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.
