Hydrological Sciences-Journal-des Sciences Hydrologiques, 46(2) April 2001
A water balance study of the upper White Nile
basin flows in the late nineteenth century
EMMA L. TATE, KEVIN J. SENE
Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK
JOHN V. SUTCLIFFE
Heath Barton, Manor Road, Goring-on-Thames, Oxfordshire RG8 9EH, UK
Abstract A water balance model of the Nile is described, working in terms of dry
season flows. The main use of the modelling studies was to extend flows back to the
start of observations at Aswan in 1869, and hence to estimate the levels of Lake
Victoria during the high flow event of the 1870s. New estimates of Lake Victoria
levels and outflows for the period 1870-1895 are deduced, extending the length of the
observed data for Lake Victoria by some 25%. Crucially, the analysis includes the
major flood event of 1878, which historical evidence suggests was the highest since
1860 or earlier. The modelling results are compared with previous estimates and a
detailed examination of historical evidence. Some prospects for future levels are
considered, taking into account the reconstructed levels of the late nineteenth century.
Key words Lake Victoria outflows; reconstructed levels; water balance; river flow
Une étude du bilan hydrologique du bassin du haut Nil Blanc
pendant la dernière partie du dix-neuvième siècle
Résumé Un modèle du bilan hydrologique du bassin du Nil est décrit, en fonction des
écoulements de la saison sèche. Les études de modélisation ont été principalement
orientées vers l'extension des données de débit jusqu'au début des observations à
Assouan en 1869, et, en amont, vers une estimation du niveau du lac Victoria dans la
période de crue des années 1870. De nouvelles évaluations des niveaux d'eau et des
flux sortant du Lac Victoria ont été déduites pour la période 1870-1895, rallongeant
ainsi de 25% environ la série des données observées. Les analyses incluent en
particulier l'événement de crue majeur de 1878, événement considéré sur le plan
historique comme le plus important depuis 1860 (voire auparavant). Les résultats de la
modélisation sont comparés avec des évaluations antérieures et avec une étude
historique détaillée. Des prédictions des niveaux d'eau futurs ont été réalisées, prenant
en compte les niveaux reconstitués de la fin du dix-neuvième siècle.
Mots clefs flux sortant du Lac Victoria; niveaux reconstitués; bilan hydrique; débit
INTRODUCTION
The White Nile is one of the; two major rivers which join at Khartoum to form the
River Nile, the other being the Blue Nile (Fig. 1). Numerous hydrological studies
(e.g. Hurst, 1952; Sutcliffe & Parks, 1999) have shown that the base flows in the
White Nile are dominated by the influence of the many lakes and swamps in the upper
basin, and in particular by Lake Victoria which is the main source of White Nile flows.
Considering Lake Victoria's dominant effect not only on White Nile flows, but
also on hydropower production in the region, this paper attempts to augment the
understanding of the variability of Lake Victoria outflows. This is achieved through
extension of the lake outflow time series by some 25%, back to 1870, incorporating the
Open for discussion until 1 October 2001
301
Emma L. Tate et al.
302
40°E
Key to dam location numbers
1 Aswan
2 JebelAulia
3 Khashm el Girba
4 Sennar
5 Roseires
6 Owen Falls
N
CONGO
Fig. 1 Map of the Nile basin.
major flood event of 1878. The time series extension is achieved using a water balance
model, which employs linear regressions to create the full time series back to 1870.
Lake level and rainfall observations within the Lake Victoria basin first started in
1896, and until the 1960s lake levels and outflows were thought to vary within a
A water balance study of the upper White Nile basin flows in the late nineteenth century
303
relatively small range. However, in the period 1961-1964, following widespread heavy
rains in East Africa, the level of Lake Victoria rose by over 2.5 m and more than
doubled the lake outflow and the area of the Sudd swamps downstream. This event
occurred shortly after the completion at the lake outlet of the Owen Falls Dam, which
supplies electricity to Uganda and Kenya, and raised questions about the future
hydropower potential of the Nile below the lake, and whether the spillway capacity
needed to be raised. Subsequent studies (e.g. Piper et al, 1986; Sene & Plinston, 1994;
Sene, 2000; Yin & Nicholson, 1998) have demonstrated the extreme sensitivity of the
lake balance to variations of lake rainfall and climate, and have highlighted the need
for caution in predicting future lake levels.
Since that time, lake levels have followed three decades of gradual decline,
punctuated by short-lived increases in the late 1970s and in 1998 when values
approached the earlier maximum. The most recent event followed heavy rainfall
during the 1998 El Nino event. This raises the interesting question of whether such
events will become more frequent, and the problem of estimating the return periods of
such events from the historical record. However, on the basis of so few events, it is
difficult to know whether sudden rises of this magnitude form part of the natural
regime of the lake, or whether they are a more recent phenomenon. This is partly
because the long response time of Lake Victoria, which is of the order of a decade,
complicates the response of lake outflows to variations of rainfall, evaporation and
inflows in the lake basin (Sene & Plinston, 1994). It is useful, therefore, to search for
additional evidence of lake level variations before the start of observations.
