Impact of Forest Service Activities on Stream Flow

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! x ~ Troendle/Nankervis/Porth Page33 5/22/2003 each time period are presented in Table 2. However, simulated changes in flow following forest disturbance using the current version of the WRENSS hydrologic model are similar to those predicted using eazlier versions although the improved process defmitions in the current version are far more robust and tend to smooth the transition across differing stand densities. For example, the trend in stream flow reduction on the North Platte River for the period 1940 to 1997 is smoother and more realistic using the current version of the model, although the prediction of the flow change from 1860 to either 1997 vs. 2000 is quite similaz. The difference in the magnitude of flow is a function of how the precipitation was adjusted. The current version of the WRENSS hydrologic model reflects all the changes in process definition tha.t have occurred since WRENSS was first developed for Hydrologic Region 4, the Snow Zone in the late 1970's. Table 2. Stream flow estimates for the DOS and SAS simulations are presented for each time interval along with the estimate of azea in each timber size class, by yeaz. Water Y ield (in) Acres by Timber Size and Year Date SAS DOS Non Seed/Sap Pole Saw Total 1860 13.41 14.8 397930 378594 297357 33712 1107593 1880 12.50 13.5 55385 697522 92137 262548 1107593 1900 13.08 14.4 285731 654913 156462 10488 1107593 1920 12.20 13.4 35860 833308 238424 0 1107593 1940 11.05 12.1 0 365292 709189 33111 1107593 1960 10.44 11.9 33057 193146 789152 92238 1107593 1980 10.23 12.1 133461 132 952339 21661 1107593 1997 9.97 11.8 15567 133593 339430 619002 1107593 2017 9.65 0 149161 9655 948777 1107593 Although not implernented as part of the analysis reported above, the concept of snow "scour", or deflation, from large openings was retained in the current version of WRENSS hydrologic model. Increased accumulation in forest openings is a function of aspect, because aspect influences interception loss and therefore net accumulation. However, we do not have a basis for adjusting scour as a function of aspect and have retained the generic function from the original WRENSS hydrologic model. The Rho function presented in figure 111.6 of Troendle and Leaf (1980) implies that as opening size (X axis) exceeds 5-H, the increased accumulation in the opening (Rhoh) is decreased based on the relationship that Rhoh =(1 - 0.044 * Opening size (H) + 0.581 *(1 - 1/ exp Opening size (H)). Rhoh represents the ratio of accumulation in the open relative to accumulation in the forest. T'he snow scour factor is the ratio of the snow pack, or precipitation, left in the opening after scour has occurred and that is estimated as Snow Scour Factor = Rho / Rho,,.x . Rho,,. taken from Figure 3, at 5-H is 1.357. Once the Snow Scour Factor is calculated, it is muhiplied by the estimate of precipitation in the opening to adjust that value for scour. A snow scour factor is calculated for all harvested areas that have less than 5 percent of the fully forested basal area. The scour function can be turned off if the user feels there is adequate roughness left in the opening to retain the snow pack. The snow scour function does not activate any time residual basal azea exceeds 5 percent of fully forested.