Biological Journal of the Lintlean Socierv, 1 0 : 5-28.\"ith 7 figure\
March 1978
Nyumba ya Mungu reservoir, Tanzania
The general features
PATRICK DENNY
Depurtiiien t oj ' Bo tail!, uiid Biocli einis t r y , Westfield College,
The Uiiiwrsity of Londort, Loridon N W 3 7ST, Etigluiitl
Accepted f o r publication June I977
Nyumba ya Mungu reservoir was cornpletcd in 1965 in northern Tanzania. By 1970 there was a
thriving tilapia fishery but it declined catastrophically in subsequent years. A team of biologists
surveyed the lake in 1974,and this paper outlines their hydrological, geographical, and climatic
findings.
'The dam, constructcd across a northkouth Ncogene fault-trough 80 km south of Mt.
Kilimanjaro. has produced a reservoir about 180 k i n * with a mean depth of 6 m. The
catchmcnt area is cxtensive but the main source of water is from Mt. Kilimanjaro. The lake is
situated in an arid area, and over one quarter of the input water evaporates from the lake's
surface. 'The outflow is regulated by a hydro-electric power station and is practically constant.
A t full capacity the retention time of the lake is one year but due to abnormally low rainfall,
the reservoir was not full, and the retention time was nearer nine months.
l h e lake is polymictic and a deep wind-driven current is maintained by the Trade Winds for
most of the year.
The general features of Nyumba ya Mungu are brietly compared with other man-made and
natural lakes in Africa.
KI;,V WORDS:-tropical man-made lake- African-physical features.
CONTENTS
. . . . . . . . . . . . . .
Introduction
Reservoir size
. . . . . . . . . . . . .
Lake surface area . . . . . . . . . . .
Lake storage volume and mean depth . . . . .
Geology of Nyumba ya Mungu region
. . . . . .
Climatc and its effect on the reservoir
. . . . . .
Windspeed
. . . . .
. . . . . . . . . . . . .
Seiches
Surface waves
. . . . . . . . . . .
Water currents
. . . . . .
Solar irradiation
. . . . . .
Air temperature
. . . . . . . . . . .
. . . . .
Water temperature and stratification
Light penetration
. . . . . . . . . .
Water balance
. . . . . . . . . . . . .
Rainfall in the catchment area
. . . . . . .
Rainfall and changes in lake level
. . . . . .
Water loss by evaporation from an evaporation pan
Estimated water loss by evaporation from lake surface
. . . .
Water loss by discharge through the dam
Estimated inflow rates
. . . . . . . . .
Water balance and retention time
. . . . . .
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P. DENNY
6
Discussion
. .
Acknowledge ments
References
. .
Appendices 1 to 4
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24
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INTRODUCTION
Nyumba ya Mungu (NYM) is a man-made lake in the Kilimanjaro region of
northern Tanzania I t was constructed for purposes of irrigation, hydro-electric
power and fisheries, and completed in December 1965.The irrigation scheme
has not been developed but by 1970 a thriving tilapia fishery had become
established. Subsequent reports (Annual Report, 1972, 1973) of a major
decline in the fish catches, and the possibility of a water-weed explosion,
caused serious concern, and following an initial study by Petr (1974),and a
report by Okorie (1974),the present biological survey was undertaken between
July and September 1974.
The reservoir is situated in the Pangani River Valley of the Masai Steppe,
about 50 km south of Moshi (Fig. 1). It is fed by two major inflows, the rivers
Kikuletwa and Rum which drain some 7500 km2 of catchment consisting of
wooded grassland, forest, true desert, and alpine desert. The Masai Steppe is an
arid zone with unreliable short rains in NovembedDecember and longer rains
from March to May. The annual rainfall rarely exceeds 500 mm a year and the
extensive hot dry season and Trade Winds produce an inhospitable environment. The vegetation before impoundment consisted of a narrow band of tall
dense Acacia woodland along the banks of the river giving way to sparse
grassland in the flood-plains heavily grazed by Masai cattle, sheep and goats; and
semi-arid Acacia scrub with an understorey of Sunsevieriu (Bailey, 1965). The
scrubland predominates around the lake shore, but to the north-west bordering
the River Kikuletwa inflow, sugar and sisal is extensively cultivated. At an
altitude of 670 m above sea level it is a natural place for a reservoir as there is a
northhouth orientated geological fault-trough, which in the Late Tertiary
period sustained an expansive lake. The trough is bounded by the Lelatema
mountain range (1623m a.s.1.) 10 km to the west of Nyumba ya Mungu, and
the North Pare mountain range (2113m a.s.l.), 20 km to the east. Approximately 80 km to the north of the lake is Mt. Kilimanjaro (5895m a.s.1.) with
its snow-capped peaks frequently veiled by clouds. Rain and snow-melt from
southern Kilimanjaro provide the main watershed which is received by the
rivers Kikuletwa and Ruvu. The headwaters of the River Kikultetwa drain the
southern slopes of Mt. Meru (4566m a.s.1.) to the west of Kilimanjaro but the
river connects with tributaries from Kilimanjaro before it enters north-west
Nyumba ya Mungu at Samanga. The river Ruvu passes through extensive Typha
and papyrus swamps before entering north-eastern NYM a t Korogwe. I t
receives runoff water from south-eastern Kilimanjaro and outflow water from
Lake Jipe, a small (c. 16 km') natural lake 35 km to the east of NYM but
geographically separated by the North Pare mountain range.
The dam, which is of the rock-filled variety with an inclined clay core near
the upstream face, was built across the Pangani river 14 km south of the
confluence of the rivers Kikuletwa and Ruvu. The lake created is roughly
triangular in shape with a broad (c. 15 km), shallow northern end narrowing to
GlNI'KAL. I:IL4TUKES OF NYUMBA Y A MUNGU
7
8
P. DENNY
Table 1. General features of Nyumba ya Mungu: location, 3" 4 5 ' S , 37" 25' E;
altitude, 670 m a.s.1.
