Investigation of the failure of the Mtera-Kidatu reservoir system.

The Mtera and Kidatu reservoirs are located in the Rufiji Basin, in the central part of Tanzania. These two reservoirs supply water to hydro-electric power generators whose production accounts for 85% of the power production in the Country. The successful operation of these reservoirs is, therefore, of critical importance with regard to the energy sector in Tanzania. During the period 1992 to 1994 water levels at the Mtera Reservoir fell very low, close to the lowest regulated water level (LRWL) of 690.0 metres above mean sea level (m.a.s.l), which adversely affected power generation from the reservoir system. The lowest level was recorded in December 1994 (690.15 m.a.s.l), slightly lower than the previous low value (690.99 m.a.s.l) recorded in December 1992. This study was carried out to establish the cause of this failure. It was noted that this failure was triggered by the inadequate refilling of the Mtera Reservoir between January and June 1991, and further aggravated by the same in January to June 1992. In this study the inflow to Mtera was estimated using the simple addition with a gain factor (ADD) model, which yielded a calibration efficiency of 54.01 % and a gain factor of 0.707. Kidatu inflow was estimated from the sum of the total discharge from Mtera and the runoff contribution from the intervening catchment between Mtera and Kidatu. A gain factor of 0.07 was obtained for the rainfall in the intervening catchment. Evaporation losses for both Mtera and Kidatu were estimated by Morton's model (WREVAP) using climatic data from Dodoma Airport. A mean annual potential evaporation estimate of 2459 mm was obtained. A water balance model was used to estimate total losses from each reservoir. The findings revealed that the total yearly and January to June losses were overestimated due to excess unrecorded spill from Mtera . Between July and December the estimated losses compared well with potential evaporation. A comparison of the inflow and discharge quantities for the periods January to June 1989, 1991 and 1992 revealed that higher levels should have been realized during the refilling phase in 1991 and 1992 for Mtera Reservoir. It is likely that the inadequate refilling was due to excess unrecorded spills from Mtera during this period. A simulation of the reservoir operation was performed based on an operating policy of minimizing spill from Mtera and Kidatu and maintaining full reseevoir water levels at Kidatu. The results of the simulation indicated that when losses were set equal to potential evaporation estimates conservative estimates of water levels were realized for the period 1983 to 1990 (calibration period) while for the 1991 to 1994 period the estimated levels were higher than the recorded levels. The findings of this study was that the failure was caused by excessive unrecorded spills from Mtera Reservoir from between January to June 1991 and 1992. The recommendation of this study is that there should be a closer control of the spills from Mtera, and the spills from Kidatu should be minimized.