Seasonal Streamflow and Sediment Forecast in the Upper Blue Nile Basin, Ethiopia

Hydrological forecasting is an essential tool for water resource management, enabling predictions of the future state of water resources in a catchment. Since some years, the forecast horizon / lead time is increasing. The demand for reliable seasonal hydrologic forecasts is significant and various applications for water resources management are increasingly important. Integrating seasonal forecast results into decision-making processes have become vital for both short- and long-term water management across various sectors, including energy, water supply, agriculture, urban planning, infrastructure, and disaster preparedness.

This paper assesses seasonal streamflow and sediment forecasting as a critical component of effective water resource management. An effective seasonal water resources forecasting system requires an evaluation of both numerical weather prediction (NWP) models and hydrological models to accurately represent atmospheric and hydrological conditions in a specific region. This study evaluates the ECMWF-SEAS5 precipitation product in conjunction with a large-scale and process-oriented hydro-sedimentological model (WASA-SED) to produce seasonal streamflow and sediment forecasts for the Upper Blue Nile Basin, home to the largest reservoir in Africa (The Grand Ethiopian Renaissance Dam, GERD with a total capacity of 74×10⁹ m³) in Ethiopia. Originating in the Ethiopian highlands, the Blue Nile River, the most important tributary of the Nile, contributes approximately 60% of the Nile’s total flow and is a critical water source for around 20 million people in Ethiopia and 200 million downstream residents in Sudan and Egypt.

WASA-SED was tested and calibrated with river flow data at a daily resolution for the 2001-2007 and validated for 2007-2011. Three different large-scale rainfall “products” were tested and compared ref. their representativity of observed rainfall. We show that such a rainfall evaluation is indispensable for hydrological simulation as well as for seasonal forecasting. We consider this step a “hydrological verification” of rainfall data. Calibration of WASA-SED with river discharge data resulted a Nash–Sutcliffe Efficiency (NSE) of 0.80 and a Relative Error (RE) of 10.32%, while validation results improved to a NSE of 0.81 and a RE of 6.82%.

Seasonal streamflow and sediment flux data were than forecasted for June to December 2024, based on the seasonal meteorological forecast in the preceding month. An ensemble of 51 regional meteorological forecast members in daily resolution and 7 months lead time, each initiating on the first day of each month, was used. Daily and monthly streamflow series were simulated for each forecast member. A post-processing step with an autoregressive model was applied to adjust for forecast biases in seasonal streamflow predictions.

To evaluate the accuracy of 6 months hydrological forecasts, ensemble-averaged monthly rainfall and discharge forecasts were compared with observed average monthly rainfall and discharge values.  Results indicate that the coupled meteorological/hydrological models reasonably predict rainfall and discharge on a seasonal scale for the Blue Nile Basin.

The forecasting system is developed in close collaboration with local research partners to facilitate its implementation and sustained use beyond the project's duration.