-
Length
Width
Surface area
Depth at dam
Mean depth of lake
Perimeter
Drawdown
Capacity
Kctention time (estimate)
Maximum
Minimum
_ _ _ _ _ ~ ___-___
2 8 km
32 km
15 km
4 km
1 1 0 km2
1 8 0 km'
41 rn
37.5 m
6 m
7 2 km
9 6 km
3.5 m
1120 x lo6 m3
1 year
~~
__
Average
30 k m
6.5 km
142 krn
39 m
7 6 km
9 months
Solar irradiation (annual mean) 2813 J Em-' day-'.
Air temperature (annual mean) 25.3" C.
Rainfall (annual mean) 4 5 3 mm year-'.
about 4 m where the Kikuletwa and Ruvu rivers previously joined to form the
River Pangani. I t is approximately 30 km in length and the depth gradually
increases from north to south to a maximum of 41 m at the dam.
The proportions of the lake are summarized in Table 1 but these are liable to
considerable change depending upon rainfall and drawdown, which can be as
much as 3 m. A spillway has been constructed to the west of the dam so that
when the lake level rises above the designed 41 m level the excess water can
overflow. Although the lake started to spill in 1968 and continued intermittently until September 1971, the rains in the last few years have been
insufficient to fill the lake to maximum capacity, and has led to exposure of
former flooded shorelines.
Trees and shrubs were cleared before impoundment in only a very few small
areas, and although in the deeper water they are now completely submerged, in
the shallower regions, especially in the north, they emerge from the water and
their dead branches provide perching and roosting facilities for a wide variety
of waterbirds. The submerged branches make boating and fishing difficult but
provide a large surface area for the periphyton which has a major importance in
the food web (Bowker & Denny, 1978).
With the development of the fisheries a previously low population area
attracted large numbers of people from other regions of Tanzania and Kenya.
In 1970/71 fishermen from lakes Victoria, Tanganyika and Malawi came to
Nyumba ya Mungu and created 26 settlements around the lake shore (Fig. 2)
with a total estimated population of 25,000 people (Ricardo, 1974). The
settlements varied in size and structure with Lang'ata and Kilimanjaro National
Co-operative Union (KNCU) becoming the largest villages on the eastern shore,
and Mikocheni a well organized township on the northern shore. An Ujamaa
co-operative was formed at Ngorika (Masai) on the western shore and scattered
villages sprang up around the lake wherever convenient. Many of these were
transient in nature. At Damsite fishing was forbidden but a community
associated with the Tanzania Electric Supply Company Limited (TANESCO)
and the Water Board became established. The influx of fishermen from such
diverse regions introduced many and varied fishing techniques including gill
GENERAL FEATURES O F NYUMBA VA MUNGU
9
Nyumba ya Mungu
I
!
I
,
,
I
,
I
,,
1
\
Legend
Notlon.1 Sorvico
*It.: 67Om.A.S.I.
L i t . : 3'45's.
Long.: 3 f 2s'E.
1
_.
0
1
2
3
4
s
Km
Figure 2. Map of Nyumba ya Mungu showing the River Kikuletwa and River Ruvu inflows, and
the River Pangani outflow. The main fishing villages and the dam are labelled.
10
P. DENNY
netting, beach seining, hand lining, and trapping; and fish catch estimates of
over 28,000 metric tonnes were quoted for 1970 (Ann. Report, 1970). By
1973/74 this had declined to an estimated 3000 metric tonnes and our visits to
the villages during the survey confirmed that the fisheries were mainly
unprofitable. Whole villages had become abandoned and in others, many huts
were derelict. Most fishermen had moved to more profitable lakes leaving a
population of about 5000 people. In 1974 Mikocheni was the largest and most
successful fishing village on Nyumba ya Mungu and obtained good catches
which were conveniently transported to Moshi. However, with the lowering of
the lake level the village was some 2 km from the shore and movements of the
larger boats and encroachment of waterweeds was a problem.
*
RESERVOIR SIZE
Lake surface area
The lake area was estimated from the siting of villages, map contours, and
the depth of water at the dam. Its shape and area were confirmed by a
lower-red (0.6-0.7pm) imagery negative relayed by the United States ERTS-1
satellite on 7 January 1974 (Welsh & Denny, 1978: fig. 1).
At spill level the water depth at Damsite is 41.15 m and Mikocheni Village at
the shallow northern end of the lake is at the water’s edge. In the drought of
1974 the water level at the dam attained a minimum of 37.64 m (2 April 1974)
and Mikocheni was isolated some 4-5 km from the shore. The map contours
gave an estimated maximum surface area of 180 km2 when the lake spills, and
contours constructed from the estimated distances of villages from the shore at
minimum water level provide an estimated minimum surface area of 110 km2.
It is therefore calculated that a one metre reduction in depth of water will
expose some 20 km2 of shore. From this the lake surface areas are calculated
for the twelve month period September 1973-August 1974 (Appendix 4). The
lake did not reach the spill level and for that period had an average surface area
of 142 km2.
Lake storage volume and mean depth
The storage volume of the lake at full capacity was estimated by Bailey
(1965) to be 1120 million cubic metres. This estimate implies a mean depth of
about 6 m. Our own observations suggested that the northern end of the lake,
which represented approximately two-thirds of the surface area, overlay
shallow water from 5 to 10 m depth whilst the southern dam end, covering
approximately one third of the surface area, overlay deep water up to 41 m
with an average depth of 1 5 to 20 m. If these are correct then the volume could
not be greater than 1600 million m3, which would substantiate Bailey’s data.
