Crop-water productions relate total dry matter (DM) or crop yield (Y) to either soil moisture content or soil water tension, evaporation, transpiration (T), evapotranspiration (ETc), or applied irrigation water (IR).
It has been shown, in container experiments , that DM production is linearly related to the ratio of transpiration to pan evaporation in climates with mostly clear skies during the growing season. The relationship is expressed as
where DM is crop dry-matter production (kg), m is a crop-specific proportionality rate (mm day-1), Tis transpiration (kg), and E is pan evaporation (mm day-1) averaged over the growing period. However, in several experiments in The Netherlands, production was
where n is a crop-specific coefficient (kg kg-1).
Arkley  showed that the ratio between T and crop production may be distorted by advection; therefore, he suggested a modified equation for crop production:
where ka is a crop-specific coefficient and RH is the mean daily relative air humidity (in percentages) during the growing season.
Arkley  also studied the relationship between crop dry-matter production and the ratio of transpiration to VPD arranged in the following form:
where ka is a crop-specific coefficient. Because H = 100ea/es,the coefficients are related as follows:
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