11. Overland flow when infiltration rate is exceeded (QXS, QOF , in). The model 

 provides for calculating overland runoff when active moisture input exceeds infiltration 

 capacity (Fl, in/wk) . This condition may occur when the soil is below saturation (QXS, 

 a rare occurrence on subalpine watersheds) or when the soil is saturated (QOF, which 

 occurs primarily during the spring snowmelt season) . Because the model is incremented 

 on weekly intervals, QXS cannot be estimated accurately. Consequently, the infiltration 

 capacity is set at a sufficiently large value to preclude any QXS. The model may be 

 set for any desired increment period, which could make QXS a more important hydrologic 

 factor . 



12. Transpiration (TRAN, in). The model treats evaporation of water via plant 

 stomates (transpiration) and evaporation of moisture from the surface soil as two dis- 

 tinct processes. To reflect the differences between grass-forb, aspen, and conifer 

 communities that are suspected to influence TRAN, the following relationship is assumed: 



TRAN = f (potential evapotranspiration , seasonal plant activity, plant rooting 

 depth, community crop coefficient). 



a. Potential evapotranspiration (PET, in) is calculated according to the model 

 described by Blaney and Criddle (1962) . _ 



b. Plant activity index (PAI). Although aspen and conifers have been shown to 

 be comparable in terms of end-of -season soil profile moisture content (Brown and 

 Thompson 1965), there is little direct research that describes relative year-round- 

 consumption patterns. Several researchers have found that conifers may actively trans- 

 pire water at times of the year when deciduous tree species are dormant (Swanson 1967; 

 Owston and others 1972; Smith 1975; and Urie 1959). Accordingly, a plant activity 

 index (PAI, fig. 3) is defined as that fraction of peak activity that a plant community 



CONIFER 



GRASS-FORB 



ASPEN 



OND JFM AMJ JAS 



MONTH 



Figure 3. — Plant activity index for the grass- forb^ aspen, and conifer types. 



6 