GEOLOGY OF THE NVUMBA VA MUNGU REGION
A geological and geomorphological survey of the Nyumba ya Mungu area
was carried out before the construction of the dam and the results are
summarized here (Sir William Halcrow & Partners, pers. comm.).
North-eastern Tanzania in the vicinity of Nyumba y a Mungu is floored by
Precambrian crystalline metamorphic rocks. These are mainly hornblendic
GENERAL FEATURES O F NYUMBA Y A MUNGU
11
gneisses with quartzofelspathic bands and more rarely pegmatitic and garnetiferous zones. There is an absence of overlying strata from the Precambrian to
the Late Tertiary period which indicates a long interval of denudation rather
than deposition. Towards the end of the Late Tertiary period there was a build
up of violent activity in eastern Africa which culminated in the formation of
the Rift. The rifting was accompanied by large scale volcanic eruptions and
during the Miocene to Pleistocene period, the largest volcano, Kilimanjaro, was
formed. The outpourings from Kilimanjaro did. not cover the Nyumba ya
Mungu area, but occur there as river borne pebbles in the alluvium, and as
Neogene 'lacustrine shore deposits such as the conglomerates found at Kiti cha
Mungu.
Perhaps the most important era for the present Nyumba ya Mungu reservoir
was the Neogene (Late Tertiary). I t was then that the block faulting in the area
formed an approximately north-south orientated fault-trough between the
elevated Pare mountains to the east, and the Lelatema mountains to the west.
A lake filled the depression and lacustrine sediments, including conglomerates,
gravelly sandstones, sandy clays, marls, and limestones were deposited. The
Neogene lake had a high calcium carbonate content, and in places the
sandstones are calcareously cemented, and the Precambrian gneisses are lime
impregnated.
A low peneplain of Precambrian gneiss with an elevation of only about 46 m
above the Pangani river occurs in the area of the present dam. In its journey to
the sea the River Pangani conveniently eroded a narrow breach through the
gneiss, and it was at this point that the dam was sited, with the peneplain
providing the southern rim for the reservoir.
With the onset of a warmer drier climate the Neogene lake disappeared and
the River Pangani meandered through the valley depositing alluvial silts, sands,
and gravels in its floodplains.
A variety of soils of varying depth occur in the region and have become
submerged by the present man-made lake. These soils are related to the rock
types, the extent of weathering, and the Neogene alluvial layers. Weathering of
the gneisses has been extensive on the higher ground but minimal in the river
bed region. In places the weathered gneisses became deeply impregnated with
lime from the Neogene lake, forming a rock rather similar in texture to the
English Chalk, and liable to slow erosion. The Noegene lake mark and
limestones which overlie the sandy clays have provided some protection against
erosion, but where these crusts have been penetrated and broken up, erosion of
the softer alluvial clays is very apparent. Consequently the limestone crusts
have only developed a very thin soil layer whilst the exposed reddish sandy
clays have the appearance of a dust bowl, and are low in essential mineral ions.
The flood plains of the Kikuletwa and Ruvu rivers, especially in the northern
region of the present lake, and towards Kilimanjaro, have a relatively deep
fertile alluvial soil, and where this soil is not submerged it is extensively
cultivated. Around the lakeshore itself, the salinity of the soil is too high for
cultivation.
CLIMATE A N D ITS EFFECT ON T H E RESERVOIR
The Masai Steppe of Tanzania occurs within the Inter Tropical Convergence
Zone. In this zone there are two pairs of similar seasons as the sun passes
12
P. DENNY
overhead through its solstices and equinoxes twice each year. As Nyumba ya
Mungu is 3'45' south of the equator the two pairs of seasons do not have
equality and the rainy seasons tend towards heavier rains in March to June and
lighter rains in November to December. The rains are interspersed with hot dry
seasons from August to November, and December to March. The climate is
modified by the presence of the high mountain ranges including Mt.
Kilimanjaro to the north and the Pare mountain range to the east. These cause
the Trade Winds to shed some of their water so that rainfall over the Masai
Steppe including the Nyumba ya Mungu area is extremely low with annual
means of less than 500 mm year-'.
Nyumba ya Mungu is directly affected by the Trade Winds that blow from
the coast. The reservoir is NNW/SSE orientated along the main geological faults
and is flanked by the two parallel mountain ranges, the Lelatema and the Pare
mountains. These mountains have the effect of funnelling the South-east Trade
Winds unimpaired from the Indian Ocean, over the long axis of the lake. The
North Pare mountains and Mt. Kilimanjaro modify the North-east Trade Winds
so that the wind is much more gusty and its direction variable (Fig. 3).
Windspeed
Windspeed data was recorded at 09.00 hrs daily at the NYM meteorological
station No. 1D/R6 from a Casella wind gauge registering statute miles; and a
hand-held anemometer was used in the field to record speeds at one minute
intervals one metre above the lake surface. Windspeeds at the meteorological
station and the lake were not identical, as over the lake, at fetches greater than
5 km, they were likely to be 1.3-1.5 times greater (Viner & Smith, 1973), but
the close proximity of the two make the data comparable.
During the 12-month period from September 1973 to August 1974 the
ten-day means of windspeed of the North-east Trade Winds from November to
April showed considerable variability from the monthly mean, whilst those of
the South-east Trades from May t o October were more uniform. There was
little difference between the overall force of each Trade Wind: an annual mean
windspeed of 1.44 m s-' represented a contribution of 1.45 m s-' from the
South-east Trades and 1.43 m s-' from the North-east Trades. The mean
monthly windspeeds were low with small peaks in October and February and
troughs in November/December and April/May which could reflect the Indian
Ocean doldrums. No correlation between the wind and the rain seasons were
apparent (c.f. Lake George, Viner 8i Smith, 1973).
