Impact of Forest Service Activities on Stream Flow

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Troendle/Nankervis/Porth Page 10 5/22/2003 types seem consistent atid reasonable wlhen compared to other accounts of stand development for those individual specie:>. The simulated pattern in stream flow decline for all species, except spruce-fir is consistent with the decline simulated using the DOS version of the WRENSS hydrologic model. Only the spruce-fir simulations differ significantly and that is largely because current stands are predc>minantly saw timber and the process of taking them back in time and then allowing them to "regrow" causes a uniform increase in density, and decrease in stream flow, over time. In contrast to the way we initially treated the spruce-fir type, it is equally, or perhaps more likely, that the spruce-fir standls that are now considered mature forest may have been mature forest stands for the entire periiod from 1860 to present and therefore contributed little to the simulated historical decline in stream flow as pi•esented above (Figure 2, Table 1) or in Troendle and Nankervis (2000). In order to assess the impact, a second set of hydrologic simulations were made, holding the spruae-fir constant duiring the 1860 to 1997 period, and the impact on the total flow was quite dramatic; (Table 2 and :Figure 9). The decline in stream low simulated to have occurred from 1860 to 1997, when the density of the spruce-fir component was adjusted, was about 3.4 area inches ('table 1 and 2). When the density of the spruce-fir component is held constant, the stream floix decline from 1860 to 1997 is 1.8 azea inches (Table 2). In addition, stream flow from the enl:ire basin during; the 1960 to 1997 is virtually constant when spruce-fir is held constant and only a moderate declxne occurs from 1997 to 2017. Based on a conservative interpretation of earlier simulations, Tmendle and Nankervis (2000) concluded that by the year 2000, the decline in flomr from Forest Service land in the North Platte River Basin resulting from increased vegetation density during the 11860 to 2000 period was approxirnately 190,000 acre feet of water per year by the yeaz 2000. The analysis using the SAS version of the model, used in this analysis, supports that c;onclusion. The estimate of decline in stream flow, holding spruce-fir constant, is reduced to approximately 166,000 acre-feet of water per year by the year 1997. T'he additional decline in stream flow simulated to occur from 1997 too 2017 is still 0.3 area inches or neazly 27,000 acre feet of water per year by 2017. In the earlier report (Troendle and Nankervis 2000), the time period used for the historical stream flow simulations was s;elected independent of the stand data. It was a coincidence in the subsequent analysis that the average age of the saw timber stands (about 180 yeazs) was such that they "originated" about -the same time as the historical simulations began (1860). This gives the impression that flows were very high at the beginning of the time period of interest and low today. The conclusion triat stream flowsi in the North Platte have declined since 1860 is correct, however, that does not imply that streatn flow had been consistently high prior to 1860. Using the same logic of moving the stands back in time, in 20-year increments, we moved them back further to the year 1700. The assumption was made that 20-years prior to any stand being "discounted' to zero age„ that stand was a mature saw timber stand completely occupying the site hydrologically. The hydrologic simulatlions were run for stand conditions for 1700 to 2017 (Figure 10). One can note that stream flow did peak in the 1860 to 1900 period and that it has declined since (Figure 10). One can alsco note that flows were equally low in the early 1800's as they are today and equally high in the 1700's as they were in the 1800's. Stream flow estimates, holding spruce-fir constant are generallw lower than the simulations that allowed the density of spruce-fir to be adjusted.