances the criterion is flow from catchments that are burnt and grazed,
a form of exploitation which does not ensure sustained farming in these
relatively steep areas with fairly high rainfall. Figure 13-aerial view of
two adjoining catchments- published by Professor Gevers (1948), illustrating the areas on Roumen and Grysappel, below Baccarat, shows one
catchment with natural forest at the top and ~ucalypt Rlantations lower
down. Although the streamflow has decreased the soil in the catchment
is well preserved and it will continue to yield timber indefinitely. The
adjacent catchment on Grysappel is almost completely denuded. It would
be interesting to learn whether the application of proper conservation
farming methods would not also ensure the "use of the water where it
falls" and thus reduce stream discharge available lower down. However,
these considerations do not affect the conclusion that in these instances
short-rotation gum crops established on bare ground, have apparently
caused a decrease in streamflow.
SoUTPANSBERG.
The water supplies of Louis Trichardt have deteriorated in recent
years and have become inadequate. One reason which has been mentioned
is the afforestation with exotics at Hanglip plantation which lies partly
in the catchment of the town's water supply.
A glance a Figure 14 will show, however, that the rainfall at Louis
Trichardt has decreased in a remarkable manner and this must be accepted
as the dominant influence on water supplies in this area. The stream
which delivers tbe town's water supply rises in indigenous forest, moreover, and it has also dried up in the forest where it could not have been
affected by exotics.
53
At Roodewal plantation further to the East along the Soutpansberg.
streamflow has been recorded over notched weirs since December. 1936.
Three weirs were established in the same catchment. In 1941-42 the
stream dried up so much that water no longer flowed over the two lower
weirs. There were however no trees whatsoever planted in the catchment.
so that the desiccation must be ascribed to the decrease in rainfall (see
Figures 14-15). Analysis of the data showed that the fluctuations in
stream discharge were closely correlated with variations in rainfall.
.
NATAL MIDLANDS.
From the Natal Midlands there are a number of allegations that trees
have dried up water. but the evidence is contradictory.
The water supply of Harding is said to have been decreased by plantations of exotics. From this same area it is also recorded, however, that
a stream rising in the indigenous forest at Ingeli. on the Border Forest
Reserve. has ceased to flow, although no exotics have been planted in its
catchment. On the other hand the original source of water supply for
the forester's use at Border plantation. has dried up. and exotics, planted
over the area, may have been the cause in this instance.
Dr. John Fisher formerly Principal of the Cedara College of Agriculture has claimed that the water supply to the college has been affected
by exotics planted in the catchment. He has also been told by farmers
that wattle plantations dried up springs which flowed again when the
wattles were felled. Here again cause and effect have not been conclusively
linked. and the evidence is too incomplete to do this. The question must
remain open especially as contradictory testimonies are also available.
Thus on his farm Broadmoor, Mr. Hunt-Holley. has observed" springs
running out of wattle plantations where none were ever seen before".
This he ascribes to the application of sound silvicultural practice in his
wattle plantations. and it is quite possible that the manner in which the
plantations are treated may be decisive in determining their effect on water
supplies.
ZULULAND.
In Zululand eucalypts are planted in swamps to help dry these
out. as a measure in the control of malaria mosquitoes. Examples may
be seen near M'tubatuba. Eucalypts have also been used for this
purpose in the Pontine marshes in Italy and the Tarsus mashes in Turkey.
and the method is undoubtedly effective.
IV. CONCLUSION.
The specific instances of alleged desiccation by exotic trees referred
to here, have been selected from widely separated parts of South Africa.
Others might be added. for example from the White River region in the
Eastern Transvaal or the Transkei. but it is doubtful whether their consideration would assist materially to clarify further the general picture
which it has been possible to derive from this review.
The fragmentary and contradictory nature of the evidence which has
been presented. demonstrates clearly that the time has not yet ·come to
draw final conclusions as to the effects of forestry on water supplies.
