Carolina was converted to pine. Such a study is needed in the West to more accurately 

 define the actual changes in watershed hydrology where aspen-conifer succession is 

 occurring. In the absence of such research, a watershed hydrologic model based on 

 recognized hydrologic processes and utilizing appropriate data from past studies and 

 modern computer technology may provide useful insights. Such a hydrologic model may 

 be of particular value in identifying critical research needs. 



A major purpose of hydrologic simulation modeling is to realistically and pre- 

 cisely represent a system (a series of processes) with a network of mathematical 

 expressions (Riley and Hawkins 1975). Models are comprised of coefficients, structure, 

 and initial conditions that interact to manipulate each piece of input data to produce 

 a desired output. Before a model can be deemed acceptable, it must be properly iden- 

 tified and formulated, calibrated to mimic observed system behavior, and verified 

 through repeated testing. Simulation models integrate the effects of a variety of 

 subprocesses in order to provide for maximum utilization of a given information base 

 in terms of predictive capability of system performance (Riley and Hawkins 1975). 



The purpose of this study is to formulate a structural watershed hydrologic model 

 that will integrate available knowledge relevant to the hydrologic impacts of aspen to 

 conifer succession. Although Leaf and Brink (1975) have written a rather sophisticated 

 subalpine hydrology model, a fundamental model sensitive to aspects of the hydrologic 

 cycle that may be influenced by vegetation changes would be useful. The model described 

 in this report begins to satisfy that need. 



DEVELOPMENT OF ASPCON 



The model describing the hydrology of aspen to conifer succession (ASPCON) consists 

 of a series of moisture storage compartments connected by transfer equations that sys- 

 tematically deal with each set of input data (fig. 1). As moisture enters and interacts 

 with a watershed, a certain amount is lost to the atmosphere via evapotranspiration, 

 while the remainder may become streamflow or percolate deep into the soil. 



Obviously ASPCON can only be as valid as the assumptions that were made as the 

 model was constructed. Literature pertaining to hydrologic behavior of grass-forb, 

 aspen, and conifer ecosystems was carefully reviewed; only key references are cited. 

 The model's transfer equations were derived from research findings that varied widely 

 in location and purpose and, therefore, often were not directly applicable. Consequen- 

 tly, many water movement equations must be considered educated guesses. This lack of 

 information points out the need for definitive research that directly relates to the 

 critical hydrologic problems associated with aspen-conifer succession. 



ASPCON is a deterministic, lumped-parameter model. The watershed is treated as 

 a single series moisture storage "tank." Model coefficients related to watershed char- 

 acteristics represent averaged values. The model calculates weekly water budgets 

 throughout 1 water-year (Oct. 1 to Sept. 30). System input includes only precipitation 

 and average weekly air temperature. The transfer functions for moisture routing within 

 the watershed are described below in the sequence of ASPCON's algorithmic logic. 



2 



