The National Weather Service is now using an innovative, experimental drought tool developed by CIRES and partners to support long-term weather forecasts. 

The Evaporative Demand Drought Index (EDDI) measures how ‘thirsty’ Earth’s atmosphere is, or how readily it sucks up moisture from plants, soil, and bodies of water. CIRES and NOAA researchers in the Physical Sciences Laboratory developed EDDI as a drought monitoring and early warning tool. EDDI has been transitioned to the National Weather Service’s Climate Prediction Center (CPC), which is responsible for issuing multi-week and seasonal climate outlooks for temperature, precipitation, and drought.

Forecasters can use EDDI to identify rapidly evolving flash droughts developing over a few weeks and monitor sustained droughts that develop over months and last up to years for agricultural and water provider communities. 

Mark Svoboda, director of the National Drought Mitigation Center at the University of Nebraska-Lincoln, said they have come to rely on the index as an integral part of the suite of products informing their approach to drought monitoring. “We find it of particular importance in our ongoing monitoring of ecological/agricultural drought conditions and for early warning of flash droughts,” Svoboda said. 

What sets EDDI apart from other drought indicators is that it takes into account the strong physical relationship between two factors. The first is evaporative demand—the "thirst of the atmosphere," estimated to be the amount of water that would evaporate from the land surface and be given off as water vapor by plants if the soil were well watered. The second is the actual loss of available moisture from the land surface through evapotranspiration—the sum of plant transpiration and evaporation from the Earth's land and ocean surface to the atmosphere. EDDI measures drought by using daily atmospheric conditions and estimating their impact on land-surface moisture and vice versa.

Two main advantages of EDDI over other drought indices are that it is independent of precipitation and it can be decomposed to identify the role individual evaporative drivers have on drought onset and persistence. At short timescales, EDDI often indicates drought onset well in advance of other tools. Forecasters could also use it for fire weather forecasting. 

“We developed EDDI to add an important dynamic to the tools used to monitor drought; what we didn’t anticipate is how well it would do in providing early warning of drought and flash drought, and how excited the wildfire community would be to integrate it into their decision-making tools,” said Mike Hobbins, a CIRES researcher in NOAA’s Physical Sciences Laboratory who helped develop EDDI.