EIS Arapahoe & Roosevelt National Forest, Pawnee National Grassland

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i Aquaric and-Riparian Resources _ Fraser Experimental Forest indicates that water yield increases from timber harvest persist at declining levels for approximately 80 years (Troendle and King, 1985). Many experiments have measured changes in streamflow from timber harvest on small watersheds (typically 100 to 1000 acres). However, as watershed size increases, it becomes progressively more difficult to measure the same changes. It is not that the additional streamflow is not present at the larger scales, but rather that as the changes in streamflow become proportionately smaller in comparison to the total streamflow, they become increasingly difficult to detect and aze eventually masked by the natural variance of the system. Since fish-bearing streams are generally larger and lower in the watershed, water yields that do not change channel stabiliry are unlikely to affect habitat for aquatic indicator species. VVater yield increases caused by timber harvest were modeled for Altemative B and for the timber benchmark, using the methods outlined in WRENSS (EPA, 1980). The timber benchmark is the estimate of the maximum timber harvest that could be sustained for all forested lands except for those lands legally withdrawn or not physically operable. Modeling water yield for this benchmark offers an estimate of how much additional water could be produced if timber harvest was maximized on the Forest. For Altemative B, water yield from timber harvest is expected to increase by 2,962 acre feet (AF) at the end of five decades. (An acre foot will cover a football field to a depth of 1 foot. It is about the amount of water used in one year by the average household in Colorado.) For the timber benchmark, water yield from timber harvest would be expected to increase by 6,690 acre feet at the end of five decades. Existin? water yield for the Forest is approximately 2 million acre feet, so increases will be on the order of 0.15 to 0.33 percent of existing yield. Because the Forest is bisected by the Continental Divide, water from the westem portion of the Forest drains into the Colorado River and water from the eastern portion drains into the Platte River. The table below shows the water yield increase from timber harvest for Altemative B and for the timber benchmark for each drainage. Table 3.16 Water Yield Increase from Timber Harvest (Ava. AF/vear) Platte .v . Altemative B lst Decade 2nd Decade 3rd Decade 4th Decade Sth Decade 484 913 1205 1418 1676 Timber Benchmark 2106 2510 3863 3911 4577 Colorado 'v Altemative B lst Decade 2nd Decade 3rd Decade 4th Decade Sth Decade 270 546 876 1130 1286 Timber Benchmark 4 276 334 1929 2113 Water yield increases every decade even though harvest levels remain relatively constant at 6.5 to 6.9 million board feet per year for Altemative B. This is because water yield persists for about Chapter Three • 126