LONG-TERM IMPACTS OF CLIMATE AND LAND USE CHANGE ON WATER AVAILABILITY OF TWO WATERSHEDS WITHIN THE UPPER BLUE NILE BASIN, ETHIOPIA

Abstract

The Upper Blue Nile Basin (UBNB) is undergoing significant land use change (LUC) due to agricultural expansion and deforestation. LUCs have implications for water resources in the region. Moreover, the UBNB is vulnerable to climate change (CC), which alters the hydrological regime and affects rainfall-dependent agriculture, increasing the risk of food insecurity during drought. The HBV-light and AquaCrop models were employed in two watersheds (Gilgel Abbay and Birr) in the UBNB to predict the impacts of these changes on water availability. The simulations were applied under baseline climate (1970 – 1999) and compared to a CC scenario (2070 – 2100). The parameters of LUC scenarios used in the HBVLight were based on calibrated and validated water balance components of prominent historical trends in LUC (eucalyptus plantations and restoration of natural forests and wetlands). The HBV model's results were then analysed to calculate the water availability indicator, the ratio (0 to 1) of actual to potential evapotranspiration (AET/PET); a higher ratio value indicates more water availability. In Gilgel Abbay, the ratio of AET/PET increased by 5.8% in CC with restoration LUC scenario compared to the baseline, and the number of days where the AET reached maximum PET (Ratio of >0.9) was higher by 137%. Both CC without LUC and with eucalyptus plantation had a reduction in the ratio of AET/PET, and the number of days (>0.9) decreased by -70% and 67%, respectively. In Birr, the CC without LUC had the highest increase in the number of days with >0.9 ratio, and the increase was by only 3%, while in restoration LUC, there was a decrease by -5%, and the most significant reduction was in the eucalyptus plantation scenario by -14%. It is worth noting that Birr generally did not have water stress since, in all scenarios, including the baseline, the ratio was over 0.9. On the other hand, the AquaCrop was used to predict how commonly cultivated crop yields (Teff and Barley) in the UBNB are affected by the projected CC. The results showed that water is not limiting for the investigated crops. Instead, the yields are predicted to increase due to the increase in rainfall and CO2 fertilisation during the growing season of those crops. The results of this paper indicate that water availability in the investigated watersheds is significantly affected by LUC at the watershed scale. The applied land-use change scenarios can help inform future land management decisions to ensure water security, such as consideration of plans to expand the eucalyptus plantation.