Agricultural Drought Assessment for Upper Blue Nile Basin, Ethiopia using SWAT

Ms. Rahel Sintayehu Tessema

Abstract

Drought is an insidious natural hazard characterized by lower than expected or lower than normal precipitation that, when extended over a season or longer period of time. Farming communities in the Ethiopia and around the world lose a lot of money every year due to drought. El Niño and southern oscillation based early warning system is relatively a recent approach used by the government agencies to assess and respond to drought in Ethiopia. It is based on weather data with out taking in account topography, soil characteristics and land use. However, agricultural drought depends on soil moisture, evapotranspiration deficits. Hence, two drought indices, the soil moisture deficit index (SMDI) and evapotranspiration deficit index (ETDI), were developed in this study based on soil moisture deficits and evapotranspiration, respectively. A geographic information system (GIS) based approach was used to simulate the hydrology using soil and landuse properties at hydrological response unit level.

The soil and water Assessment tool (SWAT) was used to simulate the long-term hydrology of Upper Blue Nile basin. Using global data from 1971 – 1994 and observed data from 95 – 2005, long-term monthly normal soil moisture and evapotranspiration were estimated. This long – term monthly normal soil moisture and evapotranpiration data were used to calculate ETDI and SMDI for 295 hydraulic response units in the watershed. Analysis of the data showed that SMDI compared well with barley yield (r > 0.72) in six zones of Amhara region during the months of critical crop growth stages. Maize, Teff and sorghum crop yields were highly correlated with ETDI (0.25 < r < 0.94) during active crop growing season but barley yield correlated negatively. In addition to major crop yield, the two indices were validated using the number of people affected by drought and other study result using remote sensing technique. The above results suggest that the indices could be a good indicator of agricultural drought and useful in giving the spatial distribution and extent of drought.

Keywords: Agricultural drought, SWAT, drought indices, hydraulic response units, remote sensing, El Niño and southern oscillation, spatial distribution