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

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r water from 1,107,000 acres of NFS land. This estimate is greater than that estimated to have occurred using existing stream flow records (Leaf 1999). Water yield from NFS lands on the North Platte is in general quite high because of the high precipitation input. Although the percentage of , forestland Suitable for Timber Harvest is less than 50 percent of the total NFS lands, sizable increases in flow appear feasible using the 120 -year rotation and appropriate silvicultural techniques for the specie. In general, the simulated trends in stream flow whether the result of in- growth, harvesting, or catastrophe appear consistent with observed changes measured at the streamgage for treatments with a comparable impact. The hydrologic model WRENSS appeared to perform well when simulating the effects of fire, insect mortality, and timber harvest. _ Simulations of hydrologic response to both clear cutting and partial cutting compare well with observed changes in flow measured at the stream gauge, for similar impacts. It would seem unlikely that the simulated changes in flow following timer harvest simulations would be detectable at any streamgage on the North Platte River. Neither is it likely that the simulated increases in flow could actually be detected as they exit NFS land, assuming a gauge were present to monitor them. It is conceivable that decreases in flow on the order of magnitude simulated for the historical trend in forest cover could be detected downstream at a USGS gauge, if the gauge had an adequately long and consistent record. Since most gauges (North Platte at Northgate and Mitchell, North Platte below Whalen, etc.) were not initiated until the early 1900's, thus making detection of the flow reduction, as simulated, questionable. , We chose not to simulate the potential response to other management scenarios as part of this effort but the reality is, instead, that the outcome of virtually any scenario can be inferred from the. range of scenario's we did simulate. For example, one can infer the relative impact of adding or subtracting Suitable acres based on simulated responses for the acreage currently available. One can also infer the impact of fully or partially implementing the management alternatives or a modification of them. Costs, in terms of water yield, can also be calculated as opportunity forgone for exclusions for Wilderness, wildlife set asides, and so on. To the degree the 44 1