EIS Arapahoe & Roosevelt National Forest, Pawnee National Grassland

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data to estimate the baseline water yield. Baseline water yield from each of the four major river basins was then summed to determine the total baseline water yield for the Routt National Forest. .? Spreadsheet Baseline 1(page 11) shows how the weighted mean annual precipitation was determined forthe watershed above each stream gage, the water yield in feet per acre for each stream gage, and the regression model. Spreadsheet Baseline 2(page 12) shows the weighted average annual precipitation for each major river basin, aiid the baseline water yield by river basin. The purpose of estimating the baseline water yield is to provide a framework for interpreting the projected water yield increases for the different alternatives. The baseline water yield does not take into account existing increases in water yield from past timber harvest. The baseline water yield represents an estimate of the quantity of water produced from the Routt National Forest if there was no timber haivest, large scale fires, etc. WATER YIELD INCREASES BY ALTERNATIVE Water yield increases due to timber harvesting were deteimined using the WRENSS model (EPA, 1980). Methods used in this analysis followed those developed by Carl Chambers, hydrologist on the Arap ahoe- Roosevelt National Forest, as described in his draft version "Methods for estimating water yields for Forest Plan, June 4, 1997° (Attachment 1). Data used to calculate baseline water yield as well as water yield increases can be found in the project tile. For the Routt Forest Plan, water yield increases for 'special cuts' were not modeled as these activities do not change the basal area. The effects of aspen harvesting were not considered as this activity is highly variable and is dependant on the demand for aspen which is currendy low. The weighted mean annual precipitation for modeling water yield increases was determined separately for lodgepole and spruce-tir timber types. This was based on the number of acres outside of wilderness areas for each timber type in each precipitation zone. Acres by timber type within wilderness areas were not considered since no harvesting is allowed within wilderness areas. Using the methods developed by C.Chambers, the average water,yield increase was determined for different timber types (lodgepole and spruce-tir) and harvest method by aspect. For two step shelterwood cuts, it was assumed that 55 percent of the basal ai•ea is removed in the seed cut with the remaining 45 percent being removed in the subsequent overstory removal. In three step shelterwood cuts, it was assumed that 25 percent of the basal area was removed in the prep cu[. 45 percent removed in the seed cut, and 30 percent removed in the overstory removal. The projected unit water yield increase was incoiporated into the FORPLAN model and tied to the solution. FORPLAN projected acres harvested by timber type and harvest method, then estimated the annual water yield increase based on these acres. Annual water yield increases from past decades were added to the annual water yield increase for the decade in question to determine the total water yield increase. A decline in the water yield ? 2