Crop evaporation, surface resistance and soil water status

Abstract

Rates of evaporation from barley and from pasture were calculated from rainfall and from changes in soil water content measured with a neutron probe. Corrections were made for drainage. The potential evaporation was calculated for ten-day periods using the Penman-Monteith equation and mean surface resistances of the crops were derived. Relationships were established between the supply of water (as measured by the soil water deficit or soil water potential) and the response to a shortage of water (as measured by the surface resistance or the ratio of actual to potential evaporation). For pasture, these relationships held for the entire growing season and for barley for most of the period from mid-May to the end of July. They did not hold for barley earlier in the year when the leaf area index was small or towards harvest when the crop was ripening. The differences in response of the two crops were apparently due to differences in the water release curves of the soils rather than to species differences.