The average maximum and minimum water contents and evapotranspiration estimates 

 for all sampling points are given in table 2. The maximum water content shown is the 

 amount measured on the initial sampling date, which varied from year to year depending 

 upon phenology and accessibility. Minimum water content is the amount measured on the 

 final sampling date, usually the minimum reached during the growing season. The 

 difference between maximum and minimum measurements represents soil moisture depletion. 

 Because the amount and distribution of summer rainfall profoundly affected these final 

 moisture measurements, precipitation was measured on or near each study area. Precipi- 

 tation during the growing season was added to soil moisture depletion to obtain an 

 estimate of evapotranspiration. 



DISCUSSION 



Treatment effects can be evaluated by considering the treatment area as a system 

 in which various inflow and outflow parameters can be measured and balanced. The 

 general and somewhat oversimplified water balance equation 



Q = P - ET - I - AGW - AS (1) 



is used frequently in forest hydrology problems of this type. In the equation, Q is 

 streamflow, P is precipitation, ET is evapotranspiration, I is interception, AGW is 

 change in groundwater aquifer storage, and AS is change in soil moisture storage. 



If treatments are confined to small plots, some estimate of effects can be obtained 

 by modifying the water balance equation and making some necessary assumptions. On a 

 small plot, streamflow and changes in the groundwater aquifer storage no longer are 

 considered because they cannot be measured readily. Treatment benefits are restricted 

 to measurable site parameters, and interception losses and reduced evapotranspiration 

 become a measure of treatment effects. Interception losses are difficult to evaluate 

 and are not characterized adequately for most vegetation types; therefore, interception 

 usually is pooled with the evapotranspiration term. The water balance equation now can 

 be written 



ET = P - R - D - AS. (2) 



In equation (1), deep seepage of soil moisture (D) and surface runoff (R) are not con- 

 sidered as separate terms. Deep seepage is assumed to be part of the soil moisture 

 storage term (S) , until such time as it is incorporated in the groundwater and reappears 

 as streamflow (Q) , or contributes to aquifer storage (GW) . In plot studies, however, 

 deep seepage is defined as the soil water that moves below the depth of rooting or below 

 the depth of soil moisture measurements and, as such, must be considered a loss to the 

 site. Because deep seepage is difficult to measure, it is assumed to be negligible or, 

 when paired plots are compared, to be uniform; then, in either case, results are not 

 unduly biased. Surface runoff also is a potential loss to the site and must be con- 

 sidered in equation (2). Surface runoff can be collected and measured, but usually is 

 negligible on well-vegetated sites in the watersheds of the Intermountain Region. On 

 the single bare plot (5c) , precipitation has been corrected by estimated runoff (table 

 2). Precipitation and soil moisture measurements can be made with relative ease and 

 accuracy on small study areas. 



3 



