Drought monitoring and analysis: Standardised Precipitation Evapotranspiration Index (SPEI) and Standardised Precipitation Index (SPI)

Although rainfall is a major indicator of the availability of water, temperature is also an important factor that can influence the availability of water as it controls the rates of evapotranspiration. Parameters such as rainfall and temperature can be used as indicators of drought. These indicators are converted to drought indices which show the different characteristics of a drought. This study compares two different drought indices in the Kafue Basin located in northern Zambia, where most of the socioeconomic livelihoods are dependent on water. The Standardised Precipitation Index (SPI) depends on precipitation as the single input variable while the Standardised Precipitation Evapotranspiration Index (SPEI) is derived from precipitation and temperature in the form of a simple water balance. By comparing time series plots (1960-2015) of the two indices, both indices were able to pick up temporal variation of droughts. SPI and SPEI agreed (R > 0.5) on the direction of change but the effect on the drought condition was different. Compared to SPI, SPEI identified more droughts in the severe to moderate categories, with extended duration and increased intensity. On the other hand, SPI identified more droughts than SPEI under the extreme category, but with a shorter duration and reduced frequency of occurrence compared to the severe to moderate droughts. The results suggest that temperature variability plays an important role in characterising droughts. SPI is useful in that it only requires rainfall as input and especially where temperature data is missing. However, the use of SPI to characterise drought should be done with caution.