Mean monthly windspeeds are of limited value when considering the effects
of wind on water turbulence, as they give no indication of windspeeds over
short durations which may affect mixing. In March 1974 for example,
although the mean monthly windspeed was 1.5 m s-l daily means ranged from
0.4 to 2.2 m s-' . Further, the wind over a 24 hour period was rarely steady.
Data from Lake George (Viner & Smith, 1973) clearly showed a diurnal
rhythm where windspeeds generally increased from 06.00 to 15.00 hrs then
dropped again during the afternoon and evening. At NYM diurnal windspeed
data was lacking but from July to September our impressions would result in a
similar conclusion.
GENERAL. FEATURES OF NVUMBA VA MUNGU
13
3b34
-
30
-
28
26
-
@/@-a/@
./ ...................................
7......
@
.......................................
Annual Mean
a‘
2422
20
-
@
-
18,16
14
-
8
/
,
@
\
/.-..~----m--8---8,
8
\.A
h l 0 l N
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1
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V
0
-
S.E.Trade
I
N.E. Trade
S I O I N I D ; J
I
1973
I
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F l M l A l M l
1974
SE Trade
Jl
J l A l
Figure 3. Air temperature, solar irradiation, and wind velocity data for the Nyumba ya Mungu
region from September 1973 to August 1974. Information compiled from data from the
Nyumba ya Mungu meterological station. No. 1 D/R6.
Seiclies
Lake level oscillations were measured on a metre rule enclosed in a
perforated drum to reduce surface wave action.
Oscillations were observed on the east shore of Damsite at five or ten minute
P. DENNY
14
intervals on 31 July 1974 (Fig. 4). The south-east wind mean speed for the 24
hours was 1.58m s-l but during the observation period, from 09.15 to
14.35 hrs the mean was 2.82 k 0.96 m s-l with fluctuations from 0.8 to
5.1 m s - l . Over this period the lake had noticeably diminishing oscillations. In
the morning a longitudinal seiche with a periodicity of approximately 2 h
40 min and an amplitude of about 12 mm was distinguishable, but after
midmorning the periodicity was approximately halved to 1 h 40 min and the
amplitude reduced. In a large lake where the average depth in relation to length
is very small the simple formula t = 2 1 m may be used to estimate the
theoretical periodicity, t, of its uninodal oscillation; where 1 is the length and h
the average depth of the lake, and g is the force of acceleration due to gravity
(Ruttner, 1963).By substitution, the periodicity of the uninodal longitudinal
oscillation of Nyumba ya M u n p should be approximately 2 h 20 min. Thus
the observed periodicity of 2 h 40 min was probably its fundamental, but later
in the day this was apparently broken into a binodal oscillation of diminished
t
LI
E
0
Y
+l
.0
4
-0a
'
-
.-C
3
3
2
1
09.0 0
lQOO
11bo
midday
Time
(h)
Figure 4. Wind velocity and lake oscillation measurements taken on the east shore of Darnsite
on 31 July 1974. Wind velocity was measured with an anemometer held 1 m above the water
surface.
GENERAL FEATURES OF NYUMBA Y A MUNGU
15
amplitude. Superimposed on the main oscillations were minor flutters which
were probably associated with transverse deflections across the bay.
The environmental factors which brought about these events are not clear.
The mean windspeed for the preceding day was very similar but strong gusts of
5-8m s-l developed between 18.00 hrs and midnight on 30 July, and this
might have been sufficient to initiate a surface seiche. After midnight the wind
suddenly dropped until next morning. The erratic winds during 3 1 July cannot
be correlated with the amplitudes or periodicities of the seiches although one
may have expected stronger winds over the middle of the lake during
mid-morning to produce the apparent bimodal sequence.
Surface waves
A number of observations on surface waves were made. During the seiche
observations 26 one-minute counts of waves gave a mean period of 0.6 s and
wavelength of 0.5 m. However, as it was a south-easterly wind the fetch was
not more than 0.5 km. In the open water, approximately halfway up the lake,
steady south-east winds of up to 5 m s-' produced waves of 3-4 m wavelength
and 20-30 cm amplitude. South east Trade Winds were generally stronger at the
north end of the lake, presumably because of the funnelling effect of the
mountains, and with the full fetch waves from 50-75 cm amplitude and
wavelengths of 6 m or so were observed with white horses (approximately 9 m
s-' windspeed). As the depth of water in the north end was considerably less
than those wavelengths there would be appreciable agitation and mixing at the
mud/water interface. At the southern (dam) end where the shore was generally
steep-sloping and the fetch minimal, wave action had much less effect on
sediment.
Water currents
Subsurface currents were assessed by measuring the drift of a drogue
attached to a fine fishing line over a timed interval, and surface currents by
replacing the drogue with a flattened matchbox (after Viner & Smith, 1973).
Care was taken to avoid the sheltering effects of the boat but reliable data was
limited as firm anchorage of the boat was difficult and submerged trees tended
to entangle the drogue and line. Measurements were made in July from
08.00-12.00 hrs over 30 m depth of water, 2 km north-west of the dam during
a prevailing south-east wind. Results are tabulated in Appendix 1.
The current flow in the top 10 m of water was in the direction of the
prevailing wind, up the lake towards the north north-west. Speeds of 7 to
10 cm s-' occurred to depths of 2 m, and below this there were speeds of 2 to
3 cm S-' down to the 10 m level. A surface current of 6 to 7 cm s-l may have
been an under-estimate caused by drag of the fishing line on a very light
matchbox float. The slow north-west directed current observed at 10 m is
considered to be genuine since the surface area of the drogue was many times
greater than that of its polythene bottle support, and since the drift still
continued when the polythene bottle was in the lee of the boat.
