9. DISCUSSION AND CONCLUSIONS
9.6 Concluding remarks in relation to research questions and objectives
The underlying goal of the study was to develop and apply a Decision Support System (DSS) so as to define alternative long-term operation rules for the Equatorial Lake reservoir system.
This was based on the premise that current operating policies based on releases in accordance to unregulated lake levels and outflows had proven to be insensitive to the demands for increased energy generation. The conclusions from the study are:
(i) The appropriate components of a DSS suitable for managing long-term operation of the system of multiple lake-reservoirs and large dams along the White Nile in Uganda, are essentially a combination of interlinked models and algorithms. These include river and lake simulation models based on previous research in the Equatorial Lake Region (WMO, 1977; Nemec and Kite 1979; Mott MacDonald, 1998). The simulation models are complemented by analytical hydrological time series software to process the reference hydrologic data (Hubert, 2000; Sveinsson, 2007). Current system data and power plant functions are incorporated from existing short-term reservoir operation models (Georgakakos and Yao 2003; WREM Inc. and Norplan (U), 2004). Output from the simulation models and power plant functions is utilized by prescriptive optimisation models (Labadie, 2003) to identify efficient operating rules. The rules could be refined using other techniques such as regression or neural network analysis.
(ii) The components of the DSS have been assembled, customised and applied to the case study to investigate regulation of the lake reservoirs. Demonstration of application of the DDS tool to the case study requires the consideration of a series of practical decisions. Among these are the compilation of long-term lake levels and outflows, underlying nature of the net inflows to the lake reservoirs and computing effort involved in application of the dynamic programming algorithms. Paucity of hydrological data in this region dictated preference be given to the utilization of long term series of observed
net rainfall. The presence of abrupt shifts in the annual net basin supply time series of each lake and lack of significant cross correlation at lag one in the monthly net basin time series is critical. It dictated the application of a uni-variate shifting mean model to generate synthetic net basin supply time series in the implicit stochastic optimisation approach. The extensive computing effort associated with fine discretization of the storage and release variables during derivation of operating rules with the multiple reservoir IDP algorithm is potentially prohibitive. It was shown that it is more feasible to define deterministic operating rules at an annual rather than monthly time step for the period 1899 – 2008.
(iii) The DSS tool has been successfully utilized to identify two alternative operating rules that maximise hydropower production and satisfy system constraints based on the observed net basin inflows to the lakes for the period 1899 to 2008. The operational policy derived for each lake reservoir is a set of rules specifying the trajectory of beginning of year storage and releases for the known net inflows. Application of the tool to the system configuration of planned dams, facilitates investigation of the effect of alternative regulation scenarios through the alteration of constraints to storage and release magnitudes. The regulation scenarios were formulated by application of the tool based on a set of two different system constraints. The results obtained from attempts to apply the implicit stochastic optimisation approach are not sufficient to define generalized annual operating rules based on all possible occurrences of net inflow.
However, general inferences on how to regulate the Equatorial Lakes can be made based on the impact of prescribed rules on lake levels during the period 1899-2008.
(iv) The comparison of results indicates that performance of the system for the two alternative regulatory options in terms of hydropower generation is marginally different. Their average annual generation over the period 1899 to 2008 would have been 1287 MW. On the other hand if the Equatorial Lakes had been unregulated the average annual hydropower production would have been 1127 MW. The two regulatory options enhance energy generation by 13% over the long term operating horizon 1899- 2008 when compared against the agreed curve. Regulation of the Equatorial Lakes has the potential to enhance power generation by up to 64% in a given year. Overall, the benefits of enhanced energy generation are markedly higher during periods of sustained low levels e.g. prior to 1961. Therefore the agreed curve mode of operation and
maintenance of natural lake levels and outflow is not an attractive operation policy particularly during periods of sustained drought if the objective is to maximise hydropower generation.
(v) Regulation of the Equatorial Lakes for purposes of maximising hydropower production can be attained without exceeding the historically observed range of lake levels and outflows as shown under Option 1. Regulation of the Equatorial Lakes is also feasible by utilizing Lake Albert as a balancing reservoir as shown under Option 2 where a wider range of releases is also permitted. The two regulation options have an identical impact Lake Victoria levels. They tend to mimic the natural variation of Lake Victoria levels but at consistently higher magnitudes. However, the two regulatory options can markedly alter Lake Kyoga and Albert levels. The natural variation of lake levels in Lake Kyoga can be wiped out over the occurrence of lengthy time periods of prevailing high levels in Lake Victoria since these policies dictate that Lake Kyoga levels be maintained at specified constant elevations during such episodes. Similarly both policies radically dictate that Lake Albert be maintained at specified constant elevations at all times. These scenarios are likely to be associated with a range of negative ecological and socio-economic consequences. Improved operating rules for enhancing hydropower generation of large dams along the Victoria and Kyoga Nile have been identified but their application has demonstrated that they would significantly modify the hydrological regimes of Lake Kyoga and Albert.
(vi) The method of application of the multi-reservoir IDP algorithm yielded practical operating rules that satisfy system constraints. Rules specified by the SDP algorithm to Lake Victoria can be implemented in a system wide network by maintaining Lake Kyoga and Lake Albert at constant elevations. This option provides a practical approach of defining approximate and rules given the dimensionality of the case study problem.
The applicability of the suggested rules should be subjected to a comprehensive environmental impact assessment to quantify the effects of maintaining constant lake levels on shore line ecology.