Based on anecdotal evidence, mainly collected by Lyons (Garstin, 1904, Appendix
III; Lyons, 1906), previous studies have suggested that levels comparable to those of
the 1960s may have been reached in 1875-1895, shortly before records began (Piper et
al, 1986; Nicholson et al., 2000a). Attempts have been made to reconstruct the time
series of lake levels during this period by correlation with low flow records at the key
gauging station at Aswan in Egypt, where records began in 1869 (Flohn & Burkhardt,
1985) and by pooling evidence from travellers' accounts, missionaries and others in
the region at the time (e.g. Nicholson, 1998). The aim of this paper is to take these
reconstructions one stage further by means of a stochastic water balance model, and
then to assess the implications, for estimating future lake levels, of combining this new
record with the existing observed record.
THE STUDY AREA
Hydrology of the Nile basin
To reconstruct Lake Victoria levels before observations started in 1896, it is necessary
to use flow data for stations downstream and this requires an understanding of the
basin hydrology. The hydrology has been reviewed many times and only the key
features will be described here, based largely on the accounts of Hurst (1952) and
Sutcliffe & Parks (1999). Figure 1 shows the main hydrological features and gauging
stations within the basin.
In the upper White Nile basin, the main contribution to river flows arises from the
effects of two rainfall seasons on Lake Victoria (March-May and October-December).
304
Emma L. Tate et al.
The lake outlet is at Jinja in Uganda, where outflows were controlled naturally at the
Ripon Falls until 1951. After the Owen Falls hydropower dam was built in 1951-1953,
the release rales were designed to mimic the natural outflow. Lake Kyoga, some 100
km downstream, is responsible for a small evaporative loss in drier periods but for
gains in the wetter period following 1961-1964. Lake Albert, some 130 km downstream, receives local inflows including the River Semliki, draining two smaller lakes
(Edward and George) which lie close to the headwater tributaries of Lake Victoria.
The local inflow is balanced by lake evaporation in drier periods, but provides a significant contribution in wetter periods. Between Lake Albert and the Sudd swamps, the
rainfall occurs in a single season between April and October, and highly seasonal
runoff occurs in the "torrents" which contributed about 15% of the total flow at
Mongalla before 1960. This flow occurs mainly between May and November, and is
responsible for the seasonal flooding in the Sudd region.
The next major influence on flows is the Sudd, where high flows exceed channel
capacities and result in widespread inundation. The Sudd is a large area of swamps and
seasonally-flooded grassland whose area varies with inflow and has ranged from less
than 5000 km2 to over 30 000 km2 between 1905 and 1980. Evaporation over inundated areas is high, and on an annual basis outflows are roughly half the inflows and
vary little seasonally or from year to year; it follows that losses are proportionally
greater in periods of high inflows. The flows of the White Nile return to seasonal
variability and higher totals with the contribution of the seasonal River Sobat, which
on average almost doubles the White Nile flow. The River Sobat derives most of its
runoff from the Ethiopian mountains, via the River Baro, and occasional contributions
from the River Pibor, which drains a wide area to the south. Downstream of the Sobat
confluence, the White Nile receives no significant inflow before meeting the Blue Nile
at Khartoum. Both flood flows and dry season flows in the lower White Nile have been
affected by the construction in 1934-1937 of the Jebel Aulia Dam some 40 km above
Khartoum.
A notable feature of the White Nile is that the seasonal variability of flows is
highly damped by lake and swamp storage within the basin; the coefficient of variation
(CV) of monthly flows immediately below the Sudd (Malakal minus Sobat flows) is
approximately 0.3. By contrast, Blue Nile flows are much more variable (CV of
monthly flows at Khartoum is approximately 1.3) with more than 80% of flows occurring in the single flood season (June-October). Artificial influences on dry season
flows in the Blue Nile arise from the seasonal irrigation/hydropower reservoirs at
Sennar and Roseires, which became operational in 1925 and 1966 respectively. Below
Khartoum, the Blue and White Niles join to form the main Nile. On an annual basis,
the Blue Nile contributed about 65% of the combined flow until 1961; this then decreased to about 55% mainly as a result of the rise in Lake Victoria outflows since that
time, but has recently returned to around 60% with the increase in Blue Nile flows
since the late 1980s.
Some 1500 km downstream, the Aswan High Dam completes the controlled
section of the Nile. Its predecessor, built in 1902, ended the natural regime of the Nile
down to its outlet below Cairo. In the reach down to Aswan, there are losses due to
evaporation, and abstractions for irrigation have increased significantly since the
1950s. The only significant inflow is from the River Atbara, which contributed about
14% of the total flow before the construction of the Khashm el Girba Dam in 1960-
A water balance study of the upper White Nile basin flows in the late nineteenth century
305
1964. Previously, nearly all of the runoff from the River Atbara occurred between July
and November, with the river usually dry from February to May.
Availability of data
Figure 1 shows the main long-term flow gauging stations in the Nile basin; note that
many of the data used in this study have been taken from The Nile Basin, volumes III
and IV (Hurst & Phillips, 1933a, b), held at CEH Wallingford. Lake Victoria levels were
observed at various sites at Entebbe and Jinja from 1896, with a gap in 1897-1898; early
records were converted to equivalent series at Jinja near the lake outlet. Level
measurements of lakes Kyoga and Albert started in 1912 and 1904 respectively. Flow
measurement in the White Nile basin began in 1905 at Mongalla, at the upstream end of
the Sudd. Between Lake Victoria and Lake Kyoga, flow measurement did not start
regularly until 1940 at Namasagali, but subsequent gaugings indicated the stability of the
outflow rating for Lake Victoria, so an outflow record can be reconstructed from the
start of level observations in 1896 until the start of operations at Owen Falls, with the
measured releases used from that time. Between Lake Kyoga and Lake Albert gaugings
started in 1907, but the most reliable record is at Kamdini between 1940 and 1979. At
Mongalla, the increase in flows following the rise of Lake Victoria coincided with a gap
in gauging, and flows in 1963-1966 appear suspect (Sutcliffe & Parks, 1999).
