Assessment of drought across central Sudan using UNEP dryness ratio

The objective of this paper is to improve our perception of drought in central Sudan. This has been realized by detailing the changes that occurred in the dryness ratio (rainfall/potential evapotranspiration) of the United Nations Environment Programme (UNEP) during the period 1941–2005. Eight representative stations of the area were selected and their dryness ratios examined on annual and monthly bases. A trend towards intensifying and more recurrent drought has been found. This trend is particularly significant in the arid areas. Statistically significant increase in areal coverage of drought has occurred. Substantial changes took place in the severity and frequency of drought between 1941–1970 and 1971–2005 over the whole area. Moreover, the early to mid-1970s, mid-1980s, early 1990s and early 2000s were determined as common drought years and were among the driest 10 years. The analysis also revealed high and generally increasing inter-annual variability of the dryness ratio. There is not a dominant short-term persistence in the data, as assessed by the lag-one serial correlation coefficient. Early forecast of the moisture condition during the rest of the season could merely be accomplished at small scale in the area. These results indicate that drought in the study area is irregular.