Using SPI and SPEI for baseline probabilities and seasonal drought prediction in two agricultural regions of the Western Cape, South Africa

Abstract

Drought is one of the most hazardous natural disasters in terms of the number of people directly affected. An important characteristic of drought is the  prolonged absence of rainfall relative to the long-term average. The intrinsic persistence of drought conditions continuing from one month to the next  can be utilized for drought monitoring and early warning systems. This study sought to better understand drought probabilities and baselines for two  agriculturally important rainfall regions in the Western Cape, South Africa – one with a distinct rainfall season and one which receives year-round rainfall.  The drought indices, Standardised Precipitation and Evapotranspiration Index (SPEI) and Standardised Precipitation Index (SPI), were assessed to obtain  predictive information and establish a set of baseline probabilities for drought. Two sets of synthetic time-series data were used (one where seasonality  was retained and one where seasonality was removed), along with observed data of monthly rainfall and minimum and maximum temperature. Based on  the inherent persistence characteristics, autocorrelation was used to obtain a probability density function of the future state of the various SPI start  and lead times. Optimal persistence was also established. The validity of the methodology was then examined by application to the recent Cape Town  drought (2015–2018). Results showed potential for this methodology to be applied in drought early warning systems and decision support tools for the  province.