A New Index for Drought Monitoring: Precipitation Evapotranspiration Difference Condition Index

This paper introduces a new climatic drought index: the precipitation evapotranspiration difference condition index (PEDCI). The PEDCI is based on the difference between precipitation and potential evapotranspiration, which reflects the climatic water balance. The accumulation of moisture deficits or surpluses at different time scales can reflect the drought conditions at these time scales. The PEDCI is similar to the standardized precipitation evapotranspiration index (SPEI), but mathematically, a normalization method is used for the PEDCI calculation, while a log-logistic probability distribution is used for the SPEI calculation. One advantage of the PEDCI as compared to the SPEI is that it does not require any empirical parameters. Time series of these two indices at different time scales over the United States for the period 1980 to 2010 were compared. Correlations between the drought indices and soil moisture, and correlations between the drought indices and net primary production (NPP) were also used for the comparison of these two indices. Our results indicate that the PEDCI is more representative of soil moisture and NPP conditions than the SPEI.