EDDI examines the current California drought: early warning and monitoring of agricultural and hydrologic drought with the new Evaporative Demand Drought Index

We have developed a physically based, multi-scalar drought index—the Evaporative Demand Drought Index (EDDI) to improve the treatment of evaporative dynamics in drought monitoring. Existing popular drought indices—such as the Palmer Drought Severity Index that informs much of the US Drought Monitor (USDM)—have primarily relying on precipitation and temperature (T) to represent hydroclimatic anomalies, leaving evaporative demand (Eo) most often derived from poorly performing T-based parameterizations then used to derive actual evapotranspiration (ET) from LSMs. Instead, EDDI leverages the inter-relations of Eo and ET, measuring Eo's physical response to surface drying anomalies due to two distinct land surface/atmosphere interactions: (i) in sustained drought, limited moisture availability forces Eo and ET into a complementary relationship whereby ET declines as Eo increases; and (ii) in “flash” droughts, Eo increases due to increasing advection or radiation. Eo's rise in response to both drought types suggests EDDI's robustness as a monitor and leading indicator of drought. To drive EDDI, we use for Eo daily reference ET from the ASCE Standardized Reference ET equation forced by North American Land Data Assimilation System drivers. EDDI is derived by aggregating Eo anomalies from its long-term mean across a period of interest and normalizing them to a Z-score. Positive EDDI indicates drier than normal conditions (and so drought). We use the current historic California drought as a test-case in which to examine EDDI's performance in monitoring agricultural and hydrologic drought. We observe drought development and decompose the behavior of drought's evaporative drivers during in-drought intensification periods and wetting events. EDDI's performance as a leading indicator with respect to the USDM is tested in important agricultural regions of California. Comparing streamflow from several USGS gauges in the Sierra Nevada to EDDI at various time scales, we find that EDDI tracks most major hydrologic droughts, with correlations to water-year streamflow that are highest at the 9- to 12-month aggregation periods, and during the summer. EDDI shows significant promise as a leading indicator of drought as measured by USDM, thereby providing a valuable planning window for growers and water resource managers.