Impact of Climate Change on Hydrology and Hydrologic Extremes of Upper Blue Nile River Basin

Abstract

The potential global warming impact on the extreme and mean streamflow of the Upper Blue Nile (UBN) River Basin was projected for the 2050s and 2080s by three hydrological models [Nedbør-Afstrømnings Model (NAM), Variable Infiltration Capacity (VIC) model, and Watflood model] driven by Representative Concentration Pathway (RCP) 4.5 and 8.5 climate change scenarios of four general circulation models (GCMs) dynamically downscaled by a regional climate model, Weather Research and Forecasting (WRF). The three hydrological models were able to capture the flow dynamics accurately in both calibration and validation periods. The mean daily maximum (minimum) temperature of UBN are projected to increase by about 1.35°C–2.38°C (1.72°C–2.74°C) under the RCP4.5 climate scenario and 2.22°C–4.47°C (2.5°C–5.1°C) under RCP8.5 climate scenario in future periods with reference to 1976–2005 as the base period. However, changes projected for the mean annual precipitation vary widely, ranging from −10.3% to 19.4%. The projected increase in evapotranspiration and increase or decrease in precipitation will result in a projected increase or decrease in streamflow of UBN. Overall, the median of mean annual streamflow of UBN is projected to decrease by 7.6% within a range of −19.7% to +17.7% in the 2050s and by 12.7% within a range of −26.8% to +31.6% in the 2080s. The ensemble mean of annual maxima (minima) of high return periods are projected by the three hydrologic models to be larger (smaller) in the 2050s and 2080s, respectively. On the whole, uncertainties due to the structure of hydrologic models, especially for low flow projections, are likely larger than uncertainties from GCMs and climate change scenarios. The results suggest that UBN is likely to experience more frequent and severe hydrologic extremes (flooding and droughts) in the future. Preliminary adaptive measures have been presented to mitigate the possible impact of droughts on UBN.