Seasonal forecast of streamflow and suspended sediment in the Blue Nile Basin, Ethiopia

The demand for seasonal hydrologic forecasts is significant and various applications for water resources management are increasing. Since some years, the lead time is going up to several months or a season. However, the uncertainty is also increasing with lead time.

We assess the potential of seasonal streamflow and sediment forecasting as a tool for management of water resources and sediment flow in the Upper Blue Nile Basin (UBNB) of Ethiopia, upstream the GERD (Great Ethiopian Renaissance Dam). A coupled hydro-meteorological seasonal forecasting system requires a performance evaluation of both numerical weather prediction (NWP) models and hydrological models to accurately represent atmospheric and hydrological conditions. We evaluate the ECMWF-SEAS5 precipitation product in conjunction with the large-scale process-oriented hydro-sedimentological model WASA-SED. The aim is to generate forecasts for streamflow and suspended sediment fluxes with a lead time of up to seven months for the UBNB.

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.

Seasonal streamflow and sediment flux data were than forecasted for June to December of the year, 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. A post-processing step with an autoregressive model was applied to adjust for forecast biases in seasonal streamflow predictions. Results indicate that the coupled meteorological/hydrological models skilfully predict rainfall and discharge on a seasonal scale for the Blue Nile Basin.