Water use by vegetable crops is mainly determined by loss into the atmosphere, rather than availability in the soil. The atmospheric demand for water is estimated by the potential evapotranspiration (PET). Four factors influence PET:

• Temperature
• Wind speed
• Sunshine
• Relative humidity

Increases in temperature can be offset by decreases in sunshine, due to cloudy conditions, or decreases in wind. Over the past few decades PET has been falling, despite rising temperature.  The interplay between temperature and other climate factors means that changes in PET are predicted to be relatively small. For 2030, the best estimate suggests only a 2-4% increase in PET in the vegetable growing regions.