Season formula (Water Overlay)

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The season fraction and index are variables used for calculating the transpiration factor in the Ground evaporation formula. It uses the configured start date of a Water Overlay and selected months and day of months to determine one of the four configured (seasonal) evaporation factors.

Evaporation month, day of month and corresponding evaporation factor array value:

Month Day of Month Transpiration value
March 15 1st
April 15 2nd
July 15 3rth
September 15 4th
October 1 1st

Evaporation factor graphs

The evaporation graphs showing the evaporation factor as a percentage per day. These are based on Table 2.10 Cropfactors[1] of the Gwz2016 book. The approximated graphs use five control points (see the season table above) per type, such that it can be reduced to four values per year. Values between the control points are obtained by interpolation.

Evaporation factors in the function table in the editor


  • The growth season dates are determined based on western european growth and transpiration graphs of the following crops and plants: grass, corn, potato, beet, grain, orchard and heather.

See also


  1. Table 2.10 Gewasfactoren, % van referentiegewasverdamping van Makkink, Bram Bot (2016) ∙ Grondwaterzakboekje, Gwz2016 ∙ Impressed druk en print