Downstream of the Sudd, flows at the outlet from the Sobat at Doleib Hill, and at
Malakal immediately downstream on the main White Nile channel, are available from
1905 but measurements on the Sobat stopped after 1983. Outflows from the Sudd are
deduced from the difference between these two sites. The White Nile flow has also
been gauged at its confluence with the Blue Nile since 1911, though flows can be
influenced by backwater effects from high flows in the Blue Nile.
The longest flow record on the Blue Nile is at Khartoum, where flows have been
recorded since 1900, but ten-day levels near the White Nile confluence have been
published for the flood season (June-October) for the period 1869-1883. There was a
gap until the present gauge was established some 4 km above the confluence in 1899;
by comparing levels at both gauges with levels at Aswan, Hurst & Phillips (1933a)
suggested an adjustment of 3.5 m at high levels between the early and later gauges.
The historic river level records for Khartoum are potentially useful in reconstructing Lake Victoria levels before 1896. The other major flow record before 1896 is
that for Aswan, which began in 1869. After the construction of the first Aswan Dam
(1902) and subsequently the Aswan High Dam (1964), various versions of the flow
have been published, including the "Water Arriving at Aswan" which takes account of
the storage in the Aswan reservoir. An independent flow record has been published for
Wadi Haifa from 1890, supplemented by Kajnarty just upstream from 1931 to 1962
and finally Dongola from 1963; the changes in location were due to successive rises in
the Aswan reservoir. Both these flow records give clear but subjective indications of
East African lake levels, especially the low flow periods, when White Nile flows
predominate. The Roda Nilometer gauge record at Cairo (Sutcliffe & Parks, 1999),
which dates from 622 AD, gives maximum and minimum levels; the record is subject
to significant channel aggradation over the years, so that evidence on flood series
cannot be transferred over too long a period.
306
Emma L. Tate et al.
WATER BALANCE MODEL
Anecdotal evidence for Lake Victoria (Piper et al., 1986) suggests that, before 1896,
the highest known levels were reached in 1878, and flow records for Aswan confirm
that a high flow event occurred on either, or both, of the Blue and White Niles in that
year. The peak monthly flow at Aswan was in September 1878. Further, Lyons (1906,
p. 103) notes a high flood on the Bahr el Jebel, and that Lado was flooded. The peak
level at G'ondokoro (just upstream of Mongalla) was recorded on 22 August 1878
(Lyons, p. 109), but the level was still high in December, indicating an unusually high
level of Lake Albert. This evidence is examined in more detail later, and led to the idea
of attempting to reconstruct the levels of Lake Victoria during the second half of the
nineteenth century.
In this water balance model, linear regressions are used to create the full time
series of lake levels back to 1870, thus extending the time series by some 25%. This
extended time series can be used to enhance understanding of the variability of lake
levels, and assist with estimating how lake levels might look in the future. Such understanding is most useful for water resources managers and planners, especially since
Lake Victoria is important for hydropower production.
It is desirable to estimate levels for the years before 1896, and, given the small
amounts of data available before 1896, a linear regression approach has been adopted.
Many previous studies have shown that, due to the slowly varying flow regime of the
Nile, the regression approach works well, provided seasonal or annual data are used
(e.g. Sutcliffe & Parks, 1999). This suggests that the influences of flow routing,
ungauged inflows and losses, and lag times are small. The resulting regression
relationships, and hydrological years used in the model, are summarized in Table 1.
To describe the model, the flow relationships will be considered for each reach
starting at Aswan and working upstream. The approach taken in this model is to use
linear regressions to extend flow series at gauged sites. In such a geographically large
basin as this, climatic and hydrological regimes vary between gauging station sites, so
that, for example, the dry season at Aswan is slightly different from that at Khartoum;
the seasons employed are listed in Table 1. It has also been important to recognize the
significant artificial influences (e.g. the construction of large dams) and hence to use
data for relevant periods to develop regression relationships.
In estimating flows of the White Nile at Khartoum, with an annual approach it is
not possible to estimate the effect of inflows from the Atbara with any certainty.
However, during the dry season, the Atbara has no significant flow, so dry season
flows at Aswan were used. Blue Nile flows are also small in the dry season, so Aswan
flows at this time must arise mainly from the White Nile.
An inspection of the Khartoum and Aswan records suggests that the period when
Blue Nile and Atbara contributions are small, or cease, is typically from April to June
at Aswan, and March to May at Khartoum. To estimate dry season flows at Khartoum
before 1896, it is necessary to relate dry season flows to the available data for the flood
season (June-October), where available, or simply to assume a typical mean value.
Although a recession model could possibly be developed for Khartoum flows, a simple
regression model captures the main relationship between March-May flows and
annual flows, whilst a second model shows a strong correlation between June-October
flood flows and annual flows. Note that, for the dry season flows at Khartoum, data
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were only used up to 1924, because low flows since then have been influenced by the
Sennar Dam (recent capacity about 0.5 km3). For annual flows, data were used up to
1965, since Roseires Dam (which is much larger than Sennar Dam, with a recent
capacity of 2.4 km3) has probably influenced flood flows since then.
