TRANSFER FUNCTIONS' 



1. Calculation of initial ground water level (GWL, in) from baseflow. At the 

 beginning of the water-year (Oct. 1) average streamflow for the last rainless week of 

 September is used to define the initial GWL. Initial GWL is the quotient of stream 

 baseflow (inl divided by a ground water recession coefficient (AGW, in/in) . 



2. Channel interception (QCHP, in). The amount of moisture falling directly into 

 the stream channel is defined as the fraction of the total watershed area consisting of 

 surface water or saturated streambanks (ACHP, in/in) multiplied by the precipitation 

 input. The value for ACHP may be determined from an areal map of a watershed. 



3. Precipitation type. Form of precipitation is determined by using a routine 

 similar to the model developed by the Army Corps of Engineers (1956): 



If TEMP < TMIN, RP = 0.0 



If TEMP > TMAX, RP = 1.0 



If TMIN <_ TEMP <_ TMAX, 



RP = (TEMP - TMIN)/ (TMAX - TMIN) 



where: TEMP is mean weekly air temperature (°F) , TMIN is a critical minimum temperature, 

 below which all precipitation is snow, TMAX is a critical maximum temperature, above 

 which all precipitation is rain, and RP is the fraction of input moisture that falls 

 as rain. 



4. Rainfall interception loss (RINT, in). Vegetative canopies are known to inter- 

 cept and retain a fraction of rainfall that is ultimately evaporated back to the atmos- 

 phere. The amount of rainfall greatly influences the amount of net moisture (moisture 

 entering the soil) for individual storms; estimates of yearly interception losses are as 

 follows: grass-forb, 9 percent; aspen, 12 percent; and conifer, 20 percent (Helvey 1971; 

 Johnston 1971; and Verry 1976). The fraction of moisture received as rainfall that may 

 be considered interception loss is assumed to be an average, weighted by areal cover of 

 each vegetation type, of three rainfall interception storage coefficients (GSTR, ASTR, 

 and CSTR, in/ in) . 



5. Snowfall interception loss (SINT, in). Researchers have many different opin- 

 ions about moisture loss from intercepted snow in coniferous canopies (Satterlund and 

 Haupt 1970; Miller 1962). Estimates of the magnitude of such losses generally range 

 between 6 and 10 percent of total snowfall (Anderson 1969). The amount of snowfall 

 interception loss from leafless aspen is assumed to be relatively minor. The fraction 

 of snowfall that becomes interception loss is defined in ASPCON simply as the weighted 

 average of two interception loss coefficients, SNA (aspen) and SNG (conifer), with 

 respective values of 0.01 and 0.07 in/in. The interception loss of snow by the grass- 

 forb type is assumed to be zero. 



Numbers correspond to the items presented in figure 1. 



4 



