Both the Standardized Precipitation Evapotranspiration Index (SPEI) and Evaporative Demand Drought Index (EDDI) (Hobbins et al. 2016) use reference evapotranspiration (Ref. ET), based on the American Society of Civil Engineers (ASCE) Standardized Ref. ET equation, which, in turn, is driven by NCEP CDAS meteorological input variables. |
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