Evaporative Demand Drought Index

The Evaporative Demand Drought Index (EDDI) exploits the strong physical relationship between evaporative demand (E0) and actual loss of water from the land surface through evapotranspiration. E0 is the “thirst of the atmosphere,” estimated by the amount of water that would evaporate from the soil and be transpired by plants if the soil were well watered. EDDI measures the signal of drought using information on the rapidly evolving (daily) conditions of the atmosphere to estimate their impact on land-surface moisture, and vice versa. EDDI’s effectiveness in reflecting the moisture conditions on the land surface is based on feedbacks between the atmosphere and land that are particularly strong during the warm season, when drought is of greatest concern. See the EDDI User Guide for more information.

The data used to generate these maps come from the North American Land Data Assimilation System Phase-2 (NLDAS-2) project, which assimilates observations of temperature, wind speed, radiation, and vapor pressure deficit. The date indicates the last day of the period of record, and the week number indicates the window size for the period of record.

The drought category shown (D0, D1, D2, D3, D4) is a function of the depth of reference evapotranspiration accumulated over a given period of record (4-week, 8-week, 12-week) with respect to a climatology of 1980-2021. The drought categories are consistent with the US Drought Monitor's Percentile Ranking Scheme.