Online Drought Indicators and Indices

Evapotranspiration Deficit Index (ETDI)


Index name: Evapotranspiration Deficit Index (ETDI).

Ease of use: Red.

Origins: Developed from through research at the Texas Agricultural Experiment Station, United States, by Narasimhan and Srinivasan in 2004.

Characteristics: A weekly product that is helpful for identifying water stress for crops. ETDI is calculated along with the Soil Moisture Deficit Index (SMDI), in which a water stress ratio is calculated that compares actual evapotranspiration with reference crop evapotranspiration. The water stress ratio is then compared with the median calculated over a long-term period.

Input parameters: Modelled data from a hydrologic model with the Soil and Water Assessment Tool (SWAT) model are used initially to compute soil water in the root zone on a weekly basis.

Applications: Useful for identifying and monitoring short-term drought affecting agriculture.

Strengths: Analyses both actual and potential evapotranspiration and can identify wet and dry periods.  

Weaknesses: Calculations are based upon output from the SWAT model, but could be calculated if the appropriate inputs were available. The spatial variability of ETDI increases in the summer months during the period of greatest evapotranspiration and highly variable precipitation.

Resources: Calculations are provided and explained thoroughly in the reference below, along with correlation studies to other drought indices. Information on the SWAT model can be found at

References: Narasimhan, B. and R. Srinivasan, 2005: Development and evaluation of Soil Moisture Deficit Index (SMDI) and Evapotranspiration Deficit Index (ETDI) for agricultural drought monitoring. Agricultural and Forest Meteorology, 133(1): 69–88.

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