Monitoring Drought with a High-Resolution Standardized Precipitation Evapotranspiration Index

The NC State Climate Office (NCSCO) has generated the Standardized Precipitation Evapotranspiration Index (SPEI) over the contiguous US for use in operational drought monitoring. SPEI is based on the climate balance between precipitation and potential evapotranspiration (P-PET). Observed sums of P-PET are related to their historical probability of occurrence and mapped to a standard normal distribution, resulting in the SPEI, which has a mean of zero and a standard deviation of one.  Positive SPEI indicate wetter conditions while negative SPEI indicate drier conditions.  SPEI has been calculated for 14 time scales (1 - 9, 12, 15, 18, 24, and 36 months) and is updated daily to reflect the most recent conditions.  High-resolution Advanced Hydrologic Prediction Service (AHPS) precipitation and Parameter-elevation Relationships on Independent Slopes Model (PRISM) temperature data are used to calculate SPEI, giving the resulting drought index an approximately 4.6km x 4.6km resolution.  This SPEI therefore has the ability to capture localized, sub-county drought conditions across a variety of time scales.  Comparisons with drought indicators (soil moisture, streamflow, and groundwater) over North Carolina, as well as other drought indices over the contiguous US, reveal this SPEI to be a valuable addition to the drought monitoring toolbox.  Maps of this SPEI are made publicly available for viewing and downloading via a website developed by the NCSCO.