The most plausible explanation of these observations is the location of a
major circulation of water along the long axis of the lake. The turbulent
16
P. DENNY
transfer of momentum from the wind energy extended down to at least 10 m
setting up a north-west directed current which must have returned in the
opposite direction along the bottom of the lake. Unfortunately submerged
trees prohibited the confirmation of the return flow. Water at the dam was
extracted at 10 m below the surface to supply the turbines and to feed the
Pangani river but this flow was insufficient to have any noticeable effects upon
the wind-driven currents.
Solar irradiation
Solar irradiation was measured daily at the NYM meteorological station
utilizing a Gunn-Bellani water distillation radiation integrator. There is some
criticism of the use of these instruments as they receive only shortwave
radiation which passes through glass, and respond mainly to the blue
ultra-violet part of the solar spectrum (Szeicz, 1968). They also have low
response to radiation for the first ten degrees above the horizon, and their
sensitivity depends upbn the ambient temperature (Monteith & Szeicz, 1960).
These criticisms are, however, largely alleviated in the tropics and Pereira
(1959) comparing a Gunn-Bellani radiometer under tropical conditions at
Muguga, Kenya (alt. 2073 m a.s.1.) with the more accurate Kipp-Moll
solarimeter obtained a correlation coefficient of 0.970 with the linear
relationship only digressing at the low energy levels. As Nyumba ya Mungu is
2" south of Muguga at an altitude of 670 m a.s.1. and has a similar but warmer
tropical climate with very brief periods of low energy levels during the day, the
radiometer data from there is probably as accurate.
The volume of water, in millilitres, distilled within the radiometer each day
were recorded and converted to ten day and monthly means. The high
correlation between the Kipp-Moll solarimeter and the Gunn-Bellani radiometer
obtained by Pereira enabled us to use the conversion factor (132 + 3 6 4 g cal
cm-' day-' where d is the volume of water distilled (Pereira 1959). Thus by
substitution of d in the formula and multiplication of the result by 4.1868 data
can be expressed in Joules cm-' day-'.
The monthly means of daily solar energy from September 1973 to August
1974 are plotted in Fig. 3. Incoming solar irradiation increased from August to
February, except during the short rains of November. The hottest month,
February, had a monthly mean of 3421 J cm-' day
but individual days
ranged from 2783 to a maximum of 3989 J cm-' day-'. From March to July
radiation steadily decreased at the onset of heavy rains and extensive cloud
cover. The minimum monthly mean radiation of 2054 J cm-2 day-' occurred
in July, and the coldest day had a radiation of only 1095 J cm-' day-'. The ten
day means for the twelve month period gave an annual mean of 2813 f 581 J
cm-2 day-'.
Daily sunshine was recorded by a sunglass on paper for five weeks during
July to September 1974. In the absence of sunglass chart paper brown
wrapping paper was found to smoulder effectively. For the first three days the
position of the sun was recorded at hourly intervals on the papers and these
papers were used subsequently in calibration. As the quality of the papers was
very variable the total lengths of the burns rather than the areas were measured.
This provided the number of hours of sunshine but no indication of the
GENERAL FEATURES OF NYUMBA YA MUNGU
17
amount of radiant energy. The weekly mean sunshine and incoming solar
irradiation from 31 July to 3 September are given in Appendix 2. It is perhaps
of interest to note that a correlation coefficient of 0.98 was obtained between
sunshine and solar irradiation data which substantiates the general validity of
the Gunn-Bellani radiometer.
Air temperature
The air temperatures of NYM recorded daily at the meteorological station on
a max. min. mercury thermometer from September 1973-August 1974 were
consistently high with an annual mean temperature of 25.3"C. The mean daily
maximum was 31.75"C and mean nightly minimum 18.8"C. Seasonal variation,
with the hot season from December to March, showed mean monthly maxima
reaching 35.5"C and individual days up to 37°C. The cool season occurred
from June to August with the maximum temperature rarely exceeding 30°C
(Fig. 3). Comparison of these data with data from the previous four years
confirmed that the annual temperature rtgime is very uniform with annual
mean variations of less than 0.4" C.
Water temperature and stratification
A Van Dorn sampler was used for vertical profile samples in deep water.
Water temperatures were measured in the field with a mercury glass-bulb
thermometer (sensitivity f 0.05"C) and oxygen determinations were by the
standard WinMer method.
Vertical profiles for temperature and oxygen in July and August 1974 at the
Damsite station, 400 m north of the dam, are pIotted in Fig. 5. On 24 July the
lake was isothermal in the morning with an overall temperature of 20.5" C, and
an orthograde oxygen profile indicated non-stratification (Fig. 5A). The
weather conditions for the preceding seven days had been cool and calm, with a
mean daily radiation of only 1969 J cm-' day'' and windspeed of 1.1 m s-l,
During the next fortnight the mean radiation increased to 2642 J cm-2 day-',
with a mean windspeed of 1.42 m s-l, and there was some evidence of
subsurface warming of the top 5 m of water during the day (Fig. 5B). Over the
same period the oxygen concentration steadily increased in the surface water
and decreased in the deeper water. Throughout August the daily irradiation
level increased (Fig. 3) whilst the mean windspeed remained low, and the
oxygen profile became distinctly clinograde, and a slight thermal discontinuity
was discernible. Small diurnal stratifications also became established but wind
action and reirradiation at night usually dissipated these before dawn.
Light penetration
Light penetration in the lake could only be measured by a secchi disc of
approximately 40 cm diameter.
Secchi disc readings from 0.6 to 1.2 m were obtained in different areas of
the lake at different times. This implies a high turbidity lake with the depth of
the photic zone not extending beyond 2.5 m. The low light penetration was
due to a high phytoplankton biomass.
P. DENNY
18
19.8.14
0
/
15
I
i
I
I18.00h
I
cn 2o
E
c.
