The Value of Long-Term Streamflow Forecasts in Adaptive Reservoir Operation: The Case of the High Aswan Dam in the Transboundary Nile River Basin

Transboundary river basins are experiencing extensive dam development that challenges future water management, especially for downstream nations. Thus, adapting the operation of existing reservoirs is indispensable to cope with alterations in flow regime. We proposed a Forecast-Based Adaptive Reservoir Operation (FARO) framework to evaluate the use of long-term climate forecasts in improving real-time reservoir operations. The FARO approach was applied to the High Aswan Dam (HAD) in the Nile River basin. Monthly precipitation and temperature forecasts at up to 12 months of lead time are used from a suite of eight North American Multi-Model Ensemble (NMME) models. The value of NMME-based forecasts to reservoir operations was compared with perfect and climatology-based forecasts over an optimization horizon of 10 years from 1993 to 2002. Our results indicated that the forecast horizon for HAD operation ranges between 5 and 12 months lead time at low- and high-demand scenarios, respectively, beyond which the forecast information no longer improves the release decision. The forecast value to HAD operation is more pronounced in the months following the flooding season (October-December). During these months, the skill of streamflow forecasts using NMME forcings outperforms the climatology-based forecasts. When considering the operation of upstream Grand Ethiopian Renaissance Dam (GERD), using streamflow forecasts minimally helps to maintain current target objectives of HAD operation and therefore result in higher operation costs as opposed to current conditions without GERD. Our study underlined the importance of deriving a new adaptive operating policy for HAD to improve the value of available forecasts while considering GERD filling and operation phases. SIGNIFICANCE STATEMENT Water management in the Nile River basin is challenged by the construction of the Grand Ethiopian Renaissance Dam (GERD) in Ethiopia. Thus, it becomes important to adapt the operation of existing dams to expected alteration in upstream flow. This study aims to use long-term forecasts to improve real-time operations of the High Aswan Dam (HAD) in Egypt. Forecasting streamflow up to 8 months ahead showed a significant improvement in the HAD operation. However, when considering GERD, using forecasts could minimally help to maintain current objectives of HAD operation. Our findings underline the importance of deriving a new operating policy for HAD to improve the value of available forecasts while considering future upstream projects.