Modeling of Hydrological Extremes Under Climate Change Scenarios in The Upper Blue Nile River Basin, Ethiopia

Abstract

The impact of climate change on modeling of hydrological extremes is needed to asses at regional and local scales since these vicissitudes are not uniform over the globe. This work stresses on climate change impact on the hydrological extremes (MMAX, CMAX, NNHF, Q10, MMIX, CMIX, MIN7, and Q90) and precipitation extremes (MLWD, MLDD, ADMT, ADMP) during the far future (2071-2100) period over the selected sites across the upper Blue Nile River basin (UBNRB), Ethiopia. The change in extreme indices were calculated based on daily precipitation and temperature data derived from the most recent CMIP5 climate projection scenarios compared to the reference period (1971-2000). The raw outputs from the climate models were corrected in order to reduce biases using distribution based quantile mapping technique. The validation and calibration demonstrates that the seasonal maximum cycle of precipitation and temperature in the reference period is reproduced reasonably in the bias-corrected climate results. The projections of climate change impacts on the hydrological extremes were evaluated using three generalize lump conceptual hydrological models: GR4J; HBV; HMETS and two objective functions: NSE and LogNSE. These hydrological models are calibrated in the period of 1971-2000 and validated in 2001-2010 period. The optimized hydrological parameters are adopted for the future hydrological simulations (2071-2100). The results indicate that the wet/dry spell will significantly decrease/increase due to climate change in some sites of the region (sites 2 and 6), while in sites 1 and 5, there is increase/decrease in wet/dry spell but not significant, respectively. Far future maximum river flows in the study area will be less extreme and more variable in terms of magnitude, and more irregular in terms of seasonal occurrence, than they are at present. Low flows may become less extreme and variable in terms of magnitude, and more irregular in terms of seasonal occurrence. Most areas of northern, western, and northeastern part of the river basin will likely become wetter in the wet season and drier in dry season. Whereas, indices that related to low flows are projected to increase most prominently for lowland sites, due to the combined effects of projected decreases in summer precipitation, and projected increases in evapotranspiration that reduce residual soil moisture in late summer. Whereas the indices related to the high flows are projected slight increase in the central and upper part of the basin. According to the evaluated scenarios, climate change may have favorable impacts on the distribution of hydrological extremes in the study area.