Laserfiche WebLink
Overall, based on the range in simulated response, water yield has decreased <br />almost 20 percent, or 3 area inches since 1860. This equates to a decrease in <br />flow of nearly 225,000 acre -feet of water from 1.3 million acres of National <br />Forest lands in the North Platte River Basin. The greatest reduction in flow <br />occurred in spruce -fir stands (3.8 inches or an 18 percent reduction) and the <br />smallest reduction occurred in limber pine. Ponderosa pine (table 7) <br />exhibited the largest initial decline (1860 to 1900) with more moderate <br />decreases since then. Given the precipitation regime estimated for each of <br />the species, the current estimates of stream flow seem reasonable for <br />lodgepole pine, spruce -fir, the forest types for which we have the most <br />stream flow data and experience. The historical change in flow, based on <br />the estimates of forest cover changes over time, also seems reasonable <br />relative to the referenced watershed experiments expressing similar changes <br />in cover density. It is difficult to say if the 20 percent change in flow, over a <br />140 -year period, would be detectable at an off -site streamgage. Perhaps it <br />would be at the forest boundary but less likely at a downstream streamgage. <br />In evaluating the historical trends in water yield, as presented in table 4, <br />several factors should be considered. There is a "lot" of fluctuation in yield <br />from 1860 — 1900 that reflects the fluctuations in stand configuration <br />mentioned earlier. Because of the fluctuations in periodic responses, one <br />should average the flow simulated for the 1860 - 1900 period at 14 or 14.1 <br />inches. At the same rime, the flow for the period from 1940 to present is <br />fairly uniform at 11.9 to 12.0 inches. Averaging flow for the 2 time periods <br />would indicate a 2.0 area inch decrease in flow occurred from 1860 to <br />present resulting in an 185,000 acre -foot decrease in water yield. The .latter <br />estimate is a more conservative but perhaps more realistic estimate of the <br />reduction in flow that might have occurred as a result of forest growth. This <br />decrease in flow is still quite large relative to total yield. Individual specie <br />responses are very consistent with how we would expect those species to <br />respond ecologically and hydrologically over time. <br />OPPORTUNITY TO INCREASE WATER YIELD FROM THE <br />NORTH PLATTE RIVER BASIN <br />Evaluation of the trends in water yield as a result of the historical changes in <br />stand structure and vegetation density was based on all NFS lands in the <br />North Platte drainage that are currently occupied by forest. Nothing was <br />assumed to have changed with respect to water, barren, grass or brush lands. <br />An evaluation of the current opportunities to increase water yield, through <br />37 <br />