1
o
.
m
00.30h
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0o.m
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8
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D
Figure 5. Vertical profiles for water temperature and oxygen concentration in July and August
1974 at the Damsite station, Nyumba ya Mungu, 400 m north of the dam.
WATER BALANCE
If it is assumed that no water is lost through subterranean leakage then the
water balance of Nyumba ya Mungu can be expressed thus:Change in volume = (inflow + direct rainfall)-(outflow + evaporation).
(expressed as water
level fluctuations)
Of these factors the only ones that could be directly measured were the
outflow, as water loss by discharge through the dam; the water gain by direct
precipitation on the lake surface; and the change in the lake level. Evaporation
was estimated from radiation data and evaporation pan measurements. The rate
of inflow was the major unknown and could only be estimated from changes in
lake level and rainfall data from the catchment area (Fig. 1).
Rainfall in the catchment area
Rainfall records from local meteorological substations were provided by the
Water Department, Moshi.
G E N E R A L 1:EATURES O F NYUMBA Y A MUNGU
19
Rainfall is seasonal with the predominant rains falling in March, April and
May, and the lesser rains in November and December. There is a pronounced
dry period from June to September. Over the last few years and particularly in
1973 the rains have been somewhat lower than normal and have caused
droughts in many places in East Africa.
The catchment area for Nyumba ya Mungu covers approximately 7500 km2
(Fig. l), but over half of this, including the surrounds of the lake, have rainfalls
of less than 500 mm year-'. This is insignificant compared with the rainfall
over Kilimanjaro. In fact the main water catchment and runoff regions for
NYM can be broadly divided into three watersheds: (i) Mt. Kilimanjaro, (ii) Mt.
Meru and (iii) The Lake Jipe catchment area, with the southern slopes of
Kilimanjaro providing the most important watershed.
Annual rainfalls in 1970-1973 from the lake area and from sites in the
Kilimanjaro watershed are given in Appendix 3 but there are no records from
the high altitudes of Kilimanjaro where much of the runoff originates. The
absence of more data and the local effects of the mountains make it difficult to
provide an accurate estimate covering the whole catchment area although a
figure of 900 mm year-' has been suggested by the Tanzania Electricity Supply
Company (TANESCO).
By reference t o Fig. 1 it can be seen that rainfall from Mkuu Parish on the
south-east side of Kilimanjaro mainly drains into Lake Jipe, whilst that from
Himo drains directly into the River Ruvu and Moshi into the Kikuletwa and
Ruvu rivers.
Rainfall and changes in lake level
The monthly rainfall data from the Kilimanjaro catchments can be compared
with the recorded lake water levels over a two year period in Fig. 6. I t will be
noticed that the monthly rainfall pattern from Nyumba ya Mungu, Moshi and
Mkuu show the same trends in that the seasonal periodicity is distinguishable
by peaks in April and November, although their magnitudes vary according to
altitude. These peaks are reflected some two weeks later by a rise in lake level.
However, the Mkuu rainfall pattern is not completely consistent with changing
lake level (see for example November 1972 and 1973) and probably the
contribution from the South-East Kilimanjaro catchment is relatively small, say
15%. (The River Lumi's average flow rate of 4-6 m3 s-' compared with a
calculated inflow into NYM of 33-36 m3 s-' supports this.) Likewise, it can be
argued that as the rain falling directly on the lake contributes only about 5% of
the total water input (Appendix 4) it can also have very little influence on the
lake level. The rainfall pattern for the Moshi region which probably represents
Southern Kilimanjaro generally and could account for well over 50% of the
total lake water is more clearly accompanied by changes in lake levels.
The inadequate rains of April 1973 followed by the virtual absence of the
November rains caused the level of the lake to fall by over three metres from
July 1973 to March 1974, and the heavy rains during April 1974 were
insufficient to replenish this (Fig. 6 ) . The changes in lake level, a fall of 9 cm
week-' for eight months followed by a sudden rise of 43 cm week-' during
April 1974, had important biological consequences which will be discussed
later (Bowker & Denny, 1978). The lowering water level was the result of
20
P. DENNY
7120
...............................................................................................
Spill
.............
14l-0
jz2-
Figure 6. Mean monthly rainfall data from the Kilimanjaro catchment area of Nyumba ya
Mungu, and estimated inflow rates and water losses from the lake. The water level at the dam is
also indicated. Rainfall data provided by the Water Department, Moshi; lake level data from the
Tanzania Electric Supply Company, Moshi.
GENERAL FEATURES OF NYUMBA YA MUNGU
21
loss of water by evaporation and discharge through the dam (Appendix 4 and
Fig. 6) which was only partially compensated for by the inflows.
Water loss by evaporation f r o m an evaporation pan
Evaporation from an open water tank has been recorded daily for several
years at the NYM Meteorological Station. The tank, based o n a design by
Pereira (1948), is a circular galvanized iron tank of 1 m diameter and 0.5 m
depth. The water level in the tank is regulated daily by a measured addition or
removal of water so that the surface meniscus just touches a hook gauge. As the
surface area of the pan is known the amount of water added or removed can be
expressed in terms of millimetres. Total daily precipitation at the same
meteorological station was measured by rain gauge and thus the increase in
water level of the pan due to precipitation can be taken into consideration in
calculating the gross evaporation (Appendix 4). The tank stands on an open
patch of grass sheltered from the direct sun but is susceptible to diurnal heating
and cooling through the walls. This will cause some inaccuracies (Pereira, 1959)
but the error is probably small.
Data obtained from the evaporation tank can be independently checked by
comparison with the Gunn-Bellani radiation data as Pereira ( 1959), testing the
relationship between the two factors at Muguga, Kenya, obtained a linear
regression with a correlation of 0.97. A similar relationship with a high
correlation coefficient could be expected from NYM data. Points seriously
deviating from the regression line would be suspect and a low correlation
coefficient would implicate errors in radiation or evaporation data. Irradiation
and evaporation data were converted t o 10 day means over the 12 month
period as the high specific heat of water will cause heat storage in the
evaporation tank and render the daily values unreliable.
