The Evaporative Demand Drought Index. Part II: CONUS-Wide Assessment against Common Drought Indicators

Abstract

Precipitation, soil moisture, and air temperature are the most commonly used climate variables to monitor drought; however, other climatic factors such as solar radiation, wind speed, and humidity can be important drivers in the depletion of soil moisture and evolution and persistence of drought. This work assesses the Evaporative Demand Drought Index (EDDI) at multiple time scales for several hydroclimates as the second part of a two-part study. EDDI and individual evaporative demand components were examined as they relate to the dynamic evolution of flash drought over the central United States, characterization of hydrologic drought over the western United States, and comparison to commonly used drought metrics of the U.S. Drought Monitor (USDM), Standardized Precipitation Index (SPI), Standardized Soil Moisture Index (SSI), and the evaporative stress index (ESI). Two main advantages of EDDI over other drought indices are that it is independent of precipitation (similar to ESI) and it can be decomposed to identify the role individual evaporative drivers have on drought onset and persistence. At short time scales, spatial distributions and time series results illustrate that EDDI often indicates drought onset well in advance of the USDM, SPI, and SSI. Results illustrate the benefits of physically based evaporative demand estimates and demonstrate EDDI’s utility and effectiveness in an easy-to-implement agricultural early warning and long-term hydrologic drought–monitoring tool with potential applications in seasonal forecasting and fire-weather monitoring.

Historical Drought Assessment Over the Contiguous United States Using the Generalized Complementary Principle of Evapotranspiration

“…Considering the importance of proactive disaster managements (Wilhite et al, ), monitoring early‐stage droughts is crucial to prepare for disastrous droughts that can follow. It has been shown that some drought events in the CONUS were led by earlier above‐normal ET p (Hobbins et al, ; McEvoy et al, ), thus emphasizing the importance of tracking the heat wave‐driven droughts. On the contrary to SEDI, a precipitation‐based drought index such as SPI and SPEI is unlikely able to detect the flash drought.…”

“…SEDI can thus supplement this weakness of precipitation‐based indices. As an alternative, it may be possible to monitor ET p for the heat wave droughts as in McEvoy et al (); nonetheless, in this case, the nonlinearity between ET p and ET a might hinder a proper indication of water deficit‐driven droughts.…”

“…In remote sensing of drought, ET has been a common indicator (AghaKouchak et al, ; Anderson et al, ; Mu et al, ; Otkin et al, ; Yao et al, ). Several indices solely depending on the potential and/or actual ETs were proposed and evaluated in drought characterization studies (e.g., Hobbins et al, ; Kim & Rhee, ; McEvoy et al, ; Vicente‐Serrano et al, ). The potential ET was combined with a precipitation‐based drought index (e.g., Beguería et al, ; Vicente‐Serrano et al, ).…”

Regional characteristics of flash droughts across the United States

“…Otkin et al (2013) initially examined flash droughts via the satellite-derived Evaporative Stress Index (ESI) (Anderson et al 2007a(Anderson et al , 2007b. Subsequently, the Rapid Change Index (RCI) was derived from temporal changes in ESI and used to explore the relationship between rapid drought development and changes in evaporative stress (Otkin et al 2014(Otkin et al , 2015 Similarly, Hobbins et al (2016) and McEvoy et al (2016) leveraged atmospheric demand via the Evaporative Demand Drought Index (EDDI) to monitor the development of flash drought while Ford et al (2015) examined the utility of soil moisture for monitoring flash drought. Each of these methods provides one to two weeks of lead time with respect to drought impacts on the environment (i.e., significant reduction of soil moisture, stress on vegetation and agriculture, etc).…”

<strong>A proposed robust approach for calculating the Standardized Evapotranspiration Deficit Index (SEDI) at the global scale</strong>

“…In fact, it has been suggested that the AED may be the single most useful variable to quantify drought severity [3]. Accordingly, drought indices based only on AED have been recently formulated under the premise that AED anomalies are strongly connected with precipitation, soil moisture and actual evapotranspiration (ET) anomalies [4,5].…”