Well-watered, vigorous crops will usually transpire at nearly the rate demanded by the atmospheric conditions (PET), but as their water supply becomes limited, physical and biological controls begin to limit the rate of transpiration. The lower limit, of course, is near zero transpiration, which causes plant decay and death if it persists. The rate of transpiration, and thus soil water depletion, is reasonably clear and agreed upon at the wet and dry end points. What rates occur at the intermediate moisture contents is not clearly known and considerable differences exist among published results and contemporary scientists.
It is apparent that plant water use rates are a function of both atmospheric evaporative demand and plant available soil water. Plants have unique abilities to control water flow rates within their vascular system and through stomatal action. They make soil water available by root extension and by creating competitive water pressure within their membranes to cause gradients and water flow. However, with our lack of understanding and the complexity of soil water uptake and biological response, there is minimal detail which is currently warranted to represent the effects of crop water stress. Therefore, a simplified approach based on atmospheric demand and plant available water has been programmed, which represents a current general understanding.
The curves of Figure 7 provide a relationship between plant available soil water, defined by the range from wilting point to field capacity, and the ratio of actual transpiration to potential transpiration. Each curve represents a different level of total atmospheric demand, i.e., PET. The general shape of the curves are modifications of those derived by Denmead and Shaw (1960, 1962) in controlled pot studies of corn.
Figure 7: Actual over potential transpiration as a function of plant available water.
These curves of Figure 7 express the effect that actual plant transpiration will decrease from potential transpiration as plant available water is decreased in a quite non-linear pattern. The curves representing different levels of PET indicate that for a given level of plant available water, the plant will transpire a greater percentage of potential transpiration when PET is low than when PET is high. This is readily observed when crops show significant wilting under a high PET day, but very little on a subsequent low PET day with very similar soil water conditions.
The exact shapes of these curves are not well defined by research results, particularly the soil water content at which transpiration reduction begins. It is particularly important to define what portion of the soil profile is used to assess the quantity of plant available water. Young plants have set roots only in the upper part of what will eventually be a full root penetration, yet some use the total "root zone" as the base for percent available water. The Denmead and Shaw results were from plants with limited root zone in which the entire pot likely had extensive root activity. In the SPAW model, the curves of Figure 7 are applied independently to each specified soil layer and the potential transpiration of that layer is a multiple expression of the PET, canopy, phenology and root density. The maximum possible available soil moisture for each layer is defined by field capacity minus the wilting point, as defined by the soil water holding characteristics specified for each layer.
The PET values assigned to each curve of Figure 7 are only approximately known, thus may require calibration for specific crop-soil-atmospheric conditions. Although experience has shown original values determined for corn were applicable to soybeans. Some modification was made for grass and dryland winter wheat, which indicates some calibration for crop may be desirable. Values for curves A to E for corn were 0.05, 0.10, 0.20, 0.30, and 0.70 inches; for grass, 0.05, 0.10, 0.25, 0.40 and 0.70 inches; and for wheat 0.0, 0.15, 0.40, 0.60 and 0.85 inches. These values provide the relative capability for plants to continue transpiration under water stress caused by a lack of available water, high atmospheric demand, or both. Generally these adjustments were not significant to the overall field hydrology and default values for those of corn are used. Currently the wilting point can be adjusted to accommodate crops with variable drought tolerance.
Plant transpiration is estimated as the combined effect of PET, root density distribution, and soil water content and distribution. The daily potential transpiration (PET minus interception times canopy) is allocated to soil layers according to the root mass of that layer, reduced according to the actual/potential transpiration curves of figure 7, and summed for the profile. Transpiration then becomes the third component of the daily actual ET. For well watered agricultural crops, transpiration can be 15-25 inches depending on crop characteristics, growth period and atmospheric demand. Crops with water stress often exhibit significantly less transpiration.