Hydrological Sciences - Journal - des Sciences Hydrologiques, 29, 2, 6/1984
Regulation of the White Nile
G. W. KITE* G.K. Associates, Consulting Engineers, 17 Spindrift Court, South Parade, West Kirby, Wirral L48 ORR, UK
ABSTRACT The paper summarizes investigations made into regulation of the White Nile during Phase III of the WMO/UNDP Hydrometeorological Survey of the catchments of Lakes Victoria, Kyoga and Mobuto Sese Seko. A number of historical regulation plans were evaluated using the mathematical model developed by the project. A series of new plans was devised to provide benefits to the riparian countries although, due to lack of data, economic and ecological effects were not included.
Régularisation du Nil Blanc RESUME Le texte résume les recherches concernant la régularisation du Nil Blanc faites pendant la deuxième phase de l'étude "Relevé hydrométéorologique des bassins versants des lacs Victoria, Kyoga et Mobutu Sese Seko" par l'OMM et le PNUD. La valeur de divers plans de régularisation hist orique a pu être estimée grâce au modèle mathématique mis au point par le projet. Une série de nouveaux plans a été préparée au profit des pays riverains mais, en raison del' absence de données, les effets économiques et écologiques n'ont pas été inclus dans cette étude.
INTRODUCTION
At one end of the basin of the River Nile (Fig.l) lies a chain of three major lakes; Victoria, Kyoga and Mobuto Sese Seko (Albert), containing an estimated 3200 km of fresh water. At the other end of the basin, and separated from the upper basin by 1500 km of desert, lies Egypt, a densely populated country totally dependent for its drinking water, its irrigation water and its water for power on the continued flow of the Nile. This situation led to the development in the early twentieth century of the first plans to regulate and control the lakes of the Nile basin to increase the reliability and usefulness of river flows. Since that time other regulation plans have been designed with similar and with different objectives. As part of Phase II of the WMO/UNDP Hydrometeorological Survey of the catchments of Lakes Victoria, Kyoga and Mobuto Sese Seko, 1976-1981, a comprehensive study was made of these historical regulation plans using a deterministic mathematical model of the lake system developed in Phase II (Nèmec & Kite, 1979). Following
*Now: Chief Technical Adviser, WMO, c/o UNDP, PO Box 107, Kathmandu, Nepal. 191 192 G.W. Kite
Fig. 1 The Nile basin.
this historical survey, a series of new regulation plans was prepared utilizing the best parts of the historic plans and attempting to improve upon their performances. Full details of the studies are available in a report presented to the participat ing governments (Burundi, Egypt, Kenya, Rwanda, Sudan, Tanzania and Uganda) by the World Meteorological Organization (WMO, 1982).
DATA
Before a regulation plan can be tested it is necessary to obtain a set of data for the lake or reservoir over some historical period. The regulation plan then operates on these water supplies to produce a set of artificial lake levels and outflows correspond ing to the internal set of control rules. These data (usually termed net basin supplies, NBS) can be obtained in two ways. Firstly, and most obviously, they can be obtained from the river flows into the lake (I), the rainfall over the lake (R) and the evaporation from the lake (E) as: Regulation of the White Nile 193
NBS* + R = It t The disadvantage of this method is that meteorological data are often available for shorter periods than hydrological data and so for Lakes Victoria, Kyoga and Mobuto sets of monthly net basin supplies were calculated for the common period of January 1912 to December 1977 as:
NBSt = ASt + 0t - It where 0+ and It are recorded natural lake outflows and inflows and AS + is the change in storage of the lake over the month, computed from the recorded beginning-of-month lake stages, Ht, as:
ASt = f(Ht - Ht_1) where f indicates the lake stage-volume relationship. Also needed as basic data were dead storages (the volumes and corresponding stages below which control of outflow is not possible), the lake stages and volumes at the start of the common period and stage-outflow and stage-area-volume relationships for each lake. Figure 2 shows these relationships for Lake Victoria.