The data, plotted in Fig. 7 with the line of best fit gave a linear regression of
Eo = 0.461d-0.50, where Eo is the mean daily evaporation in millimetres and
d is the mean daily distillation of water in millimetres within the radiometer. A
correlation coefficient of 0.95 substantiates the evaporation data and the graph
indicates that only one point, which represents data from the first ten days of
May, is seriously in error. Estimates of monthly evaporation by substitution of
d in the regression are given in Appendix 4. The mean daily evaporation using
radiometer data is calculated t o be 6.48 mm day-' as compared with a mean of
6.40 mm day-' from the pan data. The pan data are used in all subsequent
calculations of water balance.
Data from September 1973-August 1974 are given in Appendix 4. High
evaporation rates occurred in October, December, January and February with
an October maximum of 243 mm water loss and a mean rate of 7.8 mm day-'.
Low evaporation occurred from April-July with a minimum of 118 mm in July
producing a mean of 3.8 mm day-'. Water gain by precipitation exceeded
evaporation on only 21 days of the twelve months of records, and only for the
month of April was the gain greater than the loss by evaporation. The annual
precipitation was 45 3 mm compared with 23 34 mm evaporation. The mean
daily evaporation over the twelve month period was 6.4 mm day-'.
22
P. DENNY
10
-
9 -
8 -
-E
E
7 -
E
6
-
0
P
Q
5
-
4
-
0
.c
2
>
w
-Q
c
+0
3 -
r = 035
2
-
1
-
O
I
l
I
I
I
I
I
k
l
I
1
2
3
4
I
6
I
6
9
1
1
1
1
1
1
1
1
1
1
1
1
10 11 12 13 14 15 16 17 18 19 20 21
Radiometer
Distillation
(ml)
Figure 7. Correlation between water loss by evaporation from the Nyumba ya Mungu
Meteorological Station evaporation pan, and solar irradiation measured by water distillation in a
Gunn-Bellani radiometer. Data converted to 1 0 day means over a 12-month period, 1973-1974.
Estimated water loss by evaporation from the lake surface
Water loss by evaporation from the lake surface can only be accurately
estimated if the evaporation pan coefficient is known; i.e. the ratio, lake
evaporation/pan evaporation. There is an absence of pan coefficient data from
East African lakes but from the few determinations made in North America,
coefficients of about 0.70 have been suggested for unscreened pans and about
0.94 for screened pans (Linsley, Kohler 8c Paulhus, 1958). The pan at the NYM
meteorological station was unscreened and the heat absorption through the
galvanized iron walls may have enhanced evaporation. but as the lake surface
was probably more exposed to the Trade Winds than the pan the differences in
evaporation rate may have been partially balanced. With the shortage of
information there are no grounds to suggest a coefficient of around 0.7 and to
avoid confusion the rates of evaporation from the two surfaces are taken as
similar. Thus, water loss by evaporation from the lake was calculated for each
month by the multiplication of the lake surface area (in km2) and rate of
GENERAL FEATURES OF NYUMBA Y A MUNGU
23
evaporation from the pan (in mm) (Appendix 4). Estimated total monthly
water losses by evaporation from the lake surface during September
1973-August 1974 are shown in Appendix 4. As the change in lake surface area
was relatively small the shape of the graph resembles that of evaporation from
the open pan with a maximum water loss of 39 million m3 in October and a
minimum of 18 million m3 in July. In the twelve month period 329 million m3
of water evaporated from the lake with a monthly mean of 27 million m3. The
water gain by precipitation on the lake was only 57 million m3 over the same
twelve month period with 84% falling in March/April.
Water loss by discharge through the dam
Since the completion of the dam TANESCO has measured daily flow
through the turbines and over the spill. They have carefully regulated the flow
depending upon electricity demand from Nyumba ya Mungu and the
requirements of the Hale hydro-electric scheme approximately 200 km
downstream. The lake has not spilled since September 1971, and because of the
controlled discharge the mean monthly flow has been relatively constant.
From January 1972 to August 1974 the mean discharge rate was 72 million
m3 per month. The monthly discharges from August 1973 to August 1974 can
be directly compared with water loss by evaporation during the same period in
Appendix 4.
Estimated inflow rates
The difference between the expected changes in lake levels due to
evaporation and discharge and the actual changes recorded each month will
provide an estimate of the monthly rate of water inflow. Thus it is estimated
that in the dry season the water inflow was approximately 20 m3 s-l but when
the rivers came into spate towards the end of April a cumulative flow of
100 m3 s-l (Fig. 6) was probably providing a mean annual flow of 3 3 m3 s-'.
Water balance and retention time
To obtain an annual water balance 1 1 3 5 million m3 of water must be
accounted for as inflow water (Appendix 4). In order to maintain the level of
the lake this amount must be supplied from the Rivers Ruvu and Kikuletwa at
the combined rate of 36 m3 s-'. However, as was shown earlier the water level
actually fell and the mean water inflow was estimated to be only 33 m3 s-l for
the 1973/74 12-month period. An average discharge rate of 27 m3 s-' through
the dam accounted for approximately three-quarters of the total water loss,
and evaporation for one quarter.
If the storage volume of the lake at full capacity is 1120 million m3 (Bailey,
1965), and the net water loss is 1 1 3 5 million m3 then the retention time of the
lake must be one year. However, since 1971 as rainfall in East Africa has been
low, the lake has not reached full capacity and the retention time is therefore
nearer nine months.