Flows can then be estimated at Malakal. Since 1935, dry season flows have been
strongly influenced by operations at the Jebel Aulia Dam, so values from then were
excluded from the regression analysis. To follow the analysis upstream, it is better to
continue in terms of annual flows from this point, since both the Sobat and torrent
flows have, in some years, provided large inflows during the dry season, whereas
annual flows are much less variable. As on the Blue Nile, there is a reasonably consistent relationship between dry season and annual flows on the White Nile at Malakal.
To estimate flows for the Sobat, it would be desirable to represent the higher flows
which seem to occur when rainfall and inflows are also high in the upper White Nile
basin. These higher flows arise from the River Pibor, which runs almost parallel to the
upper White Nile channel with its headwater region near to Lake Kyoga. Several
possible regression relationships were investigated with inconclusive results. A weak
relationship was found between annual flows in the Sobat and the net inflow to the
upper White Nile between the outlet from Lake Victoria and the Sudd Swamp
(i.e. Mongalla minus Lake Victoria outflows). Unlike the record for Lake Victoria
outflows, this net inflow is not affected by over-year storage in the lake, and so is a
reasonable indicator of annual rainfall variations in the headwaters of the Pibor.
To complete the model, it is necessary to relate the inflows of the Sudd to the
outflows, and then to estimate Lake Victoria outflows. Previous studies of the Sudd have
suggested roughly a two to three month lag between inflows and outflows, so
hydrological years of April-March and February-January have been used for outflows
and inflows respectively to try and capture this effect. On this basis, there is a reasonably
linear relationship between Sudd inflows (Mongalla) and outflows (Malakal-Sobat),
whilst Lake Victoria outflows are strongly related to flows at Mongalla.
Using the seven regression relationships described above (Table 1), it is possible to
use the data for Aswan and Khartoum before 1896 to estimate mean annual outflows
from Lake Victoria in each year from 1869 and, via the lake outflow relationship, to
estimate mean annual levels. Due to the implicit relationship assumed for Sobat flows,
flows upstream from Malakal cannot be estimated directly (i.e. proceeding upstream),
but must first be estimated from Lake Victoria outflows following a simple algebraic
rearrangement of the regression equations. Also, the model only provides a single "best"
estimate in each year, with no assessment of uncertainty. To obtain a first impression of
the likely uncertainty, a simple stochastic model was developed. For each regression
relationship, the residuals in the relationship were assumed to be uniformly distributed
across the flow range, and normally distributed with a standard deviation equal to the
standard error in the relationship. The residuals at each site were also assumed to be
uncorrelated with those at other sites, or with those at the same site in previous years. In
some cases, there are not enough data to confirm these assumptions; however, given the
available data, the stochastic model at least provides an estimate of the uncertainty
associated with the estimate of flows at each site. Some example confidence limits are
shown on the estimates presented later, based on 10 000 realizations of the regression
model (within-year means and standard deviations typically converged to stable values
after a few hundred realizations, but 10 000 realizations provide more accurate values).
A water balance study of the upper White Nile basin flows in the late nineteenth century
309
RESULTS
Estimates from the model
The model was used to estimate annual mean Lake Victoria levels, and flows at other
sites in the basin, for the hydrological years 1870/71-1991/92. Several tests were also
made to assess the performance of the model, the first being to see how well observed
flows and levels could be estimated since 1896 using only the observed dry season
(April-June) flows for Aswan and flood season (June-October) flows for Khartoum.
Comparison plots for Malakal and Khartoum dry season flows showed good agreement over the period of the regressions (1905-1933 and 1900-1924, respectively),
although comparisons for other periods were precluded due to the major influences of
Sennar and Jebel Aulia Dams from 1925 and 1934. However, the main test was to
compare observed and predicted levels for Lake Victoria from the start of lake level
observations in 1896.
To perform this comparison, the problem arises that, since 1934, flows in the
White Nile at Khartoum have been strongly influenced by operations at Jebel Aulia
Dam, particularly in the dry season. From this time, therefore, only a partial test of the
model was possible, using the observed flows at Malakal as input to the White Nile
component of the model, rather than the estimated flows as in the period 1896-1934.
From 1934, therefore, the comparison only tests the performance of the part of the
model which estimates Sobat, Sudd and lakes Kyoga and Albert flows. Figure 2(a)
compares the observed and predicted levels for the period 1896-1992 on this basis. It
can be seen that the model captures the sudden rise in levels from 1961 to 1964, but
overestimates the decline in levels by about 0.5 m from 1964. Before 1961, agreement
is good except for the short lived rise in levels of 1918, when the observed levels are
overestimated by about 1.0 m. This appears to be due to the unusual combination of a
sudden increase in outflows from the Lake Kyoga region, with no corresponding
increase in Lake Victoria outflows.
Since the flows at Khartoum are one of the key inputs to the model, it is also worth
examining this component of the model in more detail, and Fig. 2(b) shows the flood
season (June-October) and annual flows (April-March) at Khartoum for the period up
to 1982. Khartoum flows are not shown from 1884 to 1895 since no flood flow data
were available in this period. The gradual decline in flows during the 1900s has been
noted many times before (e.g. Sutcliffe & Parks, 1999) and arises both from increased
losses at reservoirs and abstractions to irrigation, and a general reduction since the
1970s which seems to be linked to the general decline in rainfall throughout most of
Africa over this period (e.g. Nicholson et al, 2000b; Hulme, 1990). What is perhaps
more surprising is that annual flows were apparently much higher in the period 18691883. The recorded levels in this period provide a first check on this result. In this
period, the highest monthly mean level reached at Khartoum was about 17.21 m on the
Khartoum gauge in September 1878. This compares with highest values since 1900 of
16.26 m in August 1909 and 16.67 m in September 1917, implying that flows were
unusually high in 1878. In an analysis of the 1988 flood at Khartoum, Walsh et al.
