A Framework for Evaluation of CORDEX Using Dam Operation Policies for the East African Hydroelectric Power Industry

Abstract:

The productivity of hydroelectric dams along the Nile River is to a large extent determined by the level of Lake Victoria, which is primarily dictated by the rainfall and temperature variability over the lake basin. The hydrological balance of Lake Victoria is comprised of tributary inflow (15% of the water balance), rainfall over the lake (85%), evaporation from the lake (80%), and outflow from the lake (20%). The Agreed Curve Policy has been used to manage the outflow from Lake Victoria. It guarantees the natural flow of water out of the lake as if there were no dams. However, declining lake levels and population growth have resulted in chronic load shedding for residential and industrial consumers in the region, leading to the need to develop a new release policy.

The development of recently proposed water release policies for the hydroelectric dams at the outlet of Lake Victoria do not thoroughly account for projected climate change. We adopt a comprehensive approach using rainfall data from multiple CORDEX models to estimate lake levels and confidence levels for two different release policies. We then compare exceedence curves for the release rule policies based on observed rainfall data for recent decades. Exceedence curves indicate vulnerability to power supply reliability.

Factors affecting precipitation in individual models are examined. There is a distinct dry-wet-dry-wet pattern of annual rainfall over the Lake Victoria basin, which is evident in a comparison of precipitation from rain gauge stations used in the water balance model. Previous studies have demonstrated that this feature is determined by a combination of orographic forcing, the thermodynamics of the lake, and the interaction between the prevailing wind flow and lake-land breeze circulation. ENSO has the most influence on precipitation over the lake basin in terms of annual climatology. We evaluate the CORDEX models based on how well they reproduce these metrics in addition to how well they perform in the water balance model. Based on this, we provide guidance on considerations for CORDEX models improvement in order to better serve the hydroelectric provider and management community for Lake Victoria.

This study will be incorporated into the Water Resources Sector Pilot Project of the proposed GEWEX Hydroclimate project for Lake Victoria (HyVic).