Disentangling the impacts of climate change, land use change and irrigation on the Central Rift Valley water system of Ethiopia

The Central Rift Valley (CRV) of Ethiopia is a closed basin where claims on land and water have strongly increased over the past decade resulting in over-exploitation of the resources: a clear symptom is the declining trend in the water level of the terminal Lake Abyata. In this paper, we quantify the plausible recent impacts of climate change, land use change and irrigation water abstraction on water availability of Lake Abyata. We examined trends in lake levels, river discharges, basin rainfall, temperature and irrigation development (ca. 1975–2008), and computed the additional evapotranspiration loss resulting from temperature change and irrigated land. We also analysed land use change (1990–2007) and estimated the subsequent change in surface runoff. Temperature has increased linearly over 34 years (p<0.001) whereas rainfall has not changed significantly. Consequently, increased evapotranspiration consumed 62 and 145Mm3 of additional water from lakes and land surface, respectively, during 1990–2007. Furthermore, an estimated 285Mm3yr−1 of water was abstracted for irrigation in 2009 of which approximately 170Mm3yr−1 is irrecoverable evapotranspiration loss. In addition, surface runoff has increased in the upper, and decreased in lower sub-basins of the CRV associated with extensive land use change (1990–2007). However, insight in the impact of the net increase in runoff of 260Mm3yr−1 on the water availability for Lake Abyata remains partial because of data and methodological limitations. We conclude that the potential for agricultural intensification and its hydrological implications should be considered jointly to prevent further deteriorating Lake Abyata.