(1994) also note that there is plenty of anecdotal evidence to back up the claim that
floods at Khartoum were more frequent, and higher, during the 1870s and 1880s than
at any time since records resumed in 1900; in particular, that floods and rainfall were
310
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A water balance study of the upper White Nile basin flows in the late nineteenth century
311
exceptional in Khartoum in 1878, with prolonged periods of inundation in the town.
The significance of high flows at Khartoum in the period 1869 to 1883 is, of course,
that the estimated flows in the White Nile are lower than might have been expected
from an analysis which relied solely on the post-1900 data for Khartoum.
Using the model from 1869, it is also possible to estimate the overall annual water
balance of the Nile for four key locations: Aswan, the Blue Nile at Khartoum, the
White Nile at Khartoum, and at the outlet of Lake Victoria (Fig. 2(c)). Where
available, observed flows are shown, whilst in other periods the flows are estimated
from the model. Once again, Khartoum flows are not shown from 1884 to 1895 since
no flood flow data were available in this period. According to the model, in the period
1869-1883, in contrast to the Blue Nile flows, White Nile flows were high around
1878, but otherwise unremarkable. The estimated flows for Lake Victoria pre-1896
suggest a lesser, but sustained, increase in outflows in the 1890s, shortly before
observations of levels first began.
These variations are also reflected in the estimated levels before 1896, which are
shown in Fig. 2(d). The estimated mean annual level in 1878 is 13.32 m. This
compares with peak levels of about 11.6m derived from a simple regression between
Lake Victoria and Aswan flows by Flohn & Burkhardt (1985), and a value of about
14.2 m estimated by Nicholson (1998) from anecdotal evidence. Also, in the period
1870-1895, levels appear to have been slightly above the average in the period 18961960 (i.e. before the sudden rise in levels from 1961 to 1964) but, except for a brief
period in the 1890s, were less than levels from 1964 to about 1990. Figure 2(d) also
shows lines indicating the standard deviation of the estimated levels, which were
typically about 0.25 m (giving a 95% confidence limit of about ±0.5 m). The symbols
indicate estimates from anecdotal evidence which is described in the following section.
Comparisons with historical evidence
Although direct, enduring measurements of Lake Victoria levels did not begin until
1896, there are many accounts from government officials, travellers, residents and
missionaries before that time which give an indication of lake levels, and these are
summarized in Table 2 (see also Nicholson, 1998). This section reviews the main
evidence available, both for Lake Victoria itself and locations downstream, and then
compares these results with those from the model. The information is put into
perspective by the fact that Speke first visited the Ripon Falls in 1862, establishing that
a large river flowed northwards out of Lake Victoria, and that it was not until 1874 that
Stanley established the extent of the lake.
Much of the key information is collected by Garstin (1901, 1904). In particular,
Lyons contributed Appendix III to Garstin (1904) on the variations of level of Lake
Victoria and reproduced the material in Lyons (1906).
Garstin (1901, p. 49) noted that "for many years previous to 1901, the mean levels
of this lake have shown a steady fall, independent of the annual fluctuations due to the
rainfall. This general lowering of the water-levels was noticed both by Sir H. Stanley
and by Mason Bey at the time of their respective visits, while further proof is afforded
by the register of the water-marks kept by the French Catholic Mission at Uganda.
These last show that, three years ago [i.e. 1898], the mean water-level of the lake
312
Emma L. Tate et al.
averaged some 8 feet lower than it did twenty years earlier [i.e. 1878]." (our
annotations in square parentheses). This statement was made on the authority of
Mr McAllister, Vice-Consul at Wadelai and Sub-Commissioner of the Nile Province,
who met the Garstin mission in 1901. Although this statement seems clear-cut,
subsequent versions (e.g. Garstin, 1904; Ravenstein, 1901, who also misinterprets the
date) place the mission variously at Mwanza, in Tanzania; at Bukumbi, on the south
shore of the lake; and at Buganda, in Uganda. However, the key observation of an
8 feet drop (2.4 m) remains undisputed, except for a statement by Cdr Whitehouse
(Garstin, 1904) that he could not see how such a drop could be possible "unless there
was a similar rise between 1875 and 1877, which appears unlikely". His reservations
were based on photographs of Ripon falls in 1875 (by Stanley) and 1900 which
showed similar water levels at the falls.
Speaking of a visit in September 1889, Stanley (1890) himself states that the "French
missionaries, since their settlement near the Lake at Bukumbi, have ascertained that the
Lake is now three feet lower than when they first settled here—that is about eleven years
ago—that Ukerewé is no longer an island but is a peninsula". Ukerewé island/peninsula
is located in the southeast of the lake, and Stanley had noted in 1875 that the island was
separated from the mainland by a narrow channel in places only three feet deep.
Unfortunately, the French missionaries at Bukumbi "are said to have possessed a record of
the level of the lake extending over many years, but it is reported to have been lost with
much other scientific material by the sinking of a canoe on the lake." (Garstin, 1904).
