Impact of Forest Service Activities on Stream Flow

Home Browse Search

Troendle/Nankervis/Porth Page29 5/22/2003 to directly influence stream flow. Although the greatest increases in snow pack observed under partially cut stands were proportional in magnitude to increases observed in clear-cuts in both Lodgepole Pine and Spruce Fir stands. The initial conclusion drawn by Wilm and Dunford (1948) and others were that the increases in snow pack were the result of savings in what would have been interception losses from the canopy and that any significant reduction in forest canopy would result in an increase in snow pack accumulation. However, after observing snow transport processes in the clear cuts and surrounding forest on the Fool Creek watershed at FEF, Hoover and Leaf (1965) concluded that creating small openings in the forest canopy altered the aerodynamics of the stand and resulted in greater deposition of snow in the openings at the expense of that deposited in the surrounding forest. They felt that the increases in accumulation in the openings reflected a differential accumulation, or redistribution, process rather than a reduction in what would have been an evaporative loss. Hoover and Leaf (1965) assumed that the evaporative rate from snow was so low in the wintertime that evaporation of intercepted snow could not be great enough to account for the increases in accumulation observed on the ground in the openings. Since the increased accumulation in the opening was considered to be at the expense of deposition in the surrounding forest Hoover and Leaf (1967) concluded there was no net change in precipitation at the watershed, or analysis, level. Initial measurements of the average water content in the snow pack accumulated on the Fool Creek watershed, following timber harvest, did not indicate a change in water equivalent occurred, further supporting the Hoover and Leaf (1967) argument. The hydrologic model developed during this era, WATBAL (Leaf and Brink 1973), operated on the assumption that the aerodynamics of the stand were altered following the creation of small clear cuts and that these openings captured more snow pack as a result of that change. Because the growing season ET losses were reduced following tree removal, it was further assumed the increased water equivalent in the snow pack in the openings would be delivered to the stream channel more efficiently than if it ha.d been deposited in the forest where it would be subject to ET losses. Partial cutting, or thinning, of the forest was not believed to be effective in significantly increasing stream flow because it was assumed there would be no redistribution of snow and because it was also assumed that any reductions in soil moisture depletion that did occur under the reduced canopy would be used by residual vegetation. T'he eazlier observations by Wilm and Dunford (1948) and others documenting that increases in snow pack accumulation did occur under partial cutting were either ignored or attributed to other causes. In the late 1970's, the WRENSS hydrologic model (Troendle and Leaf, 1980) was developed for the Snow Zone (Hydrologic Region 4) of the Central and Northern Rocky Mountains was developed using the WATBAL model (Leaf and Brink 1973). Because the nomographic relationships presented in the WRENSS hydrologic model were developed from simulations using a calibrated WATBAL model, the WRENSS procedure appeazed to accurately simulate change. Accuracy or precision in the prediction of the absolute value, such as total stream flow, was of less concern in the WRENSS procedure than was the prediction of change. The logic was presented in the original document (EPA 1980). As the period of post treatment stream flow and snow pack observations from Fool Creek lengthened, subsequent analysis indicated that increases in snow pack water equivalent observed