STAGE-AREA STAGE-FLOW 1137 11J7- 36 3fi- „ 35 3b- E 34 34 ^"'' 33 / « 32 32 / rj; 3i 31 1 <1130 1130- ' 2 29 ?q- i « 28 28 27 27- 26 26H 1125 11?5 6 8 10 12 14 16 18 20 22 24 26 28 30 3 AREA KM «10 VICTORIA NILE FLOW m /s «10''
1137- STAGE-VOLUME 36- 35-) E 34 <ë 33 I 32-1 w 31 < 1130 5 29' ^ 28 27- 26- 1125 24 25 26 27 28 29 VOLUME KM'xIO1 Fig. 2 Lake Victoria characteristic curves.
HISTORICAL REGULATION PLANS
During the early decades of this century the Government of Egypt became concerned at the apparent conflict between a rapidly rising 194 G.W. Kite population with its increasing need for water and the finite availability of this water. As a temporary solution to this problem the Government of Egypt submitted to the Sudanese Government in 1928 and again in 1938 a plan (known as the Equatorial Nile Project) to provide more water for irrigation during that portion of the year (the untimely period, January to June) when flows from the Blue Nile were low. The increased flow was to be obtained from the White Nile by storing water in Lakes Victoria, Kyoga and Mobuto Sese Seko (termed "Century Storage" because of the design period of the reservoirs) and by diverting the flow of the White Nile around the Sudd, in southern Sudan, through two canals. This grandiose plan is described in Hurst et al. (1946). Two regulation plans were tested; the first, named Hurst 1946 (i), regulated only Lake 3 — 1 Mobutu and restricted outflows to 1157 m s during January to June 3 — 1 and to 500 m s for July to December. The second plan, Hurst 1946 (ii) regulated Lake Mobutu as in plan (i) and also restrained the outflow of Lake Victoria to its long term mean flow. It was quickly realized that these plans had two major disadvantages. Firstly, by reversing the normal seasonal fluctuations in the flow of the White Nile, considerable hardship would have been caused to the pastoral Dinka, Nuer and Shilluk people of southern Sudan. Secondly, utilizing constant outflows from the lakes would have caused large fluctuations in lake levels resulting in increased damages to lake-shore interests in Kenya, Tanzania and Uganda. Following an 8-year study the Equatorial Nile Project was dropped, Egypt and Sudan signed, in 1959, the Nile Waters Agreement allocating available waters between the two countries, and Egypt followed an alternative development path by constructing the high Aswan dam with its large reservoir capable of over-year storage. In 1947, following discussions with the Governments of Egypt and Sudan, the Government of Uganda put forward a plan to regulate Lake Victoria to maximize the benefits for the proposed Owen Falls hydroelectric plant at Jinja. This plan (Uganda 1947) proposed an 3 —1 optimum release from Lake Victoria of 666 m s with provision for modifying this flow should the lake level go above or below a 2 m allowable range. Lakes Kyoga and Mobutu were to remain unregulated. In 1968 the Government of Uganda proposed a variation on Uganda 1947 in which the allowable range in lake level and the required outflow were modified to take into account the change in lake regime which had taken place in the early I960's (Kite, 1981). The first plans to consider regulating all three lakes (Victoria, Kyoga and Mobutu) were proposed by Dr M.Amin, a consultant to the Egyptian Ministry of Irrigation, in 1958. His series of four regulation plans (Amin 1958 a,b,c and d) contained variations such as constant outflow from Lake Victoria, either constant level or constant outflow for Lake Kyoga and constant outflow from Lake Mobutu. No variations of flow with time of year were considered, reflecting the changed interests of downstream countries following completion of the high