The Appendix by Lyons in Garstin (1904) quotes several other descriptions of
events in 1877 and 1878. For example, Wilson noted that in February 1877 there was
exceptionally heavy rainfall to the south of the lake and an unusual rise of two feet.
Hutchinson, quoting Wilson, states that the lake continued to rise and reached a
Table 2 Anecdotal and other evidence of Lake Victoria levels before 1896.
Date
April 1891
Level on
Jinja gauge (m)
10.8
11.8
12.0
12.1
12.7
12.7
12.7
13.8
11.0
11.8
11.3
June 1892
January 1895
June 1895
13.1
11.8
13.3
January 1896
1898
11.57
11.4
End of year 1860
March 1875
June 1875
February 1877
May 1877
12 Jan. 1878
15 Mar. 1878
Aug.-Sep. 1878
1884
Early 1886
September 1889
Description available
Peney's gauging
Stanley noted Ukerewe island channel 3 ft deep
Stanley poled through channel 3 ft+ deep
2 ft rise from mean
Further 2 ft rise
Same as previously observed
Same as previously observed
Levels 2.4 m higher than in 1898. Flooding in the Sudd swamps
Rains failed in northeast of lake
Rains failed in northeast of lake
Stanley noted Ukerewe no longer an island
French mission says lake level has fallen 3ft in the past 11 years
Level 5-6 ft below highest level in recent years
Rugezi straits impassable
Level 6 ft above normal
Lake rose 1.5 m in 1895
Level higher than in 1892, less than in 1878. Flooding in the
Sudd swamps
First observed level at Entebbe (corrected to Jinja gauge)
Based on Aug-Dec observations at Entebbe
A water balance study of the upper White Nile basin flows in the late nineteenth century
313
maximum value in May 1877 of two feet above its February level. On his return to
Kagehi (near Mwanza, on the southeast corner of Lake Victoria) on 12 January 1878,
the level was within 1-1.5 inches of its May 1877 level, showing that the NovemberDecember rains had been particularly heavy. The level was the same on 15 March
1878, and a few days later in Uganda he received evidence of unusually high levels.
Dr Felkin of the Royal Scottish Geographical Society, who visited Emin in Lado (near
Mongalla) in 1878, recorded that the lake rose three feet above its normal level in
August and September 1878 following exceptionally heavy rains.
Further evidence of later changes is given in the same Appendix by Lyons in
Garstin (1904). Fischer noted that the rains failed to the northeast of the lake in early
1886, and also in 1884. This corresponds to Lyons' conclusion that 1880-1890 was a
period of falling lake levels. In 1891, Gedge noted that levels were said to be about
eight to nine feet below the highest water mark, but Dermott noted in April 1891 that
south of Ukerewe the water level was five to six feet below the high level marks on the
rocks; the Rugezi straits between Ukerewe and the mainland were not passable. In the
following year, Lugard noted in June 1892 that following exceptionally heavy rainfall
between November 1891 and February 1892 there had been a marked rise in the lake
to "some 6 feet perhaps above its ordinary level". In 1892, Baumann also noted that
the Rugezi straits were now passable and thus levels had risen since April 1891
following heavy rains. In 1895, Père Brard stated that the lake level on the southern
coast had risen 1.5 m after heavy rains in March, and that plantations along the
southern shore were destroyed; it was claimed that no such level had occurred for 30
years, but it was noted that 1878 was certainly as high or higher.
There are some inconsistencies in these observations, but an attempt has been
made in Table 2 to interpret them in terms of Lake Victoria levels. There is also
important evidence of an upper bound to lake levels in the recent past. An excavated
cave at Hippo Bay near Entebbe (Brachi, 1960) revealed that beach sands overlain by
occupation debris contained water-rolled fragments of charcoal distributed throughout
the sands; a sample of charcoal was dated as 3720 ± 120 years BP (before present)
(Bishop, 1969), and Bishop noted that "the recently recorded rise in the level of Lake
Victoria between 1961 and 1964 is of interest as, at their maximum, the lake waters
were within two feet of the dated beach gravel in Hippo Bay cave.. .This cave fill is so
unconsolidated that even a few weeks would suffice to remove the whole deposit. It is
certain that at no time since 3720 ± years ago has Lake Victoria risen to re-occupy the
10- to 12-foot cliff notch." An increase of 10-12 feet (3.05-3.66 m) over the Jinja
gauge level for 21-31 August 1959 (10.80 m) gives a maximum of 13.85-14.46 m on
the Jinja gauge as the highest level recorded in the past 4000 or so years.
Sudd inflows Narratives from the Sudd are also important as high inflows are an
indication of high outflows from Lake Victoria. However, information on early
measurements of Sudd inflows is limited. Garstin (1901, p. 49) refers to a measurement by Dr Peney on 27 February 1863 above Bedden rapids (above Rejaf), with a
total of 550 m3 s"1, but Garstin (1904, p. 127) refers to 1861 and Lyons (1906, p. 91)
gives a flow of 565 m J s'1 on 27 February 1861. Although the accuracy of this
measurement has been criticized, its significance is that, in February, at the end of the
dry season, most of the flow downstream of Lake Albert originates from Lake
Victoria, with inflows from the Semliki typically offset by losses in lakes Kyoga and
314
Emma L. Tate et al.