The danger of over generalisation has also been demonstrated. Certain
tentative general conclusions do, however, appear to be justified and these
are set forth here, although it is fully realised that they may have to be
revised when more complete data are available. There has been much
54
loose talk: on this subject and, on the other hand, reliable results from
controlled analytical research are not yet available. The following conclusions therefore merely attempt to summarise briefly the least problematic results of general experience and observation. I t is hoped, that the)
will approximate reasonably closely to reality and so usefully replace the
unrestrained and irrational speculation. which has too often played a role
in the consideration of this very involved and controvertial problem: I. Plantations of exotic trees, grown to timber size. will probabl)
not use more water than indigenolls forests. if they are on
comparable sites.
') Plantations of exotics and indigenous forests, will probably use
more water the fynbas (sclerophyll scrub) or grass communities. The magnitudes of the differences are not known, hut
in. the case of plantations they will probably be greater the
more conditions deviate from those in true, moist, high-foreq
regions.
3. The consumption of water by plantations, forests and other
plant communities, will depend chiefly on the amount of water
available in the soil.
4. Plant communities of the same ecological order. that i'.
occupying siniilar positions in the succession of vegetation.
will probably use approximately equal volumes of wllter.
5. Swamps and vleis tend to be dried up if trees are planted ill
them, and also if the natural succession progresses as far a~
the forest climax. The water is constantly available or accessible to the roots of the trees, because it is stagant or nearly S0.
6. There is no evidence that fast-growing tree species use more
water than slow-growing ones- all other factors being equal.
7. The removal of vegetation- natural or artificial- from catchments, especially along stream banks, will cause an increased
discharge from the streams. The advantage is probably temporary because it depends on the retention of deep soils, rich in
humus, which is impossible without a good cover of vegetation.
8. Heavy ground-cover provided by plantations and forests retard
floods, build up and conserve soils.
The ultimate validity of these conclusions will be tested in time. For
the present it is essential that they should be synthesized into a series of
practical recommendations in regard to tree-planting in catchment areas .
The development of the country cannot be delayed until further information
is available; it must proceed to keep pace with increasing popUlation .
industrial expansion, and, in native territories, also with a rise in standard
of living. Where tree-planting is contemplated it is therefore advisable tha t
the following recommendations should be considered: -I. Extensive afforestation should not be undertaken. unless there i ~
~ reasonable chance of making a good profit on the mone),
Invested, or if there is not some other convincing local reason
demanding the establishment of trees. for eXallple, hut-pole
growing in native territories to reduce exploitation of natural
forests; aesthetic reasons; firewood production on farms: or
provision of shelter for stock.
') Afforestation should as far as possible be restricted to forest
regions with high rainfall. There are man.y areas in South
Africa where extensive afforestation should not be encouraged.
55
3. From the water conservation point of view, long rotation timber
crops should be preferred to short rotation, quick-profit crops
such as eucalypt-coppice for mine-props, or wattle for tanning
bark. In making this statement the important economic role
of such crops is not lost sight of. Sites for these crops should
however be selected with care.
4. Where the discharge of streams is used for irrigation, industrial
or municipal purposes, moist areas along streams should not
be planted up. This has been Forestry Department policy since
1932.
5. Tree-planting is not Forestry. Sound silvicultural practice and
management on a sustained yield basis, are essential, and if
these are not introduced the effects may be harmful on even
the most suitable sites.
*
*
*
*
*
In recent years, forestry has become accepted as an industry of
premier importance, which is making a valuable con.tribution to the
national welfare of South Africa. If in the expansion and practice of
forestry, the recommendations enumerated above are observed. it will
continue to yield great ben.efits in the' future, without jeopardising the
production of farming or industry by adversely influencing the water
supplies, indeed, it should through the conservation and building up of
soils and the reduction of floods, especially in the vital mountain areas,
assist to stabilise and safeguard the water resources of the country.
ACKNOWLEDGEMENTS.