Aswan dam. Another plan published by Amin in 1960 proposed that all three lakes have outflows restricted to their respective long-term mean flows. The difficulties with plans such as these are twofold: firstly, time series of lake levels and outflows are often not stationary; that is, statistics such as mean and variance are not time-invariant Regulation of the White Nile 195 (when Amin published his 1960 plan the mean outflow of Lake Victoria over the evaluation period which he used, 1904-1957, was 670 m3s-1 whereas the mean outflow over the period used in the present study, 1912-1977, is 794 m3s_1). The second difficulty is that, as mentioned earlier, use of a single constant outflow results in an excessive range of lake levels. The latest and most sophisticated of the historical regulation plans is Plan 100-A proposed by the Egyptian Organization for Nile Waters. This plan allows a wide range of outflows for Lake Victoria with a desirable flow (draft) of 750 m s-1 but provision for higher or lower flows should the lake level go beyond aim allowable range. Lake Kyoga was to be maintained at a constant level for as much of the time as possible, but with a provision for change in level when necessary. The change in level of Lake Kyoga was to be controlled by reference to a scale made up by adding up the active storages contained in Lakes Victoria and Mobutu. If the 9 3 combined active storage exceeded 200 x 10 m then the level of Lake Kyoga could be raised to accommodate some of the excess flow. Similarly, if the combined active storage went below 70 x 10 m then the level of Lake Kyoga could be lowered to augment the reduced outflows from Lake Victoria. The outflow of Lake Mobutu was also dependent on the combined active storages of Lakes Victoria and Mobutu. A peculiarity of this plan is that it utilized artificially high starting levels e.g. instead of using the recorded January 1912 Lake Mobutu level of 619.57 m, the plan assumed an initial level of 630.60 m, which is nearly 7 m higher than the maximum recorded lake level . The result was to store a "bank" of water from which withdrawals were made at times of low water supply enabling the plan to survive periods it would otherwise have failed.
EVALUATION OF HISTORICAL PLANS
The regulation plans' were run on a monthly basis over the period January 1912 to December 1977 using the lake component of the Mathematical Model of the Upper Nile Basin. This model was prepared during Phase II of the Hydromet Project incorporating lake routing routines. Many of the plans utilizing only one regulated outflow failed during the first period of low water supplies when the lakes were drained to the minimum permissible level. In order to have all the plans operate over the entire evaluation period, these plans were modified so that when lake levels approached their minimum the outflows were reduced to those which would have occurred naturally. A similar procedure was needed to revert to natural outflows at high lake levels (Fig.3). Plans were evaluated using two criteria: (a) How the regulated levels and outflows compared to the ranges of levels and outflows recorded under natural conditions. A plan which drastically alters the range of lake level or outflow would probably not be a good plan; the costs and social disadvantages of such a plan might well outweigh the benefits. (b) A set of reliability indices suggested by Kleme's (1980) was used to compare the performances of the various plans : 196 G.W. Kite
(i) Time reliability Rt = (m x 100)/n where n is the total number of regulated time periods (months), and m is the number of time periods in which the lake outflow is equal to or greater than the required draft.
