Evaporative Demand Drought Index (EDDI): The Physical Basis.

Mike Hobbins (1,2), Daniel McEvoy (3), Justin Huntington (3,4), Andrew Wood (5), James Verdin (6)

Abstract
Many operational drought indices suffer from poor representation of evaporative dynamics, with precipitation and temperature (T) alone representing hydroclimatic anomalies: typical is the Palmer Drought Severity Index (PDSI) that lies at the heart of the US Drought Monitor (USDM). To address this shortcoming, we have created a simple drought index—the Evaporative Demand Drought Index (EDDI)—that both improves the representation of drought evaporative dynamics and acts as a leading indicator of both flash and sustained droughts. At its simplest, EDDI is estimated as standardized anomalies from a climatological mean evaporative demand (Eo) accumulated across a given time-window; positive EDDI values indicate drier than normal conditions. EDDI measures Eo’s response to surface drying anomalies resulting from two distinct land surface-atmosphere interactions: (1) ET and Eo vary in parallel due to energetic or advective increases before surface moisture limitations—as in “flash droughts”; and (2) a complementary relationship between Eo and ET results from moisture becoming limited at the land surface—as in sustained droughts. That Eo rises in response to both drought types suggests EDDI’s utility as a robust leading indicator of drought. This presentation outlines the physical basis for EDDI; summarizes different drought responses and how they combine to make EDDI a useful drought indicator; examines EDDI’s long-term performance against USDM in basins from across CONUS’s hydroclimatic spectrum; and demonstrates EDDI as a leading drought indicator.