Albert, and the torrent flows being negligible. Thus the outflow at Jinja at the end of
1860 must have been of this magnitude, corresponding to a level of about 10.8 m on
the Jinja gauge.
Within the Sudd, Johnson (1992) discusses large floods in 1878 and in 1895,
damaging sorghum crops to the east of the Bahr el Zeraf. Luac and Ghol Dinka
traditions recall that the flood arrived from the south, along the Bahr el Jebel, with
little narrative evidence of unusually high flows on the Sobat. The flood of 1878 was
dramatic, but did not produce any long-term displacement. A flood of 1895-1896
made an area to the east of the Bahr el Zeraf untenable.
Eighteen months of flooding began in October 1916, which submerged whole
districts from Bor to Malakal and from Kongor to the Pibor River. The first flood of
1916-1917 came from the Bahr el Jebel, flooding Kongor in October. The flood of
1917 was higher and more prolonged, and was observed to come from two directions:
from the main river and also from the east. This suggests that there was a large
contribution from the Pibor, as the Sobat was high but the Baro at Gambeila was not
high. There was also high rainfall around the Sudd in 1917-1918. This is some
confirmation of the observation that the 1917-1918 event may have derived mainly
from north of Lake Victoria, rather than the lake basin.
Floods of 1946-1947 are described by Johnson as due to heavy Sobat flows and
subsequent "creeping flow", rather than runoff from the lake basin. The 1961-1964
flood was caused by heavy rainfall in 1961-1962 over a wide area. It can be noted that
there was high flow on the Pibor in 1961-1962, which reached the Sobat in 19621963, before the peak outflow from Lake Victoria. The baseflow component at
Malakal was higher and longer than in 1917.
There are few direct observations of Sudd areas. Those which do exist are mainly
based on air photography (1930-1931), maps (from reconnaissance in 1950-1952) and
satellite imagery (1979-1980). In order to estimate Sudd areas in those years for which
no direct observations are available, water balance models are commonly used. The
model used by Sutcliffe & Parks (1987) suggested that areas of the Sudd between 1905
and 1980 varied between approximately 5000 to 40 000 km2. In fact, the 1961-1964
event, when there was an approximate 20% increase in rainfall above average over
Lake Victoria, caused a doubling of lake outflows, and caused the area of the Sudd to
increase almost six-fold. Therefore, it is entirely reasonable to suggest that the high
outflows from Lake Victoria in 1878 and 1895 must also have caused significant
flooding.
Comparisons with the model results
Although there are contradictions in some of the early evidence of lake levels, Table 2
summarizes the results from these various descriptions and Fig. 2(d) shows the
inferred values as points. Given that the model predicts annual mean values, on the
graph the observations in Table 2 have been adjusted upwards by 0.25 m for dry
season observations, and reduced by 0.25 m for wet season peaks, since the annual
range (max to min) of Lake Victoria levels is typically about 0.5 m. The agreement
between the model and historical observations is generally good, except for the 1895
event, when the anecdotal evidence suggests a peak about 0.9 m higher than given by
A water balance study of the upper White Nile basin flows in the late nineteenth century
315
the model. However, given that the first observed level in January 1896 was only
11.57 m, perhaps the anecdotal value of 13.3 m in June 1895 is rather high, since that
would imply a drop of 1.6 m in only six months.
For the main peak of 1878, the value of 13.3 m from the model agrees well with
the inferred value of 13.55 m from the evidence of the French missionaries. The
agreement in the general trend in levels is also good; for example, that levels rose
suddenly in 1878, but dropped over the next 11 years, with rains failing in 1884 and
1886. Levels rose again in 1892 and 1895, before dropping back to the low levels
recorded in the first few years of observation from 1896. The historical observations
therefore add some credibility to the model results, and the implications for future
flows of assuming this sequence of levels from 1869 are assessed in the following
section.
DISCUSSION AND CONCLUSIONS
This study has provided new estimates of Lake Victoria levels and outflows for the
period 1870-1895, extending the length of the observed data for Lake Victoria (1896
onwards) by some 25%. Crucially the analysis includes the major flood event of 1878,
which anecdotal evidence suggests was the highest since 1860 (and possibly earlier);
possibly linked to the El Nino event of 1878, which was one of the strongest of the
past few centuries. This is supported by evidence from the Roda Nilometer, which
indicates that the flows in 1878/79 were the highest for the previous fifty years or so,
and therefore the highest in the nineteenth century.
Net basin supply
To assess the implications of these results for future levels and outflows at Lake
Victoria, it is necessary to estimate the net basin supply to the lake, which is the sum
of the inflows from rainfall and tributary flows, less the evaporation from the lake. The
net basin supply is a true measure of climatic variability around the lake, whereas the
lake level and outflow time series are not independent, and are influenced by storage
effects in the lake, which can persist for the order of a decade.
A first estimate of the annual net basin supply can be obtained from the annual
storage changes in the lake and the mean annual outflow. Observations suggest that the
surface area of the lake only varies by a few percent about a mean value of 67 000
km2, so annual storage changes can be estimated by assuming a constant surface area
equal to the mean. End-of-year levels have been estimated by assuming that the annual
mean values calculated by the model are indicative of mid-year levels, and assuming a
linear variation in levels between years. Although this is only a crude approximation,
comparisons with the more sophisticated estimates of the annual net basin supply
derived by Sene & Plinston (1994) showed good agreement over the period for which
these estimates were available (1925-1990). Sene & Plinston (1994) also estimated the
annual lake rainfall from raingauges around the lake, and Fig. 3(a) shows that there is a
reasonable correlation between annual rainfall and net basin supply (expressed as a
depth over the lake surface) over the period 1925-1990, with an uncertainty of about
316
Emma L. Tate et al,
±100 mm year" . The net basin supply estimates can therefore also be used to infer the
lake rainfall, and Fig. 3(b) shows this comparison for the period 1870-1996.
