Testing two different precipitation datasets to compute the standardized precipitation index over the Horn of Africa*

Abstract

Severe droughts are a frequent problem for the Horn of Africa. In this article, we test the possibility of using the standardized precipitation index (SPI) for monitoring drought conditions in this region. The SPI is a statistical indicator evaluating the lack or surplus of precipitation over different time scales, thus allowing the time-related impacts of the moisture deficit to be distinguished. The SPI is calculated as a function of long-term average precipitation, using continuous, long-term series of historical accumulated monthly precipitation records. The SPI for a given rainfall amount is then given in units of standard deviation from the mean of an equivalent normally distributed probability distribution function. As a consequence, wetter and drier climates and periods can be represented and monitored in the same way. For this study, the 3, 6, 9 and 12 month SPI values have been calculated for the period from 1985 to 2008 over the Horn of Africa using Global Precipitation Climatology Centre (GPCC) data and European Centre for Medium Range Weather Forecasting (ECMWF) ERA-40 reanalysis data interpolated to a 0.5° grid. Both SPI datasets have been compared for selected time periods by means of a root-mean-square-error, correlation-coefficient and contingency-table approach. First results demonstrate the feasibility of using the SPI for drought monitoring at regional scales over Africa, but further investigation is needed in order to identify the most suitable precipitation dataset for the SPI computation over Africa.