Interannual Variability and Ensemble Forecast of Upper Blue Nile Basin Kiremt Season Precipitation

Abstract

Ethiopian agriculture and Nile River flows are heavily dependent upon the Kiremt season (June?September) precipitation in the upper Blue Nile basin, as a means of rain-fed irrigation and streamflow contribution, respectively. Climate diagnostics suggest that the El Niño?Southern Oscillation phenomenon is a main driver of interannual variability of seasonal precipitation in the basin. One-season (March?May) lead predictors of the seasonal precipitation are identified from the large-scale ocean?atmosphere?land system, including sea level pressures, sea surface temperatures, geopotential height, air temperature, and the Palmer Drought Severity Index. A nonparametric approach based on local polynomial regression is proposed for generating ensemble forecasts. The method is data driven, easy to implement, and provides a flexible framework able to capture any arbitrary features (linear or nonlinear) present in the data, as compared to traditional linear regression. The best subset of predictors, as determined by the generalized cross-validation (GCV) criteria, is selected from the suite of potential large-scale predictors. A simple technique for disaggregating the seasonal precipitation forecasts into monthly forecasts is also provided. Cross-validated forecasts indicate significant skill in comparison to climatological forecasts, as currently utilized by the Ethiopian National Meteorological Services Agency. This ensemble forecasting framework can serve as a useful tool for water resources planning and management within the basin.