Eddi a powerful Tool for Early Drought Warning Released December 2015 What is eddi?

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EDDI

A Powerful Tool for Early Drought Warning

Released December 2015

What is EDDI?

EDDI, which stands for Evaporative Demand Drought Index, is 

a drought index that can serve as an indicator of both rapidly 

evolving “flash” droughts (developing over a few weeks) and 

sustained droughts (developing over months but lasting up 

to years).

Why use EDDI?

EDDI has been shown to offer early warning of drought stress 

relative to current operational drought indicators, such as 

the US Drought Monitor (USDM) (see Figure 1). A particular 

strength of EDDI is in capturing the precursor signals of water 

stress at weekly to monthly timescales, which makes EDDI a 

potent tool for drought preparedness at those timescales. 

EDDI also uses the same classification scheme as the USDM 

to define drought conditions, so it is easy to read EDDI maps.

Does EDDI work in real time?

Yes. At present, EDDI is generated every week by analyzing 

a real-time atmospheric dataset. There is also an ongoing 

effort to forecast EDDI based on seasonal climate-forecast 

information.

What is the physical basis for EDDI?

EDDI exploits the strong physical relationship between 

evaporative demand (

E

0

) and actual loss of water from the 

land surface through evapotranspiration. 

E

0

 is the “thirst of the 

atmosphere,” estimated by the amount of water that would 

evaporate from the soil and be transpired by plants if the 

soil were well watered. EDDI measures the signal of drought 

using information on the rapidly evolving (daily) conditions 

of the atmosphere to estimate their impact on land-surface 

moisture, and vice versa. EDDI’s effectiveness in reflecting the 

moisture conditions on the land surface is based on feedbacks 

between the atmosphere and land that are particularly strong 

during the warm season, when drought is of greatest concern.

EDDI is sensitive to two distinct land-surface atmosphere 

interactions: (i) increased 

E

0

 drives increased evapo-

Figure 1

Development of a flash drought in the Midwest in 2012. The 

2-week EDDI (right) is compared at 5-week intervals to the 

US Drought Monitor (USDM) (left). EDDI captures the severe 

drought condition two months ahead of the USDM. Image: 

Mike Hobbins.

transpiration until the available soil moisture becomes 

limiting, potentially leading to flash droughts; and (ii) as 

surface water becomes increasingly scarce in sustained 

droughts, evapotranspiration declines, which leads to higher 

air temperature and lower humidity, and thereby increases 

E

0

.

Green River, Wyoming. Photo: K. Miller, USGS.

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EDDI

A Powerful Tool for Early Drought Warning

Is the calculation of EDDI sensitive 

to land-surface type? 

No, EDDI is based on relative changes in 

E

0

, so it 

is not sensitive to land-surface type and is a valid 

drought indicator for all regions.

How is EDDI calculated?

EDDI is a measure of the departure of 

E

0

 

aggregated across a time-window of interest 

relative to historical conditions (from a 30-

year climatology). 

E

0

 is calculated as reference 

evapotranspiration based on the FAO-56 

Penman-Monteith formulation. At each point in 

space, the rank of aggregated 

E

0

 relative to its 

climatology is converted to a percentile, which 

is then assigned to different drought categories 

– e.g., ED0, ED1, etc. – that are equivalent to the 

categories used by the USDM. The time-window 

of interest could be specific weeks or months in 

any given year, up to the present. For example, a 

2-week EDDI will aggregate two weeks of 

E

0

 and 

estimate its departure relative to the historical 

aggregation of 

E

0

 for those two weeks. Figure 2 

shows an example of 2-week EDDI for the Wind 

River Indian Reservation in central Wyoming 

between May 26, 2015, and September 29, 2015.

What time-window information on 

EDDI is most appropriate?

The optimal time-window for EDDI will be user- 

and sector-dependent. For example, an irrigator 

may be interested in short-term EDDI – say 

across a 2-week window – to track and respond 

to weather-scale changes, whereas a reservoir 

operator more interested in interseasonal 

variations in snowpack may find more utility 

in a 6-month EDDI that examines the behavior 

across the snow accumulation and snowmelt 

periods. Given the natural and physical linkages 

between 

E

0

 and wildfire risk, work is ongoing to 

establish the optimal timescales for fire-weather 

prediction. Indeed, the US Forest Service Rocky 

Mountain Research Station uses 1-month EDDI 

in their seasonal forecasting of the numbers 

of large fires and of fire-suppression costs. For 

other cases, consideration of EDDI information 

at multiple timescales would be useful.

Where can I get EDDI data and maps?

EDDI is an experimental product. To receive 

EDDI data and maps, please contact Mike 

Hobbins (

[e-mail]

). Currently, 

EDDI maps for the Rocky Mountain region are 

being presented in the weekly NIDIS Upper Colorado River Basin Drought 

and Water Assessment (

http://climate.colostate.edu/~drought

). In the near 

future, we hope to make EDDI data available through the WWA Climate 

Dashboards and, looking further out, to have EDDI data distributed 

nationally by the National Weather Service.

Figure 2

Drought development in the Wind River Indian Reservation across the 

irrigation season, as observed by a 2-week EDDI at 2-week intervals. Drying 

did not appear in the US Drought Monitor until September 29 (blue-outlined 

map at bottom right). Image: Mike Hobbins.

References

Hobbins MT, Wood A, McEvoy D, Huntington J, Morton C, Anderson M, Hain C, and 

Verdin, The Evaporative Demand Drought Index: Part I - Linking drought evolution 

to variations in evaporative demand. Journal of Hydrometeorology (In revision).

McEvoy D, Huntington J, Hobbins MT, Wood A, Morton C, Anderson M, Hain C, and 

Verdin J, The Evaporative Demand Drought Index: Part II - CONUS-wide assessment 

against common drought indicators. Journal of Hydrometeorology (In revision).

Hobbins, MT (2014), Measuring the Atmosphere’s Thirst. Dry Times: National 

Integrated Drought Information System Newsletter 4 (Apr. 2014): 14-15.

http://www.drought.gov/media/pgfiles/NIDIS-Newsletter-April-2014.pdf

Authors

Imtiaz Rangwala

1,2,3,4

(

[e-mail]

), Mike Hobbins

1,2,5

, 

Joe Barsugli

1,2,3,4 

and Candida Dewes

1,2,3,4

1 Cooperative Institute for Research in Environmental Sciences (CIRES), 2 NOAA 

ESRL Physical Sciences Division (PSD), 3 North Central Climate Science Center 

(NCCSC), 4 Western Water Assessment (WWA), 5 National Integrated Drought 

Information System (NIDIS)

Acknowledgements

Ami Nacu-Schmidt (CIRES/WWA) for design and layout of this document.