The relationship between evapotranspiration and moisture content

Abstract

In development of forage and pasture crop management practices it is important to make effective use of the soil moisture in production of vegetative growth. Although irrigation in New Zealand is on the increase, there seems little conclusive evidence on how much water should be applied to obtain optimum levels of production. With the cost of water likely to increase as more use is made of it, these optimum levels of supply become of economic importance. Information must be available regarding how much water can be used productively for a point is reached where the extra water added costs more than is gained by the increase in yield. It is necessary to learn whether plants having unlimited moisture supply tend to "waste" water as compared with plants having less ample supplies of soil moisture. Within the present meaning a "wastage" of water would be indicated by a relatively high amount of water loss through transpiration per unit of dry matter produced by the plants. The quantity of water removed from the soil by plants is variously termed "water requirements" or “water use” in the literature.

A study of the effect of soil moisture content on the evapotranspiration rate is important in crop water requirement studies. The results will influence the timing of irrigation. If evapotranspiration does not decrease as moisture content decreases it may be possible to replenish water less often. On the other hand, if evapotranspiration rate does decrease with decreasing moisture content a more precise optimum point could be found from the relationship, for the timing of irrigation.