(ii) Volume reliability Ev = {kD + ZQ (iii) Effectiveness of hydro-plant En = {l - [EQ>T(Q - T)]/ nQ} x 100 where T is the total capacity of the turbines. The percentage of water lost over the spillway is 100 - En. (iv) Exceedance of maximum non-damaging flow Effl = (t x 100)/n where t is the time for which regulated flow exceeds the maximum non-damaging flow and n is the total regulated time. All plans were operated under stationary conditions. To achieve stationarity for a particular plan the model would be operated using recorded net water supplies. The final lake levels (December 1977) produced by the plan would then be used as initial lake levels (January 1912) in a second run. This would be repeated in a third run and so on until the final lake levels were the same as the initial levels. Generally only two computer runs were necessary to achieve stationarity. Table 1 shows the results of comparisons of regulated and recorded lake levels and outflows for Lakes Victoria, Kyoga and Mobutu and Table 2 shows the results of the reliability statistics for the various regulation plans for the three lakes. DESIGN OF NEW REGULATION PLANS The objectives of a regulation plan were assumed to be: (a) to maximize the power output at the Owen Falls dam, 1137 - 1136-26 / ^ / M 36 • y y "~T M35-T6 / "? / E / y m y S il 35 - / r u L / ""3 / ro ,_S 1134 • / aj / 1133-76 / UJ / UJ 1133-26 / H33 - o ^ % o M 32 - 0 500 1000 1500 2000 2500 Outflow (ïï^s"1) Fig. 3 Lake Victoria plan Uganda 1947. Regulation of the White Nile 197 (b) to maximize power potential in the Kyoga Nile, (c) to meet the forseeable water resources requirements of. the riparian countries of the Upper Nile basin, (d) to provide maximum guaranteed annual yield downstream of Lake Mobutu, (e) to minimize impact on riparian peoples. The design of a regulation plan for Lake Victoria was straight forward. The estimated year 2000 water needs (supply, irrigation, Table 1 Maximum and minimum regulated levels and outflows 1912-1977 Regulation Maximum Minimum Maximum Minimum 3 plan level Lake Victoria Hurst 1946 (a) 1136.18 1133.08 1721 347 Hurst 1946 (b) 1136.61 1132.66 2140 270 Uganda 1947 1136.71 1133.30 2140 507 Amin 1958 (a) 1136.83 1133.17 2300 640 Amin 1958 (b) 1136.83 1133.17 2300 640 Amin 1958 (c) 1138.09 1133.45 1157 460 Amin 1958 (d) 1138.09 1133.45 1157 460 Amin 1960 1136.61 1132.66 2140 270 Uganda 1968 1136.20 1133.14 1700 645 100-A 1136.38 1133.18 1730 395 K6 1136.49 1132.97 1990 350 K8 1136.48 1132.90 1990 350 Recorded 1136.18 1133.08 1721 347 Lake Kyoga Hurst 1946 (a) 1034.11 1030.31 1991 280 Hurst 1946 (b) 1034.47 1030.08 2218 244 Uganda 1947 1034.72 1030.73 2345 322 Amin 1958 (a) 1031.62 1031.62 2847 30 Amin 1958 (b) 1035.15 1030.03 2500 230 Amin 1958 (c) 1031.62 1031.62 2445 0 Amin 1958 (d) 1034.62 1034.62 2445 0 Amin 1960 1034.83 1029.24 2500 230 Uganda 1968 1034.31 1030.80 2067 332 100-A 1034.62 1030.62 2428 141 K6 1034.85 1030.07 2500 230 K8 1034.77 1029.92 2500 230 Recorded 1034.11 1030.31 1991 280 Lake Mobutu Sese Seko Hurst 1946(a) 624.72 618.37 2160 0 Hurst 1946 (b) 625.00 618.20 2160 0 Uganda 1947 622.96 619.57 1859 338 Amin 1958 (a) 625.91 618.53 2160 0 Amin 1958 (b) 625.63 618.54 2160 0 Amin 1958 (c) 624.83 618.45 2160 0 Amin 1958 (d) 624.56 618.40 2160 0 Amin 1960 625.41 618.35 2160 0 Uganda 1968 624.11 619.57 2087 340 100-A 647.98 619.57 1230 200 K6 621.65 618.81 1500 500 K8 621.49 619.16 1500 450 Recorded 623.97 618.75 2050 343 198 G.W. Kite Table 2 Performance indices for regulation plans under stationary conditions Draft Time Volume Exceedance of Hydro-plant Regulation D reliability reliability non-damaging effectiveness 3 1 plan (m s' } Rt (%) Rv (%) flow, EM (%) Eh (%) Lake Victoria Hurst 1946 (a) — — 0 Hurst 1946 (b) 794 95 97 7 91 Uganda 1947 666 100 100 6 86 Amin 1958 (a) 640 100 100 15 79 Amin 1958 (b) 640 100 100 15 79 0 84 Amin 1958 (c) 507 100 100 Amin 1958 (d) 507 100 0 84 Amin 1960 794 95 100 7 91 Uganda 1968 645 100 97 0 81 100-A 750 71 100 0 87 K6 750 96 93 4 100 K8 650 