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

Quantifying the intensity of a drought is not an easy task, as one has to take into
account multiple parameters, for example rainfall, temperature, soil water content, etc.
Numerous indices have been developed to tackle this issue, and some of the most widely
used are standard precipitation index (SPI) and standard precipitation evapotranspiration index
(SPEI), which, as the name implies, standardize precipitation and evapotranspiration over a
point 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 theoretical
difference between the two indices is in the fact that SPEI takes (potential) evapotranspiration
into account, while SPI does not. Since sufficient measured evapotranspiration data was not
available, potential evapotranspiration was used, calculated by the Thorntwaite method
(taking into account temperature and geographical position). Precipitation and temperature
data was acquired from the station network ran by the Republic Hydrometeorological Service
of Serbia, for the time period of 1961-2020. Since the SPI and SPEI can be computed for
different 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), their
behavior in regards to drought events shows an important difference. This is visible in the
temporal spread of the dry months. SPEI depicts the last two decades as much drier than SPI
does, and inversely, the sixties and the seventies as much wetter. Also, for the whole time
period, 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 consequence
of rising average temperatures.
The conclusion is that the indices should not be used interchangeably and with
temperature data being as available as precipitation data, SPEI is more representative of
water budget in the area in question.