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, <br />approach of moving stands into and out of various age classes over time as was <br />done by Troendle and Nankervis (2000) and Troendle et al. (2003) in the <br />assessment of the North Platte River Basin. <br />b) Current forest stand conditions were projected into the future for three 20 year <br />time cycles to evaluate the effect of a no human intervention or no natural <br />disturbance scenario on future water yield: In concept, this scenario simulates the <br />progression of all forest land to a state of complete hydrologic utilization with <br />mature stands remaining mature and young stands moving toward maturity. <br />c) Current stands were projected into the future under a no human intervention <br />scenario but allowing stand conditions to be altered to reflect the impact of <br />natural disturbance on current vegetation: Examples of natural disturbance <br />include the 2002 Hayman Fire and the current mountain pine bark beetle <br />infestation in lodgepole pine. <br />d) The impact of management activities imposed on NFS land in the South Platte <br />River Basin from 1997 to 2005. <br />The updated version of the WRENSS Hydrologic Model (Troendle and Leaf 1980; <br />Troendle et al. 2003; Swanson 2004) was used for the simulations. Changes made to the <br />original WRENSS Hydrologic Model were reported in an earlier report on the North <br />Platte River Basin (Troendle et al. 2003). As noted in a through d above, the primary <br />objective of this assessment was to document changes in water yield that may have <br />occurred because of various changes in vegetation density or forest cover over time in the <br />South Platte River Basin. The WRENSS Hydrologic Model (Troendle and Leaf 1980) <br />was developed to focus on predicting change in water yield, or water available for <br />streamflow, annually. Less emphasis is placed on the accurate simulation of total water <br />yield as this is more complex, requires calibration, and errors in the simulation process <br />can make change detection more difficult (U.S. Environmental Protection Agency 1980). <br />For that reason, although this and the earlier reports do simulate total water yield from <br />forest lands, emphasize is on the potential changes in water yield that may have, or might, <br />occur as the result of past or proposed activities that alter vegetation composition or <br />density. Grassland areas, water bodies, and other non-forested surfaces are not included <br />in the simulations simply because they do not contribute to the estimate of change. <br />Emphasis is placed on simulating potential changes in water yield that may result from <br />changes in forest vegetation. <br />3 <br />2/2/2007