93 98 4 100 97 Lake Kyoga Hurst 1946 (a) — — 0 Hurst 1946 (b) - — 2 Uganda 1947 — - 3 Amin 1958 (a) — — 12 96 Amin 1958 (b) 627 93 15 Amin 1958 (c) — — 1 Amin 1958(d) — — 1 Amin 1960 771 92 95 7 Uganda 1968 - - 2 100-A — — 2 K6 700 96 98 7 K8 600 91 95 6 industrial) for the portions of Burundi, Kenya, Rwanda, Tanzania and Uganda were derived as part of a separate study. These values 3 —1 (in m s ) were subtracted from the recorded water supplies over the historical comparison period and the plan written to pass 3 — 1 optimum output (750 m s ) through the Owen Falls power plant while keeping lake levels within the recorded range. Lake Kyoga was used, in the new plans, as a regulating reservoir to smooth outflows for maximum utilization of any future power plants to be built on that stretch of the River Nile between Lakes Kyoga and Mobutu. With a fall of about 400 m and an average flow 3 -1 of about 800 m s the power potential of the Kyoga Nile could be around 2600 MW. The new plans retained one of the features of Plan 100-A, regulation of Lake Mobutu depending on the combined active storages in Lakes Victoria and Mobutu. The advantage of this is that it is possible to make use of knowledge of the condition of the upstream lake (Victoria). For example, if Lake Mobutu is low a direct regulation plan would reduce the outflow from Lake Mobutu accordingly. If, however, Lake Victoria is high, then using the combined active storages, outflows from Lake Mobutu can be maintained at a high or normal level in the knowledge that increased supplies from Lake Victoria are en route. Regulation of the White Nile 199 The mechanism of combining the active storages was modified. The ratio of total storage in Lake Victoria to total storage in Lake Mobutu is about 7:1. If the active storages are simply added together, as in Plan 100-A, then the influence of Lake Victoria is much too great and it would be possible for such a regulation plan to drain Lake Mobutu. Instead, the new plans used a weighting factor to give equal influence to each lake. RESULTS A series of new plans was developed and tested; the results of the comparisons with the historical regulation plans and with recorded lake levels and outflows are shown in Tables 1 and 2. When comparing the regulated lake levels and outflows with those recorded under natural conditions (Table 1) it can be seen that most plans operate fairly close to nature for Lake Victoria which, because of its great size (70 000 km ) is very well regulated naturally anyway. Plans Amin 1958 (c) and (d), which maintained constant lake outflows resulted in maximum lake levels nearly 2 m above maximum recorded levels, while plans Amin 1958 (a) and (b), which would have required outflows much less than mean water supply to the lake, required outflows higher than natural to keep the lake level below the maximum specified in the plan. On Lake Kyoga the Amin 1958 plans (a), (c) and (d) all result in greatly increased ranges of outflow because of attempts to maintain a constant lake level. The big differences arise on Lake Mobutu where most historical plans have tried to maintain too high an outflow. The result has been to draw the lake down to a point at which the plan has to be abandoned and outflows reduced to natural (zero, in some cases). Similarly, those plans which use only a single prescribed outflow were unable to cope with years of high water supply (e.g. 1962-1964), the plans had to be abandoned and recourse made to the natural stage- discharge relationship. Plan 100-A, which started at an artificially high level reached a maximum level 24 m above the maximum recorded level. The results of the performance indices (Table 2) may be summarized as follows: (a) Lake Victoria. All regulation plans have time and volume reliabilities close to 100%. This is to be expected because of the large degree of natural regulation. Those plans with lower drafts (e.g. Amin 1958) have higher time and volume reliabilities but have lower hydroplant effectiveness