Drought Monitoring over Africa using the Standardized Precipitation Index (SPI)

Abstract

The Standardised Precipitation Index (SPI) is a statistical indicator evaluating the lack or surplus of precipitation over different time scales, allowing distinguishing time-related impacts of the moisture deficit, for example on agricultural production (soil moisture), surface hydrology (run-off), groundwater, and economy. SPI is calculated as a function of the long-term average precipitation, using continuous, long-term series of historic accumulated monthly precipitation records. Since rainfall is not normally distributed for aggregation periods of less than 12 months a gamma distribution is fitted to the frequency distribution. The SPI for a given rainfall amount is then given, in units of standard deviation, by the precipitation deviation from the mean of an equivalent normally distributed probability distribution function with a zero mean and a standard deviation of one. For this study, the 3, 6, 9 and 12 months SPI has been calculated for the period from 1985 to 2008 over the Horn of Africa using GPCC and ERA-40 data. Both SPI datasets have been compared for selected time periods by means of root mean square error and correlation coefficient approach demonstrating possibilities of SPI for drought monitoring at regional scales over Africa.JRC.DDG.H.7-Land management and natural hazard