Description of Maps and Underlying Data
The EDDI maps displayed here use atmospheric evaporative demand (E0) anomalies across a timescale of interest relative to its climatology to indicate the spatial extent and severity of drought. This page provides access to near-real-time maps (with a five-day latency due to the most recent information being five days old) depicting EDDI (EDDI Category Maps), changes in EDDI (EDDI Change Maps), and attribution of EDDI to individual meteorological drivers (EDDI Attribution Maps), with timescales that measure E0 anomalies across the 1 to 12 weeks and 1 to 12 months prior to the most current date.
The E0 is calculated using the FAO-56 Penman-Monteith reference evapotranspiration formulation (0.5-m tall reference crop), driven by data on near-surface temperature, humidity, wind speed, and incoming solar radiation, with these data extracted from the operational North American Land Data Assimilation System (NLDAS-2) dataset. For a particular time-window, EDDI is estimated by standardizing the E0 anomalies relative to the same accumulation time-window in a historical period of record (1980–2021), using a rank-based non-parametric method described in Hobbins et al. (2016; see the "Resources" tab). EDDI data are available at a ~12-km resolution (0.125° lat and long) across CONUS since January 1, 1980, and are updated daily.
On the EDDI Category Maps, colors indicate the frequency at which the observed E0 anomaly has occurred in the climatology, with warm colors indicating conditions that are drier than normal and cool colors indicating wetter-than-normal conditions. As an example, the ED4 category indicates that the current E0 anomaly has only been observed less than 2% of the time in the past 42 years (1980–2021), which represents the most severe drought conditions; the EW4 category means indicates that the anomaly has been exceeded 98% of the time, which represents the wettest conditions. For plotting purposes, EDDI values are binned into different percentile categories analogous to the US Drought Monitor plots—however, in case of EDDI plots, both drought and anomalously wet categories are shown.
On the EDDI Change Maps, the evolution in EDDI is depicted over the period indicated in the title. For instance, the "1-month EDDI: Changes during the 7 days ending on March 10, 2021" shows by how many categories 1-month EDDI has changed from March 3 to March 10. Brown colors indicate deepening drought conditions (say from D0 to D1, which would show as a category change of -1); green colors indicate ameliorating drought conditions (say from D2 to D1, which would show as a category change of 1). In each case, deeper shades indicate greater changes in EDDI across the period. Only a few change periods are shown for each EDDI category map. Note these maps only depict regions either starting or ending the change period in categories D0–D4; regions that are either not in drought during the change period or that leave drought during the change period are shown as blank.
On the EDDI Attribution Maps, the observed E0 anomaly is compared with the contributions from four near-surface meteorological drivers—temperature, humidity, wind speed, and incoming solar radiation—plus a closure error, to quantify their separate effects. The maps are provided for 7 timescales: 1-week, 2-week, 1-month, 2-month, 3-month, 6-month, and 12-month. They use a first-order linear decomposition where each driver's contribution is computed as the product of the anomaly in the driver and the sensitivity of E0 to that driver. The decomposition is first performed daily and then aggregated to the timescales of interest. This newly developed, user-requested tool provides the first-ever decomposition of atmospheric evaporative demand, and is described in Hobbins et al. (2026, in preparation).