than those plans which aim for higher drafts. Plans Amin 1958 (a) and (b) result in the highest percentage of flows considered damaging to the channel of the Victoria Nile because the high regulated lake levels lead to high channel flows above the regulation ranges. (b) Lake Kyoga. Only plans Amin 1958 (b), Amin I960, K6 and K8 regulate Lake Kyoga in such a manner that reliability statistics can be meaningfully evaluated. Those plans which keep the lake level constant cannot be considered to have any planned draft. The percentage of outflows considered damaging varies from plan to plan 200 G.W. Kite by the ratio of 15:1. Since there is no existing hydroplant, no effectiveness statistics could be calculated. (c) Lake Mobutu. Again, on Lake Mobutu, all plans (except Uganda 1947 and Uganda 1968 which do not regulate this lake) result in high time and volume reliabilities. The percentages of time for which flows are damaging are relatively high (up to 19%) for most plans although 100-A, K6 and K8 avoid these high flows. The benefits of Plans K6 and K8 can be summarized as follows: K6 K8 Flow Relia- Flow Relia- (m3s~1) bilitu (m3s-1) bility Power, Lake Victoria 750 96% 650 93% Power, Lake Kyoga 700 96% 600 91% Water supply, d/s Mobutu 500 100% 450 100% Consumptive use u/s countries - -85 100% The costs of the new regulation plans would include the following: (a) A structure at the outlet of Lake Victoria (possibly a modification to the existing Owen Falls dam) to control the lake level within the range 1132.50-1137.00 m a.s.l. (Khartoum datum) and having an outflow capacity of at least 2000 m s-1. (b) A regulatory structure at the outlet of Lake Kyoga to control that lake within the range of 1029 and 1035 m and having an outflow capacity of at least 2500 m3s-1. (c) A regulatory structure at the outlet of Lake Mobutu to control that lake within the range 618.50-62.00 m with an outlet 3 —1 capacity of at least 1500 m s The new plans K6 and K8 were also tested using generated sequences of net water supplies to the three lakes so as to subject the plans to differing sequences of events. The plans were able to cope with all sequences used. CONCLUSIONS Regulation of the Upper Nile basin lakes could lead to substantial benefits for the people of all the riparian countries. A number of historical regulation plans designed for specific interests were evaluated and the results compared with those of two new regulation plans. These new plans consider the water supply and hydroelectric benefits available to the riparian countries. At the moment a large portion of any gain in reliable flow down stream of Lake Mobutu would be lost in increased evaporation in the Sudd. When the Jonglei canal is completed in 1985 then it will be of more interest to study again the benefits of lake regulation. One of the problems to be faced will be to develop synthetic sequences of net water supplies which can incorporate large jumps in supply such as occurred in 1960 so that proposed regulation plans can be tested over many such sequences. Regulation of the White Nile 201 ACKNOWLEDGEMENT The author gratefully acknowledges the permission of the World Meteorological Organization to publish this material. This does not mean that WMO necessarily agrees with the opinions expressed in this paper. REFERENCES Hurst, H.E., Black, R.P. & Simaika, Y.M. (1946) The Nile basin. Physical Department Paper no. 51, Ministry of Public Works, Egypt. Kite, G.W. (1981) Recent changes in level of Lake Victoria. Hydrol. Sci. Bull. 26 (3), 233-243. Klemes, V. (1980) Assessment of regulation studies for the equatorial lakes in the Upper Nile basin. Consultant Report to WMO, Geneva. Nemec, J. & Kite, G.W. (1979) Mathematical model of the Upper Nile basin. Chapter in: Logistics and Benefits of Mathematical Models in Hydrology and Water Resources. Pergamon Press, London. WMO (1982) Project findings and recommendations, hydrometeorological survey of the catchments of Lakes Victoria, Kyoga and Mobutu Sese Seko. WMO, Geneva.