Minimum moisture contents and resulting soil moisture depletion values vary con- 

 siderably both between sites supporting a given vegetation type and between years on 

 any given site. These data reflect both moisture withdrawal and precipitation char- 

 acteristics prior to final measurement. In some years, summer storms repeatedly 

 replenish moisture deficiencies in the surface few feet of soil, but this water usually 

 is lost through evapotranspiration within a few days. Most of the late-season rainfall 

 contributes to the replenishment of soil moisture deficits. 



Growing season rainfall fluctuated greatly between the study years of 1963 and 

 1967. For example, summer rainfall on site 5 averaged 4.61 inches, but ranged from 

 1.13 to 10.43 inches. This type of variation is advantageous because it provides a 

 good estimate of the extremes that can be expected in soil moisture depletion and 

 evapotranspiration values. Measurements from site 5a during 1963 and 1965 illustrate 

 how rainfall can influence soil moisture measurements and contribute to interpretation 

 problems. Maximum water content in the 9-foot profile was essentially the same (37.73 

 and 37.55 inches) during each year. Minimum moisture contents were 18.61 inches in 

 1963 and 25.72 inches in 1965. Rainfall in 1965 was nearly twice as great as in 1963. 

 When precipitation was added to the soil moisture depletion figures, evapotranspiration 

 was estimated to be 1.89 inches greater for the 1963 growing season. 



To make the results more comparable, total evapotranspiration values cited in 

 table 2 were converted to inches of evapotranspiration per foot of measured soil 

 profile. Then the highest and lowest evapotranspiration value for each vegetation type 

 for all years and sites were used to construct figure 2. The graph illustrates the 

 wide range of evapotranspiration values that might be expected over several years on 

 many mountain sites. These results should not be considered conclusive as data on 

 several of the vegetation types were taken for a limited number of sites and years of 

 measurement . 



Tree and shrub types show a generally greater loss than sagebrush-grass, grass- 

 herb, or bare types. Estimates for both sagebrush-grass (site 8) and bare types 

 (site 5c) are based on but one site each and so are not reliable. Certainly, we would 

 not expect the same minimum evapotranspiration value per foot of soil from both bare 

 and well-vegetated sites, and--when legitimate comparisons are made--our expectations 

 are substantiated. Comparative losses from adjacent grass-herb and bare plots (5b and 

 5c, table 2) show evapotranspiration losses nearly always 0.5 inch or greater per foot 

 of soil on the vegetated plot. 



In all instances, sprout vegetation has a narrower range than respective mature 

 types; so sprout ranges are found on the lower side of the evapotranspiration scale, 

 although there is considerable overlapping (fig. 2). 



The range of evapotranspiration losses from Douglas-fir ( Pseudotsuga menziesii ) 

 appears to be too small, but the information available is meager, representing data 

 recorded for 2 years from site 16 and for a single year from site 3b. 



Most of the information assembled is for aspen, Gambel oak, and grass-herb types, 

 consequently, the range of evapotranspiration losses per foot of soil is widest for 

 these types. The higher evapotranspiration losses from the grass-herb type overlap all 

 the other tree and shrub types, especially in the 1.75- to 2.75-inch range. Maximum 

 rates of evapotranspiration are 3.26 and 5.10 inches per foot of soil from the mature 

 aspen and oak types, respectively. 



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