Monitoring the California Drought with a novel drought index: the Evaporative Demand Drought Index (EDDI).

Tuesday, April 21, 2015
Mike Hobbins1,2, Daniel McEvoy3,4, Justin L Huntington3, Andrew W Wood5 and James P Verdin6, (1)National Integrated Drought Information System, Boulder, CO, United States, (2)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (3)Desert Research Institute, Reno, NV, United States, (4)University of Nevada Reno, Reno, NV, United States, (5)National Center for Atmospheric Research, Boulder, CO, United States, (6)USGS/EROS, Boulder, CO, United States
Abstract:
We have developed a physically based, multi-scalar drought index—the Evaporative Demand Drought Index, or EDDI—that leverages the inter-relations of evaporative demand (E0) and actual evapotranspiration (ET) to improve the treatment of evaporative dynamics in drought monitoring and offer drought early warning.

EDDI measures E0’s physical response to surface drying anomalies due to two distinct land surface/atmosphere interactions. In sustained drought, limited moisture availability forces E0 and ET into a complementary relationship whereby ET declines as E0 increases, due to the energy balance tipping to favor sensible heating. In “flash” droughts, increasing advection, radiation, and/or temperature or decreasing humidity increases E0. That E0 rises in response to both drought types suggests EDDI’s robustness as a leading indicator of drought.

To drive EDDI, we use for E0 daily reference ET from the ASCE Standardized Reference ET equation forced by NLDAS drivers (temperature, specific humidity, downwards shortwave radiation, and wind speed). EDDI is derived by aggregating E0 anomalies from its long-term mean across a given window and normalizing them to a Z-score. Positive EDDI then indicates drier than normal conditions.

Here, we summarize EDDI’s physical basis and decompose the evaporative drivers of the current CA drought. We assess EDDI’s performance as a drought monitor here, observing the development of the CA drought, including recent wetting events. Initial results show that while the current CA drought originates in Prcp deficits, EDDI reports a more-intense drought than do Prcp-based indices, in line with the USDM—indicating that E0 and its drivers can exacerbate ongoing drought. EDDI shows significant promise as a leading indicator of CA drought as measured by USDM (e.g., in the Russian River above Ukiah, the drought signal from 2- to 3-week EDDI leads the USDM by 10 to 12 weeks), thereby providing a valuable planning window for water resource managers and growers. Across the west, longer lead-times are available at longer aggregation periods (e.g., the 2- to 4-month lead-time for 10- to 12-month EDDI in the snowmelt-dominated Upper Colorado basin), indicating the need for hydroclimatically appropriate optimization of the EDDI aggregation window.

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