| dc.description.abstract | The community′s livelihood in the Awash River basin (ARB) is mainly reliant on rainfall-dependent agriculture. Effects of rainfall anomalies such as reduction of agricultural productivity, water scarcity, and food insecurity are becoming more prevalent in this area. In recent years, the ARB experienced more frequent and intense spatio-temporal rainfall anomalies, which make the shift and trend analyses of rainfall associated with sea surface temperature crucial for providing guidance to improve food security. Change-point detection tests (e.g., the von Neumann ratio, Pettit′s, the Buishand′s Range (BR), and the Standard Normal Homogeneity (SNH)), and the Mann-Kendall trend-test analysis (M-K) of temperature and rainfall were carried out for the ARB from 29 meteorological stations during the period 1986-2016. The frequent drought reoccurrences were also characterized using five drought indices, including the Water Storage Deficit Index (WSDI), the Evaporative Stress Index (ESI), the Standardized Precipitation Evapotranspiration Index (SPEI), the Standardized Precipitation Index (SPI), and the Evaporative Demand Drought Index (EDDI) during the period 2002-2017. Moreover, the impact of climate change on the hydrology of upstream ARB was assessed by Hydrologiska Byråns Vattenbalans-avdelning (HBV) model using six scenarios during 2021-2050 and 2071-2100.
The M-K trend test over the entire ARB showed a significant increasing trend in annual and seasonal temperature. Temperature change-points for the major rainy season (MRS) (June-September) and the minor rainy season (mRS) (February-May) were detected in 2001/02 and 1997/98, respectively. For rainfall, the downstream part of the ARB experienced high variability, significant decreasing trend, and shift in mean values. The BR and SNH test results showed that the mRS rainfall change-point was around 1997/98, with a subsequent annual decrease of 52.5 mm/yr. The increase (decrease) of rainfall in the annual and MRS periods is attributed to effects of the La Niña (El Niño) events. The significant decreasing trend and change-point of rainfall in the mRS is associated to the steady warming of the Indian and Atlantic Oceans, local warming, and the La Niña events, which explained that rainfall anomalies in the ARB are posing a serious challenge to agriculture productions in the area. It is therefore essential that appropriate integrated water management and water-harvesting technologies should be established in the ARB, especially in the downstream areas.
The M-K test from 2002 to 2017 detected a significant increase in annual and seasonal terrestrial water storage (TWS), which is advantageous to irrigation management and other increasing economic activities in the basin. Based on water storage deficit (WSD), the most severe drought that lasted for 15 months was detected around 2005/01-2006/03 with a total WSD of -411.8 mm with a peak deficit of -46.24 mm in 2005/03, indicating an extreme shortage of TWS in the basin. Persistent droughts were identified in 2002, 2008-2009, 2012-2013, and 2015 by SPI, ESI, SPEI, and EDDI, showing less intensified drought after 2009. In addition to the conventional drought, flash drought was also examined in ARB using ESI and EDDI indices. Results showed agricultural/grass-lands, vegetation, and irrigational cropland areas were prone to flash drought in ARB. The extent of flash drought detected by the EDDI was more extensive as compared to those by the ESI due to soil moisture and vegetation coverages were considered in the ESI. The downstream part was found to be highly susceptible to flash drought, especially in the MRS and the last two months of mRS. The EDDI can early detect the start of the flash drought, which can be used as an early warning precursor to support the planning of adaption measures to reduce agricultural crop losses and drought-related risks in the basin. Overall, the 3- and 6-monthly drought indices can best predict the onset and severity of meteorological and agricultural droughts in ARB.
Considering impacts of climate change, the upstream ARB (MK subbasin) is highly vulnerable due to high population density, diverse ecology, and mainly rainfed agriculture. The projected increase in temperature under all GCMs considered in this study will increase the evapotranspiration that induces more water loss in MK subbasin. Nevertheless, the risk of crop chilling damage will be reduced as the projected increase in minimum temperature. The annual and MRS rainfall and streamflow are projected to increase by all GCMs, excluding ECHAM-A2. Under RCP8.5 scenarios, annual rainfall (streamflow) is expected to increase by 38% (23%) and 57% (49%) during 2021-2050 and 2071-2100, respectively. The projected streamflow increase by most of the GCMs may increase flood risk mainly in August, while the streamflow decrease by ECHAM-A2 will exacerbate the existing water shortage, especially in the mRS. Overall, water harvesting during the MRS would be vital to minimizing the adverse effects. These findings will help the community and water managers of the subbasin to establish suitable adaptation measures for viable water resources management | en_US |