13. Evaporation of rainfall from surface soil (RVAP, in). The model allows for a 

 portion of rainfall to be evaporated from the surface soil. Generally in these forests, 

 rain that falls during the growing season readily evaporates after each storm and seldom 

 contributes to soil moisture recharge. A function was s}Tithesized to reflect this 

 phenomenon : 



AK = RAIN/PET 



If AK > 1, then AK is assigned the value of 1 

 RVAP = RAIN - (RAIN x AK) 



where: RAIN is net rainfall (in). The value of RVAP is then subtracted from the soil 

 moisture content. As a consequence of defining RVAP as a function of rainfall amount 

 as well as PET, significant amounts of rain are evaporated from the soil only during the 

 growing season. 



14. Soil profile interflow (QF, in). When soil moisture is above the water con- 

 tent for field capacity (FC, in), moisture may move laterally through the soil profile 

 until it reaches the stream channel. Soil moisture in excess of ' field capacity is 

 multiplied by an interflow coefficient (FQF, in/in) to define interflow. 



15. Deep percolation (QI, in). The quantity of water that percolates through 

 the soil profile and enters the ground water reservoir is calculated similar to QF 

 except that a deep percolation coefficient (FK, in/in) is applied instead of FQF. 



15. Deep seepage (SEEP, in). A portion of the water entering the watershed may 

 leave the area without contributing to local streamflow. In other words, a fraction 

 of moisture is routed via deep seepage into aquifers. The deep seepage storage compart- 

 ment receives moisture when the ground water level reaches a certain maximum (TOP, in) . 

 Mien this maximum is reached, the ground water level is multiplied by a deep seepage 

 coefficient (DPSP) to calculate the amount of water added to SEEP. 



17. Subsurface flow from ground water storage (QGW, in). The amount of water 

 entering the stream channel from the ground water reservoir is defined as the product 

 of the ground water level and a ground water recession coefficient (ACW, in/in). 



18. Channel routing of flow. Moisture for streamflow that is generated by the 

 model may be expected to experience a timelag before passing through the gaging station 

 at the mouth of the watershed. Therefore, the model provides for fractions of generated 

 runoff to be delayed up to 5 weeks. 



ASPCON computes weekly and yearly water budgets by summing all components of 

 streamflow, evapotranspiration , and changes in soil moisture and ground water storage. 



MODEL CALIBRATION 



The model was calibrated for an "average" water-year on the West Branch Chicken 

 Creek Watershed (CCW) , Davis County Experimental Watershed in Utah. The present vegeta- 

 tion status on the 217-acre CCW is approximately 20 percent grass-forb, 78 percent 

 aspen, and 2 percent conifer (Johnston and Doty 1972). A total of 47 inches of precipi- 

 tation fell during the modeled year, of which 11.6 inches was rain and 55.4 inches vi/as 



8 



