Abstract
The Upper Blue Nile (UBN) basin is less-explored in terms of drought studies as compared to other parts of Ethiopia and lacks a basin-specific drought monitoring system. This study compares six drought indices: Standardized Precipitation Index (SPI), Standardized Precipitation Evaporation Index (SPEI), Evapotranspiration Deficit Index (ETDI), Soil Moisture Deficit Index (SMDI), Aggregate Drought Index (ADI), and Standardized Runoff-discharge Index (SRI), and evaluates their performance with respect to identifying historic drought events in the UBN basin. The indices were calculated using monthly time series of observed precipitation, average temperature, river discharge, and modeled evapotranspiration and soil moisture from 1970 to 2010. The Pearson’s correlation coefficients between the six drought indices were analyzed. SPI and SPEI at 3-month aggregate period showed high correlation with ETDI and SMDI (r > 0.62), while SPI and SPEI at 12-month aggregate period correlate better with SRI. The performance of the six drought indices in identifying historic droughts: 1973–1974, 1983–1984, 1994–1995, and 2003–2004 was analyzed using data obtained from Emergency Events Database (EM-DAT) and previous studies. When drought onset dates indicated by the six drought indices are compared with that in the EM-DAT. SPI, and SPEI showed early onsets of drought events, except 2003–2004 drought for which the onset date was unavailable in EM-DAT. Similarly, ETDI, SMDI and SRI-3 showed early onset for two drought events and late onsets in one-drought event. In contrast, ADI showed late onsets for two drought events and early onset for one drought event. None of the six drought indices could individually identify the onsets of all the selected historic drought events; however, they may identify the onsets when combined by considering several input variables at different aggregate periods.
Highlights
Drought is abnormally dry condition sustained for a prolonged period
The spatial extent of SWAT based evapotranspiration and soil moisture were in hydrologic response units (HRUs)
The time series (1970–2010) of the drought indices were produced for all the stations considered
Summary
Drought is abnormally dry condition sustained for a prolonged period. Drought can be characterized by its severity, duration, and areal extent. In the past few decades, a number of devastating drought events have been occurred. In 2009, severe drought affected different parts of the globe, but more people were affected in eastern Africa than anywhere else. About 20 million people were estimated to have been suffering from food insecurity in eastern Africa [3]. The year 2009 was recorded as a severe drought year in Ethiopia; six million people were affected and needed food aid from international emergency services [3]. Other historic drought events have occurred in Ethiopia in the past few decades, most notably 2003/2004 and
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