Management and Disturbance Effects on Water Yeild

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ist ric ! ater YieId The WRENSS Hydrologic Model (Troendle and Leaf 1980; Troendle and Nankervis 2000; Troendle et al. 2003; and Swanson 2004) was used to simulate water available for streamflow, or water.yield, from the South Platte River Basin. As was determined for the Nocth Platte River Basin (Troendle and Nankervis 2000; Troendle et al. 2003), water yield from NFS lands in the South Platte River Basin has steadily declined since the early 1900s (Table 3). As noted earlier, two scenarios for spruce-fir were simulated for each of the 20-year intervals from 1860 to 1997, and a single scenario was used to extrapolate current conditions without disturbance from 1997 to 2060. In the first simulation scenario, the current spruce-fir stands were adjusted to reflect change over time in the same manner as for the other species. In the second scenario, the spruce-fir stands were maintained at their current condition throughout 1860 to 1997. Simulated results for both scenarios are presented (Table 3). The consensus of the R2 Silviculture and Ecology staff was that the current spruce-fir stands were most likely, on average, to be present on the same sites during the entire time from 1860 to 1997 because spruce-fir is a climax tree species. This does not imply the stands were static but it does assume the stands were not "reset" during that period. Given that spruce-fir is less likely to replace itself in a stand- replacing scenario and is more likely to sustain itself over time, the decision to simulate the no-change scenario seemed reasonable. Similar assumptions are less likely to be the case for the balance of species present. Simulated water yield from NFS land in the South Platte River Basin was greatest ca 1920 (Table 3) and has declined in proportion to the subsequent vegetation density increases (see Table 2). The net reduction in water yield occurring from 1920 to 1997 is equivalent to about 1.8 area inches or 261,000 acre-feet from the 1.7 million acres of NFS land in the scenario that holds spnice-fir constant (Table 3). The simulated decline in water yield was slightly greater at 2.1 area inches (about 300,000 acre-feet) with spruce- fir stands altered similarly to the other species. Decreases in water yield from NFS land in the North Platte River Basin were estimated to be 3.4 area inches from 1860 to 1997 (Troendle et al. 2003). Average precipitation and streamflow are much lower in the South Platte River Basin than in the North Platte River Basin, so changes in flow resulting from either increases or decreases in vegetation are expected to be smaller. Numerous uncertainties, or risks, are inherent in the hydrologic simulations. Some uncertainties are more or less constant for each simulation while others vary. Uncertainties associated with the application of the hydrologic model probably do not vary from application to application. The strength of the WRENSS Hydrologic Model is in the assessment of changes in water yield. Prediction of absolute water yield is less reliable. The uncertainties associated with the vegetation descriptions input to the model probably have a greater impact on the outcome than the model itsel£ The most reliable estimate of water yield is likely to be that for the year 1997, the reference year, because that is the year best represented by the available stand description data. Although we have equal confidence in the capability of the hydrologic model to simulate water yield for each of the 20-year time increments, those simulations used projected, or adjusted, stand characteristics. The reliability of the historical and future water yield simulations, relative to current conditions, are directly related to the reliability of the projection of forest stand conditions back or forward in time. Our approach to changing stand conditions over time 12 2/2/2007