Estimating Additional Water Yield From Changes in Management of National Forests in the North Platte Basin

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currently being generated. Simulation of harvesting 1:120th of the area in the year 2001 does not indicate any change in flow (rounding error) occurs. By the end of year 2005, when 5:120ths are harvested, total flow would increase to 11.4 inches (or a 0.3 inch increase in water yield from the entire over 355,354 acres of lodgepole pine). By 2015, when 15:120th of the area would be harvested, simulated flow increased to 12.0 inches reflecting a 0.9 inch increase over the 15 -year period. This equates to an increase of 26,651 acre - feet of water per year in year 2015. It would be at this point in time that as we proceed in the harvesting alternative we would also begin to simulate a decrease in flow or hydrologic recovery from the area harvested in 2001. The hydrologic recovery period can be expected to take a total of 60 to 70 years (Troendle and King 1987), so the efficiency of additional cutting in subsequent years beyond 2015 would continue to exceed declines due to re- growth in earlier harvest areas, for many years; perhaps 50-years. Increases in flow should eventually reach and sustain 40,000 acre -feet of water per year from lodgepole pine throughout the rotation. It would be possible to accelerate the harvesting and generate greater water yields at least in the near term, because over 50 percent of the lodgepole cover type., is in sawtimber class. However, an accelerated production rate now would not be sustainable either in terms of timber production or water yield over the long term. Like lodgepole pine, aspen is also best managed using even aged management and like lodgepole pine, clear cutting on a 120 -year rotation was the silvicultural tool simulated. There are currently 7,278 acres of aspen considered suitable for timber harvest. WRENSS simulations indicate 13.3 inches of water is being generated from that cover type at present. Clear cutting 1:120th of the area (61 acres) did not alter flow in year 2001. By year 2005, 303 acres had been harvested yielding a 0.4 inch increase in flow. By year 2015, 915 acres had been harvested, yielding an additional 1.2 inches of water for a total increase in yield of 725 acre -feet of water per year. Depending on site quality and initial stocking (sprouting) density, aspen can recover hydrologically in anywhere from 15 to 45 years. This estimate is based on unpublished simulations of growth response, using GENGYM (Edminster 1978) and assuming complete hydrologic recovery occurs at a leaf area index of 5.25 (Troendle and Leaf 1980). If we assume an average 30 -year recovery; the increase in flow from aspen can be sustained at about 1000 acre -feet per year over the 120 -year rotation. 39