Impact of Forest Service Activities on Stream Flow

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Troendle/Nankervis/Porth Page 12 5/22/2003 simulations presented in Figure 9 assume the stand cliaracteristics of spruce-fir are constant for the period. E:stimated V1later Yield from Forest Service Lands in the Norl:h Platte Basin: 1700 -1997 14 13 - _ • . .. c 12 v • ? • d « ? 10 , . . g -? Spruce-Fir BA ChangEs pruce-Fir BA Constaiit . 8 . . 168 170 '172 174 176 178 180 182 184 186 188 190 192 194 196 198 200 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Figure 10: Simulated annual water yield from National Forest lands in the North Platte River from 1700 to 1997 showing the cyclical pattern in yield that would exist if current stand conditions were discourited back thru time in 20 year increments. Simulation of activity in,lpacts was fairly straightforward. In Tables 1 and 2 above, it can be noted that the simulated uvater yield frorri NFS land on the North Platte River was approximately 10.0 area inches in 1997 based on the S-AS simulations. In the projection of the adjusted stand conditions to 2017, the s,imulated flow decreased to 9.7 area inches, indicating a 03 area inch decline will occur as the iresult of additional re-growth for all species, including spruce-fir during the period 1997 to 2017. Flow would be slightly greater (9.8 inches) if spruce-fir are not adjusted. We simulated that water yield at the time existing stands all reach maximum hydrologic utilization will be 9.1 areat inches of water yield indicating a nearly 0.6 inch additiona.l decrease in flow could occtu• in the future. The simulated decrease in flow, as the result of increased stand density from 1997 to 2017, is approximately 27,000 acre-feet of water. Again, because of the dominant influenc;e of increased density in the spruce-fir forest type, this estimate may be high as discussed earlier. There are two significant points that can be made from the results of the historical flow analysis. First, average water yieldl is declining as vegetation density increases over time. Water that may have been available for appropriation wh.en agreements and compacts were made in the past may not be available to meet those commitnnents today, making water short falls and the need for long term planning more critical than ever. Also, current levels of stream flow may not be stable into the future and subject to further decllines, at least for the next 30 to 50 yeazs. A second point