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Troendle/Nankervis/Porth Page28 5/22/2003 <br />interception loss. Consequently, water available for stream flow, (estimated as precipitation <br />minus evapotranspiratioii) is also reduced. However, the original intent of the entire WRENSS <br />procedure was to estimate the change iri response that would occur as the result of silvicultural <br />activities and in this cor.?text the currenl: version of the WRENSS hydrologic model appears to <br />have been improved. The evolution in hydrologic thinking that resulted in the evolution of the <br />WRENSS hydrologic mcidel, as used in t:his report, is presented below. <br /> 100% <br /> <br />= 95% <br />0 <br />:r <br />.? 90% <br />'a <br /> <br />? 85% <br /> <br />a <br /> <br />y $0% <br />O <br /> 75% Ix • - • - - - <br />c <br />v 70% •9K <br />? <br />a <br /> 65% <br /> <br /> 60% <br /> 0% 20% 40% 60% 80% 100% <br /> PF:rcent Complete Hydrologic Utilization (BAIBAmax'?100) - <br />Figure l. Percent gross precipitation reaching the forest floor as a function of stand density by <br />cover type and aspect. <br />Historically, interpretations drawn from the results of the hydrologic research at the Fraser <br />Experimental Forest (FEF) in central Colorado have had a significant influence on the <br />development of hydrolc?gic tools (models) used to estimate flow change following forest <br />disturbance. The most significant factor influencing the pathway that hydrologic model <br />development has taken h,?s been the shifts in our understanding of how forest disturbance affects <br />snow pack accumulationt processes. Unlike most other hydrologic regions in the USA, forest <br />disturbance in the Central and Northern Rocky mountains has been shown to greatly alter what <br />would be winter evaporative losses and this reduction in evaporative process represents a major <br />component in the observe;d flow change. <br />Initial studies on the effects of timber harvest on snow pack accumulation (e.g. Wilm and <br />Dunford 1948, Meiman 1987) indicated that virtually all reductions in stand density resulted in <br />some increase in snow ??ack accumulation. These increases in snow pack were considered to <br />reflect a reduction in whit would have t?een an interception (or vaporization) loss and presumed <br /> <br /> <br /> <br /> `_X--- - <br />-?- Aspen: All Aspects <br />• • • - Conifer: East/West <br />- -? - Conifer: Nort:h <br />- - X - - Conifer: South \ ? ?, <br /> <br />?--- -