Management and Disturbance Effects on Water Yeild

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is simplistic. All hydrologic simulations used the average 30-year average monthly precipitation obtained from the Oregon State climate maps. Although this allows for comparing responses, particularly change in response under a constant climatic scenario, this approach did not address the dynamics in response (both total yield and change in yield) that would occur under the changing and dynamic climate regime from 1860 to 1997. For example, under a wetter than average regime, both total yield and the simulated changes would have been greater while under drier conditions the simulated flows and changes would have been less than those presented. As expected, the actual or measured streamflow would have strongly reflected the climatic variability and precipitation regime that occurred during the period and not the long term average scenario that we simulated. Reductions in water yield that may have resulted from vegetation change may or may not have been apparent in the measured flow because of the inherent variability in flow that may have occurred during the period. However, using a constant precipitation regime in the modeling effort does allow the effect of vegetation change to be identified more clearly. Fewer risks are associated with the projection of future stand conditions than with projections into the past. Projecting current stand conditions into the future based on the age/basal area relationships simply allows younger stands to continue to mature while holding mature stands constant. As such, the projections of future stand conditions and water yield reflecting a no-disturbance scenario, with one exception, seems reasonable. The impact of the 2002 Hayman Fire is included in the simulations for 2020 to 2060 and is discussed separately. As was found for the North Platte River Basin (Troendle et al. 2003), water yield from the South Platte River Basin will continue to decline assuming the natural succession of the current forest vegetation is not interrupted (Table 3). As current stands continue to mature, water yield from NFS land can be expected to decline another 0.5 area inches, or an additional 72,000 acre-feet by the year 2060 (Table 3, Figure 8). In the North Platte study, projections indicated future water yields would decline an additional 0.32 area inches or 29,000 acre-feet from 1997 to 2017. Water yield from NFS land in the South Platte River Basin can be expected to decline an additional 0.2 area inches or 28,000 acre-feet during a similar timeframe (1997-2020). As noted earlier, the stratification of all stands into one of three size classes for each period appears discontinuous (Table 2). In contrast, the basal area estimates for the stands that were aggregated into one of the three age classes were continuous, and that uniformity is expressed in the trend of simulated water yield from the South Platte River Basin for the 1860 to 2060 period (Figure 8). Water yield, and particularly the change in water yield, can be expected to be cyclical as the forest stands evolve and reset due to human or natural intervention. The impact of the resetting process is most dramatic at the level of the stand or hillslope and becomes more integrated and less obvious as the landscape scale. On average, the simulated water yield from NFS land in the South Platte River Basin appears to have been moderate during the mid- to late 1800s, then increased to a high during the early 1900s in response to declines in vegetation density. Water yield has since declined in relation to the increase in vegetation that has occurred since the early 1900s. Barring vegetation disturbance, this decline in water yield will continue at a more moderate rate into the near future (Figure 8). In the North Platte River Basin, stand characteristics were projected back to the year 1720 to show that the historical trends in water yield are cyclical. This projection was not done for the South Platte River Basin; 13 2/2/2007