The Evaporative Demand Drought Index (EDDI): a New Drought Monitoring and Early Warning Tool

NOAA’s Physical Sciences Division (PSD) and the Desert Research Institute have developed a new, multi-scalar drought index—the Evaporative Demand Drought Index (EDDI)—to improve the treatment of evaporative dynamics in drought monitoring and to provide drought early warning. The EDDI is available in quasi-operational mode from PSD, and will be operationalized at the National Water Center through a NOAA Research Transitions Acceleration Program (RTAP) grant. Existing popular drought indices—such as the Palmer Drought Severity Index that informs much of the US Drought Monitor (USDM)—primarily rely on precipitation and temperature (T) to represent hydroclimatic anomalies. When evaporative demand (E₀) is considered—for example, to derive actual evapotranspiration (ET) from land surface models—it is often estimated from poorly performing, T-based parameterizations. Instead, EDDI leverages the interrelations of a fully physical E₀ and ET, measuring E₀’s physical response to surface drying anomalies due to land surface/atmosphere interactions in both sustained and “flash” droughts. EDDI shows significant promise as a leading indicator of drought as measured by USDM, providing a valuable planning window for land and water resource managers and agricultural producers.

Uniquely, EDDI are that it relies solely on readily available meteorological data while obviating the need for both precipitation and surface moisture data. Therefore, EDDI provides a perspective of drought from the atmospheric demand side that can provide a perspective previously missing from the “convergence of evidence” approach to drought monitoring. EDDI has demonstrated the most predictive skill in retrospective analyses for the onset of agricultural drought (low soil moisture) and hydrologic drought (low streamflow). Future research will examine the potential of EDDI in predicting wildfire weather risk. A compelling example of EDDI’s early warning potential is that it would have signaled the onset of the 2012 flash drought in the Midwest months ahead of the US Drought Monitor (see figure). Further, the physically based reference ET that EDDI uses as an E₀-estimator can be decomposed to attribute drought explicitly to its demand-side meteorological drivers.

The EDDI is currently available (e.g., ftp://ftp.cdc.noaa.gov/Public/mhobbins/EDDI/USDM/)  and is also disseminated via various portals such as the Western Water Assessment RISA’s Climate Dashboards, at western NIDIS Drought Early Warning Systems’ webpages, and, soon, on the U.S. Drought Portal. The current user base is expanding rapidly, and stakeholders are drawn from both operational and research communities (e.g., state climatologists, NWS Weather Forecast Offices, NIDIS DEWS, the USGS North Central Climate Science Center, and US Forest Service wildfire experts), who in turn serve land and water resource managers and agricultural producers.

EDDI is slated to be operationalized at the NOAA National Water Center by May 2019, as part of the RTAP program, for national dissemination to all NOAA stakeholders. The steps needed to achieve this goal include: refining and meeting user needs through a process of active engagement with stakeholders affected by agricultural and hydrologic droughts and fire weather, using integrated social science research, to inform the rollout program, and educating users as to EDDI’s promise and functionality; cooperating with scientists and users to verify the index across a wide variety of sectors, hydroclimates, and time scales; further extending the stakeholder base to include more WFOs and to include RFCs, other federal and state land and water management agencies, and to more closely integrate EDDI into the USDM; shortening the latency of EDDI products from five days to one, and increasing its spatial resolution from 12 to 4 km.

Future plans beyond this research-to-operations arc include using EDDI in wildfire prediction (first in CA, supported by a SARP grant); linking EDDI to operational seasonal forecasting of E₀; and extending EDDI globally, particularly to the Famine Early Warning Systems Network (FEWS NET) food-insecure countries.