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<br />003120 <br /> <br />Page 19 <br /> <br />is for this reason that a separate analysis of water use in the headwater region <br /> <br />of the Colorado mainstem, including trans basin diversions to Denver and other <br /> <br />Front Range cities, was conducted through the use of a more detailed model). <br /> <br />C. Detailed Descripton of the Models <br /> <br />I. Representation of increased runoff <br /> <br />The runoff increases were assumed to be the result of removal of 50 <br /> <br />percent of the treatable area in clear cut strips (see Leaf, 1975, for a <br /> <br />description of such a treatment). <br /> <br />Of greatest importance for this study was that the runoff increase <br /> <br />reflect the stochastic nature of streamflow. Modeling the relationship <br /> <br />between pre-increase flow and the increase was more important than accurately <br /> <br />predicting the volume of flow increase that is possible from a given land <br /> <br />area. Annual fluctuation in increase is largely a function of the same <br /> <br />J <br /> <br />hydrologic factors affecting pre-increase flow. In order to adequately take <br /> <br />into account their stochastic nature, runoff increase estimates were tied to <br /> <br />pre-increase flow on the Colorado at Glenwood Springs (downstream from the <br /> <br />treatment area) based on the precipitation conditions that were assumed to <br /> <br />create the pre-increase flow. <br /> <br />The general approach to estimating the runoff increases to be associated <br /> <br />with pre-increase flows was to (1) estimate treatment area annual <br /> <br />precipitation associated with downstream flow for all years of record, (2) <br /> <br />estimate annual per-acre runoff increase as a function of annual <br /> <br />precipitation, (3) aggregate to total annual runoff increase from all of the <br /> <br />treatable acreage involved, and (4) seasonally allocate the annual runoff <br /> <br />increase. Each of these four steps is explained below. <br />