USGS logo USAID logo

Daily Potential Evapotranspiration (PET)


Daily Potential Evapotranspiration (PET)

Daily Period

info icon

Available Tools


The GeoWRSI is a geo-spatial, stand-alone implementation of the Water Requirements Satisfaction Index (GeoWRSI), as it is implemented by the USGS for the FEWSNET Activity. The program runs a crop-specific water balance model for a selected region in the world, using raster data inputs. The program produces a range of outputs which can either be used qualitatively to help assess and monitor crop conditions during the crop growing season, or can be regressed with yields to produce yield estimation models and yield estimates. Other tools are available to post-process the GeoWRSI outputs so that they can be used in yield estimation models.

Product Documentation

Global Potential Evapotranspiration (PET)
(updated: February 2007)

The daily global potential evapotranspiration (PET) is calculated from climate parameter data that is extracted from Global Data Assimilation System (GDAS) analysis fields. The GDAS data are generated every 6 hours by the National Oceanic and Atmospheric Administration (NOAA). The GDAS fields used as input to the PET calculation include air temperature, atmospheric pressure, wind speed, relative humidity, and solar radiation (long wave, short wave, outgoing and incoming). PET is computed for each 6 hour period and then summed to obtain daily totals. A day is defined as beginning at 00 hours GMT.

The daily PET is calculated on a spatial basis using the Penman-Monteith equation (the formulation of Shuttleworth (1992) for reference crop evaporation is used). These equations were standardized in accordance with the FAO publication 56 for the 6-hourly calculations (Allen et al, 1998).

The daily PET values are scaled up (i.e. multiplied) by a factor of 100 to preserve the precision to .01 mm. The PET data have a 1 deg. ground resolution and are global in spatial extent (i.e. -180 to +180 longitude by -90 to +90 latitude).

References for Calculations:

Allen, R.G., L. Pereira, D. Raes, and M. Smith, 1998. Crop Evapotranspiration, Food and Agriculture Organization of the United Nations, Rome, Italy. FAO publication 56. ISBN 92-5-104219-5. 290p.

Shuttleworth, W.J., 1992. Evaporation. In D. Maidment, Handbook of Hydrology. McGraw-Hill.

Smith, M. 1992. Expert consultation on the revision of FAO methodologies for crop water requirements. FAO, Rome, Italy. 60pp.