The effects of forestry on water supplies has been discussed with a very
large number of persons from all parts of South Africa, including eminent
scientists, experienced farmers and keen and enightened observers from
other walks of life. It is impossible now to define to what extent the
author's thinking has been influenced by those discussions, but their influence
has undoubtedly been considerable. He wishes therefore to acknowledge
the assitance he has received in this way from so many people that they
cannot all be mentioned here. Although fully occupied with their own
work they were nevertheless prepared to sacrifice time and energy in order
to contribute what they could to the elucidation of this wide and involved
problem.
Much information has been gained from replies received to a circular
letter sent out by the Director of Forestry in 1947, in which he asked for
opinions and evidence on this subject. It is no fault of those who replied
that the "expresison of opinions" predominated over the "presentation
of reliable evidence". All replies were valuable even when they were
entirely negative or contributed little to the solution of the main problem.
Thus when a recognised scientist with an intimate knowledge of a particular
region, replied that he had no evidence that exotic trees had dried up water.
this was accepted as a very valuable contribution, because the trees could
not then obviously have had that effect. Replies to the Director's letter
were received from: Prof. J. D. Scott, formerly in charge of Estcourt
Agricultural Research Station, n.ow Profesosr at the Pietermaritzburg
College of Agriculture; The Organising Secretary. National Veld Trust:
Prof. J. H. Wellington, Faculty of Geography, University of Witwatersrand:
The Department of Botany, Orange Free State University College, Bloemfontein: Dr. 1. Fisher. formerly Principal of the Cedara Agricultural
56
College, Natal; Dr. T. Schumann, Director of Meteorological Office.
Pretoria: Dr. J. S. Henkel, Pietermaritzburg, Natal; Dr. J. C. Ross.
Director: Soil Conservation and Extension, Pretoria, who also arranged
for replies from Mr. E. du Toit of his Division, and the Chief Regional
Officers at Queenstown, Stellenbosch, Pietermaritzburg and Bloemfontein.:
Mr. D. R. D'Ewes, Cape Town; Mr. D. Gordon Mills, Cape Town; Prof.
N. J. G. Smith, Faculty of Botany, Rhodes University College, Grahams·
town; The late Dr. Alex. L. du Toit, F.R.S. Geologist: Prof. R. H.
Compton, Director, National Botanic Gardens, Kirstenbosch, Newlands:
Dr. R. A. Dyer, Director: Division of Botany and Plant Pathology.
Pretoria; Prof. R. S. Adamson, Faculty of Botany, University of Cape
Town; Mr. T. B. Bowker, M.P., Grahamstown; Dr. E. A. Nobbs, Stellenbosch; Mr. H. R. Roberts, Department of Native Affairs. Pretoria; Mr. J. A.
Norton~ Planning Committee, Qumbu, Transkei: Mr. N. S. Pillans, Bolus
Herbarium, Cape Town: Mr. J. P. Botha, Athole Agricultural Research
Station, Ermelo; Dr. J. W. Rowland, Principal Pasture Research Officer.
Pretoria; Prof. W. J. Talbot, Faculty of Geography, University of Cape
Town: Mr. C. H. Wood, Grahamstown; Mr. N. Shand, Consulting Engineer, Cape Town; Prof. J. T. R. Sim, Agronomy Section, Stellenbosch
Elsenburg College of Agriculture; Dr. H. Merensky. Geologist, Westfalia.
Eastern Transvaal; The. City Engineer, Cape Town; The Director of
Agricultural Education and Research; Dr. S. H. Houghton. Director:
Geological Survey, Pretoria; and the Director of Irrigation, Pretoria.
Special acknowledgement is due to Dr. H. Merellsky. who discussed
the problem with the author, demonstrated the areas under observation on
his farm Westfalia in the field. and submitted valuable data of rainfall
and the flow of springs; and to Prof. T. W. Gevers. who prepared a
comprehensive memorandum setting forth his conclusions.
The indispensable data provided by the Meteorological Office are
gratefully acknowledged, and also the help given by the Hydrographic
Surveyor of the Irrigation Department.
Finally the trouble taken by fellow officers of the Forestry Department
to gather information, and the assistance given by them in the field. are
especially appreciated and gratefully acknowledged.
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