The Evaporative Demand Drought Index. Part I: Linking Drought Evolution to Variations in Evaporative Demand

Abstract

Many operational drought indices focus primarily on precipitation and temperature when depicting hydroclimatic anomalies, and this perspective can be augmented by analyses and products that reflect the evaporative dynamics of drought. The linkage between atmospheric evaporative demand E0 and actual evapotranspiration (ET) is leveraged in a new drought index based solely on E0—the Evaporative Demand Drought Index (EDDI). EDDI measures the signal of drought through the response of E0 to surface drying anomalies that result from two distinct land surface–atmosphere interactions: 1) a complementary relationship between E0 and ET that develops under moisture limitations at the land surface, leading to ET declining and increasing E0, as in sustained droughts, and 2) parallel ET and E0 increases arising from increased energy availability that lead to surface moisture limitations, as in flash droughts. To calculate EDDI from E0, a long-term, daily reanalysis of reference ET estimated from the American Society of Civil Engineers (ASCE) standardized reference ET equation using radiation and meteorological variables from the North American Land Data Assimilation System phase 2 (NLDAS-2) is used. EDDI is obtained by deriving empirical probabilities of aggregated E0 depths relative to their climatologic means across a user-specific time period and normalizing these probabilities. Positive EDDI values then indicate drier-than-normal conditions and the potential for drought. EDDI is a physically based, multiscalar drought index that that can serve as an indicator of both flash and sustained droughts, in some hydroclimates offering early warning relative to current operational drought indices. The performance of EDDI is assessed against other commonly used drought metrics across CONUS in Part II.

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.…”

“…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 Climate Response Collaboratives: Multi-Institutional Support for Climate Resilience

“…Partnerships among the High Plains Regional Climate Center, NDMC, NIDIS, and NCCSC have enabled the coproduction of quarterly drought and climate summaries for WRR and the surrounding area (Wind River Indian Reservation Drought and Climate Summary). The partnership with WWA is supporting the testing of innovative drought tools such as the EDDI for the WRR (Hobbins et al 2016), and providing an overall evaluation of the project. The summaries and EDDI together provide the infrastructure for monitoring and early warning systems, and support decision-making on the ground.…”

The Variability of ASCE Standardized Reference Evapotranspiration: A Rigorous, CONUS-Wide Decomposition and Attribution

“…NOAA's ET r has been validated across western CONUS (Lewis et al, 2014) and the Texas High Plains (Moorhead et al, 2015), and while not yet deemed sufficiently accurate for the traditional use of ET r in field-scale irrigation scheduling, it nonetheless remains a highly leveraged product with uses across many other agricultural and water resource sectors. For example, it is used in irrigation support in agriculture away from agrometeorological stations; for water resource managers charged with estimating landscape-scale actual ET such as the U.S. Geological Survey's National Water Census for which it is under consideration (James Verdin, personal communication, 2015); as a climatology for the National Weather Service's (NWS) Forecast Reference ET product (NWS, 2015); and in multiple monitoring and prediction activities, including underpinning an agricultural drought index (the Evaporative Demand Drought Index, EDDI; Hobbins et al, 2016) and to support the U.S. Forest Service's seasonal forecasting of wildfire-suppression costs (Ham et al, 2014).…”