To assess the spatial and temporal availability of blue and green water for up- and downstream stakeholders, the hydrological responses of the upper Blue Nile basin in the Ethiopian Highlands was modelled and analysed with newly generated input data, such as soil and land use maps. To consider variations in the seasonal climate, topography, soil, land use, and land management, the upper Blue Nile basin was modelled in seven major sub-basins. The modelling showed significant spatial and temporal differences in the hydrological responses of different sub-basins and years. The long-term mean annual drainage ratios of the watersheds range from <0.1 to >0.65, and the annual drainage ratio of one sub-basin can vary from 0.22 to 0.49. Steep slopes, shallow soils, and cultivated areas increase the drainage ratios due to high surface runoff, low soil moisture content, and a smaller share of evapotranspiration. Various climate change scenarios predict more precipitation, and land use change scenarios foresee a higher share of cultivated areas due to population growth. In view of these trends, results from our study suggest that drainage ratios will increase and more available blue water can be expected for downstream stakeholders.
