Developing a hybrid drought index: Precipitation Evapotranspiration Difference Condition Index

Drought indices are widely used for drought quantification. The objective of this study is to introduce a hybrid drought index, the Precipitation Evapotranspiration Difference Condition Index (PEDCI), and to compare its performance in Oklahoma to existing drought indices. The PEDCI is based on a simple water balance model, which accounts for the difference between water supply (precipitation) and water demand (potential evapotranspiration). While it is similar in this respect to the Standardized Precipitation and Evapotranspiration Index (SPEI) and the Palmer Drought Severity Index, it uses a different method to normalize the index in time and space which was inspired by Vegetation Condition Index. The performance of the PEDCI is evaluated quantitatively using SPEI, soil moisture data, and crop yield data. The performance of the PEDCI is also evaluated qualitatively using six criteria. Results show that the PEDCI is highly correlated with SPEI and that it can represent drought conditions in Oklahoma as effectively as the SPEI. Correlations between the PEDCI and soil moisture/crop yield also demonstrate good correspondence. The qualitative evaluation of PEDCI shows that there are two key strengths in comparison to the SPEI. First, it is simple to calculate and interpret. Second, the normalization method does not require any empirical parameters or fitting a probability distribution function. The PEDCI can be used to characterize drought conditions on a variety of timescales and it has utility for monitoring agricultural and meteorological drought, especially in a changing climate.