---
name: "i.evapo.pt"
section: "1grass"
platform: "linux"
package: "grass-doc"
version: "8.4.1-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/i.evapo.pt"
---

# i.evapo.pt(1grass)

Computes evapotranspiration calculation Priestley and Taylor formulation, 1972.

## 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*

## AUTHOR

Yann Chemin, GRASS Development Team, 2007-08

## SOURCE CODE

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

Accessed: Monday Feb 24 17:51:33 2025

Main index \| Imagery index \| Topics index \| Keywords index \| Graphical index \| Full index

© 2003-2025 GRASS Development Team, GRASS GIS 8.4.1 Reference Manual