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

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Summary of Data Set Data provided to us, by the U.S. Forest Service, described approximately 23,000 polygons, or homogeneous units of area, that in aggregate represent the total U.S. Forest Service ownership in the North Platte River Basin. For each polygon, the management class, cover type (including: specie composition, size class, basal area, age, and cover density, when appropriate), aspect, and elevation, were provided. For each polygon, data are also available to estimate mean monthly precipitation. When simulating hydrologic response, the polygons, or unit areas, expressing similar characteristics were aggregated to reduce the number of computer runs. In anticipation of the alternatives that would be simulated, and the questions that might arise, we interrogated the distribution of the polygons with respect to certain characteristics. We found, for example, little difference exists in the percentage of U.S. Forest Service ownership that falls in the saw log size class, by management class. Wilderness areas do not appear to be any more dominated by larger (older) stands than areas designated as Suitable for Harvest. Although the largest polygons, or unit areas, are as large as 3000 acres, more than 95 percent of the total forested area, regardless of management class, is characterized by polygons less than 1 square mile, or 640 acres in size with the majority of the units less than 60 acres. Because of the relatively small size of the units, on- the - ground management opportunities will be influenced by the dispersal of the units as well as availability of access to them. The alternatives we will propose will not address that problem but experience with Coon Creek tells us that the real opportunity for timber harvest and therefore water yield augmentation will not be as great as anticipated or simulated. Appendix A presents the current database, less the annual precipitation data. HISTORICAL TRENDS IN WATER YIELD As noted earlier, there are currently 1,107,593 acres of forested land, owned by the U.S. Forest Service, in the North Platte River Basin. Species composition is predominantly lodgepole pine and spruce fir (table 2). There are significant amounts of ponderosa pine and aspen with lesser amounts of douglas fir and limber pine (table 2). Fifty -five percent of all stands are in the sawtimber class with the majority of the balance in the pole size class. 27