6. Snowpack evaporation (SVAP, in). Dotv and Johnston (1969) found evaporative 

 losses from snowpacks in winter as follows: open ground, O.OS in/ in, under aspen, 0.034 

 in/in, and under conifers, 0.026 in/in. A weighted average of three snowpack evapora- 

 tive loss coefficients, GSV, ASV, and CSV, is assumed to be the fraction of snowfall 

 that is evaporated during the year, 



7. Snowpack accumulation. Research suggests that vegetative canopies influence 

 snowpack in western watersheds (Garv and Coltharp 1967; Thies 1972; Dunford and 

 Niederhof 1944; and Meiman 19701. Accordingly, snowpacks in the model are accumulated 

 differently for each vegetative type. Snowpack accumulation is assumed to be a fraction 

 of total net snowfall for each community type (99 percent in grass-forb areas, 106 per- 

 cent in aspen areas, and 95 percent in conifer areas). This approach is simple yet 

 does provide for a redistribution of snowfall within the watershed that is consistent 

 with field observations. 



8. Snowpack melt. Just as snowpack accumulation patterns vary between watershed 

 cover types, the timing and rate of snowmelt may also be expected to change as a func- 

 tion of vegetative succession. Snowpack ablation may be expected to begin first in an 

 open area and last in a coniferous forest. Snowmelt rates should be about the same 

 for open and aspen areas but significantly slower for coniferous types (Thies 1972; 

 Federer and others 1972). Snowmelt in ASPCON is indexed by mean weekly air temperature 

 in a manner similar to the Army Corps of Engineers (1960) model. Figure 2 shows that, 

 for each vegetative type, the amount of snowmelt is a function of a melt rate coeffi- 

 cient, CMC, AMC, and CMC (in/°F wk) , and a base temperature coefficent, GBASE , ABASE, 

 and CEASE (°F) , for grass-forb, aspen, and conifers, as well as mean weekly temperature 

 (°F). 



9. Channel inflow from snowmelt (QMCH, in). Part of each increment of snowmelt 

 may be expected to occur on saturated soil adiacent to stream channels and, therefore, 

 to readily enter the stream channel. The fraction of snowmelt thus contributing to 

 streamflow is equivalent to the product of the amount of snowmelt and a melt inflow 

 coefficient (TMCH, in/in). TMCH functions similar to ACHP and may be estimated from 

 an areal map of a watershed. 



10. Active moisture input. The term "active moisture" is defined as the sum of 

 net weekly rainfall and snowmelt. Active moisture is capable of entry into the soil 

 system (depicted as the large "tank" in fig. 1) for subsequent evapotranspirat ion , 

 deep percolation, or direct contribution to streamflow. 



5 



