Comparison and analysis of drought indices SPI and SPEI for Belgrade (Serbia)

Abstract

Quantifying the intensity of a drought is not an easy task, as one has to take intoaccount multiple parameters, for example rainfall, temperature, soil water content, etc.Numerous indices have been developed to tackle this issue, and some of the most widelyused are standard precipitation index (SPI) and standard precipitation evapotranspiration index(SPEI), which, as the name implies, standardize precipitation and evapotranspiration over apoint for a certain time period and use that as a reference for an estimate of available water.The goal of this work is to analyze these drought indices for a long-term time period,take a look at their correlation and compare them with available records. The theoreticaldifference between the two indices is in the fact that SPEI takes (potential) evapotranspirationinto account, while SPI does not. Since sufficient measured evapotranspiration data was notavailable, potential evapotranspiration was used, calculated by the Thorntwaite method(taking into account temperature and geographical position). Precipitation and temperaturedata was acquired from the station network ran by the Republic Hydrometeorological Serviceof Serbia, for the time period of 1961-2020. Since the SPI and SPEI can be computed fordifferent timescales, in this study the indices for 3-, 6- and 12-months were compared.The results show that despite high correlation between SPI and SPEI (r > 0.9), theirbehavior in regards to drought events shows an important difference. This is visible in thetemporal spread of the dry months. SPEI depicts the last two decades as much drier than SPIdoes, and inversely, the sixties and the seventies as much wetter. Also, for the whole timeperiod, SPEI shows a trend towards more dry conditions, while SPI shows no clear trend.This is a consequence of rising potential evapotranspiration, which in itself is a consequenceof rising average temperatures.The conclusion is that the indices should not be used interchangeably and withtemperature data being as available as precipitation data, SPEI is more representative ofwater budget in the area in question.