Slatyer 3 suggests that the water balance equation be reduced to ET = AS during dry 

 weather, because surface runoff . and downward drainage are closely linked to soil water 

 recharge. Throughout much of the West, including Utah, summer precipitation on well- 

 vegetated land seldom exceeds the immediate storage capacity of the surface few inches 

 of soil. Therefore, it is readily available to loss by evapotranspiration . For this 

 reason, the water balance equation becomes ET = AS + P during the summer months. 



Results presented in table 2 were taken from several independent studies and 

 reflect water loss estimates from a large variety of vegetation covers and sites. Both 

 soil moisture depletion and evapotranspiration are influenced by a large number of 

 factors, including aspect, elevation, and several variables (e.g., climate, vegetation, 

 and soil). Therefore, comparisons of water losses from dissimilar sites are suspect 

 (table 2). However, this is not to say that all trends and observations are invalid. 

 We simply want to caution against indiscriminate use of the data and to point out that 

 comparisons of water withdrawal by various cover types are most valid when based on 

 results obtained from paired plots. 



For example, certain trends in water consumption associated with single variables 

 were noted and reported in the independent studies mentioned above. These observations 

 were perfectly valid within the context of a particular study. However, as the scope 

 of available information broadens with increased sampling, these trends are confounded 

 and masked by the influence of other variables. To cite an instance, when aspen sites 

 11 and 12 were considered separately, data indicated a definite trend in water use at 

 two elevations. But, when evapotranspiration losses for all aspen sites during a single 

 season are considered, no trends attributable to elevation are apparent as shown below. 



Elevation 



Soil moisture depletion 



Precipitation 



Evapotransp 



(Ft.) 



(In.) 



(In.) 



(In.) 



6,100 



8.51 



4.29 



12.60 



7,000 



11.55 



4.30 



15.85 



7,640 



9.29 



1.01 



10.30 



7,800 



9.51 



1.01 



10.52 



7,900 



5.67 



4.80 



10.47 



8,400 



17.39 



1. 13 



18.52 



9,200 



7.62 



5.55 



13. 17 



Our estimated losses probably are conservative, because the moisture that moved 

 laterally into the plots subsequently is lost by evaporation or transpiration. It 

 should also be noted that depletion figures represent only the soil moisture changes 

 within the depth of measurement (4 to 9 feet). This depth does not necessarily rep- 

 resent the maximum depth of root penetration on a given site. Because moisture with- 

 drawal by roots below the depth of measurement is not included in the depletion figures, 

 we may have underestimated depletion and evapotranspiration losses. 



Maximum moisture contents are constant on a given site, rarely varying more than 

 1 or 2 inches for the entire measured profile. Seldom, if ever, in the mountain water- 

 sheds of this region does winter precipitation fail to fully recharge the soil profile. 

 Hence, differences in maximum moisture contents merely reflect the difficulty in 

 measuring field moisture capacity. Slight differences in moisture storage recorded for 

 the particular day that initial measurements were made each year are due to gravi- 

 tational water still present in the profile or to previous water withdrawal by 

 evapotranspiration . 



Slatyer, R. D. Plant water relations. New York: Academic Press. 366 pp. 1967. 



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