A scientific-based protocol for the calculation of two standardized drought indices using NASA POWER data

Abstract

The standardized precipitation (SPI) and standardized precipitation–evapotranspiration (SPEI) indices are widely used drought indices. However, two factors constrain their use in several regions on the planet. The first is the scarcity of weather stations capable of providing 30-year records for precipitation and evapotranspiration. The second is related to the regional climate conditions, which may prevent the proper calculation of the indices at specific time scales. To overcome these challenges, we developed a scientific-based protocol that helps users to properly calculate the indices using data from the NASA POWER project. The protocol evaluates how well NASA POWER data represents the ‘real-world’ conditions and if the index estimates meet all conceptual assumptions expected from standardized drought indices. The protocol was applied in a tropical/subtropical Brazilian region and was able to assist users in selecting suitable time scales for calculating each index. For the SPI, the range of suitable time scales varied from 2 to 5 months. For the SPEI, this range varied from 14 days to 8 months. The protocol also indicated that the generalized extreme value distribution outperformed the generalized logistic for calculating the SPEI, and the accuracy of NASA POWER data for estimating precipitation and evapotranspiration totals varied according to the time scale.