Figure 4. — Effect 

 of limiting soil 

 moisture on 

 potential 



evapotranspiration . 



1.0 



ZD 



y— 

 o 

 < 



0.5 



PERMANENT 

 WILTING POINT 



0.5 X FIELD 

 CAPACITY 



FIELD 

 CAPACITY 



may reach when water is not limiting growth. The PAI is thus defined to reflect the 

 week-to-week influence of day length and soil temperature on a plant's ability to 

 transpire water. 



A correction is applied to PET to account for the effects of limiting soil moisture 

 on transpiration. The relationship outlined in figure 4 adjusts PET according to the 

 following rule: PET' = PET x (SM-PWP)/AWH where: SM is volumetric soil moisture content 

 (in), PWP is water content at permanent wilting point fin), and AWH is one-half of the 

 profile's available water or the difference between the water content at field capacity 

 (FC, in) and PWP. The adjustment of PET for limiting soil moisture is made according 

 to a model by Hanks (1976) , which is similar to the approach taken by Leaf and Brink 

 (1975) . 



c. Plant rooting depth (RDP) . The capacity of different plant communities to 

 occupy the root zone and the differences in mean soil depths for different watersheds 

 are reflected in a plant rooting depth coefficient. The RDP is defined as that fraction 

 of the total available rooting zone in the soil profile that contains 90 percent of all 

 live plant roots. 



d. Community crop coefficient (CC) . The crop coefficient is included in the model 

 to reflect differences in consumptive use rates of water by different vegetation types 

 when all other factors are held constant. The grass-forb community is given the value 

 of 0.9. Although forested communities may be expected to transpire greater amounts of 

 water than nonforested areas, it is questionable whether crop coefficients for aspen and 

 coniferous forests should be different. Unlike coniferous forests, aspen forests gen- 

 erally have a highly productive understory which contributes to transpiration losses. 

 However, coniferous forests have a larger leaf area index and increased quantity of 

 aboveground biomass than do aspen forests. As a result of the above mediating consider- 

 ations, the crop coefficients for aspen and conifer types are set at 1.25. 



Watershed transpiration loss is weighted according to areal vegetation composition 

 and is calculated as the product of PET', PAI, RDP, and CC values. 



7 



