i.evapo.pt - GRASS GIS manual
Author: GRASS Development Team
Description: i.evapo.pt: Computes evapotranspiration calculation Priestley and Taylor formulation, 1972.
Keywords: imagery, evapotranspiration

## NAME

i.evapo.pt - Computes evapotranspiration calculation Priestley and Taylor formulation, 1972.

## KEYWORDS

imagery, evapotranspiration

## SYNOPSIS

i.evapo.pt
i.evapo.pt --help
i.evapo.pt [-z] net_radiation=name soil_heatflux=name air_temperature=name atmospheric_pressure=name priestley_taylor_coeff=float output=name [--overwrite] [--help] [--verbose] [--quiet] [--ui]

### Flags:

-z
  Set negative ETa to zero
--overwrite
  Allow output files to overwrite existing files
--help
  Print usage summary
--verbose
  Verbose module output
--quiet
  Quiet module output
--ui
  Force launching GUI dialog

### Parameters:

net_radiation=name [required]
  Name of input net radiation raster map [W/m2]
soil_heatflux=name [required]
  Name of input soil heat flux raster map [W/m2]
air_temperature=name [required]
  Name of input air temperature raster map [K]
atmospheric_pressure=name [required]
  Name of input atmospheric pressure raster map [millibars]
priestley_taylor_coeff=float [required]
  Priestley-Taylor coefficient
  Default: 1.26
output=name [required]
  Name of output evapotranspiration raster map [mm/d]

## DESCRIPTION

i.evapo.pt Calculates the diurnal evapotranspiration after Prestley and Taylor (1972). The Priestley-Taylor model (Priestley and Taylor, 1972) is a modification of Penman's more theoretical equation.

## NOTES

RNETD optional output from i.evapo.potrad is giving good results as input for net radiation in this module.

Alpha values:

- 1.32 for estimates from vegetated areas as a result of the increase in surface roughness (Morton, 1983; Brutsaert and Stricker, 1979)
- 1.26 is applicable in humid climates (De Bruin and Keijman, 1979; Stewart and Rouse, 1976; Shuttleworth and Calder, 1979), and temperate hardwood swamps (Munro, 1979)
- 1.74 has been recommended for estimating potential evapotranspiration in more arid regions (ASCE, 1990). This worked well in Greece with University of Thessaloniki.

Alpha values extracted from: Watflood manual.

## SEE ALSO

i.evapo.mh, i.evapo.pm, i.evapo.time, i.eb.netrad, r.sun

## AUTHORS

Yann Chemin, GRASS Development Team, 2007-08

## SOURCE CODE

Available at: i.evapo.pt source code (history)

Accessed: Sunday Jun 21 00:55:00 2026

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