P. DENNY
24
DISCUSSION
Man-made lakes for hydro-electric power and irrigation are a recent and ever
increasing feature of the African landscape. The larger lakes such as lakes Volta
(8500 km2), Kariba (4300 km2) and Kainji (1280 km2), have received considerable public attention and their magnitudes are sufficient to have
far-reaching effects upon the environment. By comparison, Nyumba ya Mungu
is a small reservoir (k180 km2) although considerably larger than any lake in
the United Kingdom.
The Nyumba ya Mungu reservoir was constructed in a very arid area and
appears to have features more akin to Lake Turkana in the dry Savannah of
northern Kenya (Ferguson & Harbott, in press) than to the natural lakes in
closer proximity. The annual rainfall onto both lakes is negligible (<SO0 mm
year-') and their calculated water loss from surface evaporation is over 2.0 m
year-'. Even Lake George, which is in one of the driest regions of Uganda,
receives >800 mm year-' rainfall whilst evaporation is calculated to be
approximately 1.8 m year-' (Viner & Smith, 1973). In contrast Lake Victoria
which is nearby, receives approximately 1450 mm year-' in direct precipitation
and loses about the same amount from evaporation. This precipitation accounts
for five-sixths of its total water requirement (Talling, 1966). Nyumba ya
Mungu receives its main water supplies from the catchment areas of
Kilimanjaro but over one quarter of this is lost by evaporation from the lake.
The larger man-made lakes, such as lakes Volta, Kariba and Nasser, have
retention times ranging from two to nine years, but Kainji lake and the smaller
reservoirs, including Nyumba ya Mungu, usually have retention times of less
than one year. These shorter retention times potentially have two effects.
Firstly the chemistry of the lake water may be more closely correlated with the
inflow waters, and secondly, the seasonal fluctuations and changes in rate of
inflow may produce rapid and sometimes dramatic changes in lake level. In
NYM the drawdown can be as much as 3 m. The effects of this and the water
chemistry are discussed in subsequent papers (Denny, Harman, Abrahamsson &
Bryceson, 1978; Welsh & Denny, 1978).
The high solar irradiation encountered in the tropics encourages stratification and stability of lakes and the 1.5-2.0 Co difference between the surface
and bottom water of NYM is potentially sufficient to produce a stable
stratification at these high water temperatures (Denny, 1972b). If we examine
the vertical profiles for July and August (Fig. 5 ) , the 20 m vertical profile on
the 24 July must have had the centre of gravity at 10 m as the lake was
isothermal. However, as the lake tended towards stratification the centre of
gravity would have fallen as the more dense cooler water settled below the less
dense warmer water. The stability of this stratification can be expressed as the
amount of work required to raise the centre of gravity back to the isocentric
point; i.e. the amount of work required to totally mix the lake and bring it
back to the isothermal state. The stability is therefore a function of the
densities and depths of the two discontinuity layers, and can be expressed
S = (D, - D1 ). (2h - z). z / ,
.
.
where S is the stability coefficient; D, is the mean density of the hypolimnion
water at mean temperature f z ; D1 is the mean density of the epilimnion water
G E N E R A L FEATURES OF NYUMBA Y A MUNGU
25
at mean temperature t , ; 2h is the total depth of water and z is the depth of the
epilimnion (Ruttner, 1963: 32-34). By substitution of the data from the
vertical temperature profiles in Fig. 5 the stability coefficients o n successive
days can be calculated. On the 24 July obviously S was zero; five days later it
was 0.15 kg-m m2 but by 19 August it had increased to 7.9 kg-m m2. This can
be compared with Ranu Lamongan in Java; a lake of similar depth and climatic
features. Eight days after an overturn and complete mixing, the surface water
layers of Ranu Lamongan had warmed to 27°C compared with a hypolimnionic temperature of 26"C, and a stability coefficient of 8 kg-m m2 had
developed (Green, Corbet, Watts and Ouey, 1976). Although the lake has the
potential of developing a stability coefficient of 35 kg-m m 2 (Ruttner, 1931),
Green et al. (1976) demonstrated that even with a stability coefficient of
8 kg-m m2 it had developed biological characteristics consistent with stable
stratification. The vertical oxygen profile of NYM on 19 August likewise shows
that the stability coefficient was sufficient to induce oxygen and other
chemical discontinuities. The coefficient, however, would need to increase
considerably t o establish any degree of permanence (cf. Lake Bunyonyi,
south-west Uganda, a permanently stratified lake (Denny, 1972a) with a
calculated coefficient of approximately 90 kg-m m2),and our data indicates
that the energy in the stability was generally less than the stirring forces of the
Trade Wind-driven water currents. Thus regular mixing of the main body of
water occurred.
At Nyumba ya Mungu in the cool season the daily mean air temperatures
frequently fall below those of the hypolimnion water and there must be a net
loss of heat from the water, and a greater tendency towards instability. An
important consequence of this regular mixing is the improbability of sudden
overturns with upsurge of anoxic hypolimnion water, and subsequent mass fish
deaths, which have been reported from other African man-made lakes.
ACKNOWLEDGEMENTS
The biological survey of Nyumba ya Mungu could not have been undertaken
but for the goodwill and financial support from a large number of institutions,
government departments and individuals. Our thanks and gratitude to all these
should in no way be lessened by their not being mentioned individually here.
The reader is referred to Denny & Bailey (1978) for information on those
who helped and participated, and to whom we extend our thanks and best
wishes.
In this publication I should particularly like to thank Mr M. Anwar, Mr S.
Moor, and Mr Stoppa from the Tanzania Electric Supply Company (Moshi),
who provided data on the hydro-electric dam, and Mr E. Led0 and Mr Singh
from the Water Department (Moshi), Ministry of Water Development and
Power, Tanzania, who kindly provided meteorological data.
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26
P. DENNY
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