Several interesting conclusions can be made from this plot. The first is that, during
the 1878 event, both the rainfall and net basin supply rose rapidly to an exceptional
level, but fell back to very low values in the next two years. This is in contrast to the
1000
-500
(b)
0
500
1000
Net basin supply (mm/year)
1500
3000
Lake rainfall
2500 -
-Net basin supply
2000 1500 1000 500
1E70 1880 1890 1900 1910 1920 1930 1940 19501960 19701980 1990
-500
Year
0.3
0.25
-1896-1996
-1870-1996
0.2
3 0.15
(c)
0.1
0.05
-400 -200
0
200 400 600 800 1000 1200 1400 1600
Net basin supply (mm/year)
Fig. 3 Results of modelling of Lake Victoria's net basin supply (a) correlation
between annual rainfall and net basin supply; (b) net basin supply estimates used to
infer rainfall over Lake Victoria; and (c) frequency distribution of the net basin
supply.
A water balance study of the upper White Nile basin flows in the late nineteenth century
317
1917 event, which built up over two to three years, and the 1961-1964 event, which
arose initially from a rapid increase in rainfall, which was almost sustained for the next
two to three years, and was followed by several years of above average rainfall. These
three major events were therefore all qualitatively different, and the anecdotal evidence
confirms this to some extent: i.e. the 1878 event arose from exceptional rainfall,
affecting both the Blue and White Nile basins; the 1917 event arose from heavy rainfall
to the north of the lake, with unusually high flows in the Pibor; whilst the 1961-1964
event arose from prolonged heavy rainfall in the Lake Victoria region, but not in the
Blue Nile catchment. The smaller increase in levels during the 1890s appears to have
been due to several years of slightly above average rainfall. It is also worth noting that,
for all these events, the variations in areal rainfall are not particularly large, with a total
range of only 1400-2400 mm year"1 over the period 1870-1998. However, due to the
extreme sensitivity of levels to minor changes in rainfall, in terms of levels and outflows,
these events appear much more dramatic than suggested by the rainfall data.
The statistical characteristics of the rainfall and net basin supply are summarized
for several averaging periods in Table 3, using the standard WMO 30-year periods. In
the period 1871-1900, the lake rainfall appears to have been slightly above average,
and perhaps slightly more variable than usual. Only the years 1961-1990 stand out as
having unusually high rainfall. The estimated maximum lake rainfall and lake levels
reached are also shown; according to the model, the lake rainfall reached comparable
peaks of 2300-2400 mm in 1878 and 1961, but a much lower peak in 1917. Annual
mean levels in 1878 and 1961 were also similar. However, following the 1961 event,
levels took much longer to decline due to the sustained increase in rainfall, and the
lake levels more clearly exhibited the quasi-exponential decline expected from
idealized models of the lake water balance (e.g. Sene & Plinston, 1994).
Table 3 Estimates for annual lake rainfall and net basin supply over various periods.
Period
Net basin supply
(mm)
Mean
St. dev.
Max. rainfall in
period
Value (mm) Year
Max. annual level in
period (m)
Value (m) Year
190
145
176
179
407
319
302
598
385
306
238
323
2314
2015
2236
2370
1878
1917
1951
1961
13.3
11.7
11.5
13.0
1878
1917
1942
1964
171
175
404
407
311
328
2370
2370
1961
1961
13.0
13.3
1964
1878
Lake rainfall
(mm)
St. dev.
Mean
Thirty year periods
1871-1900 1753
1901-1930 1707
1931-1960 1716
1961-1990 1837
Full record
1900-1996 1752
1871-1996 1754
Possible future levels of Lake Victoria
In all stochastic models for net basin supply, the frequency distribution gives a useful
first indication of the likely range of predicted levels, and Fig. 3(c) shows the
estimated values for the periods 1896-1996 and 1870-1996 (i.e. excluding and
including the results from the present study). In terms of future levels, a preliminary
conclusion must be that these additional 26 years of data (1870-1895) do not greatly
318
Emma L. Tate et al.
change ideas about the natural flow regime of the lake, and stochastic projections of
future flows are likely to be similar to previous estimates. Given the large financial
investments in the Owen Falls Dam, and current plans to expand hydropower potential
at sites further down the Nile, this is perhaps reassuring, although a much more
detailed investigation would be required to draw conclusions which could be used in
any economic assessments or detailed design studies. The additional 26 years of data
therefore provide further evidence that Lake Victoria levels are highly variable, and
can both rise and fall rapidly in periods of only a few years. Also, the 1961-1964
event, which at the time was a major surprise to those involved with development of
the Nile, can be seen to be consistent with the natural rainfall regime of the region,
with a return period which can be tentatively estimated as of the order 100 years.
Acknowledgements This paper was prepared with support from the UK Natural
Environment Research Council (CEH Wallingford). The diagrams were designed and
produced by Val Bronsdon and Sonja Folwell.
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Received 21 June 2000; accepted 13 November 2000