Impact of Forest Service Activities on Stream Flow

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Troendle/Nankervis/Port11 Page30 5/22/2003 in the openings did in fac;t represent a significant increase in accumulation at the watershed level (Troendle and King, 198:5) implying thei-e has been a significant reduction in winter interception loss following timber harvest. More importantly, the increases in snow pack accumulation were a significant part of the measured increase in stream flow. Troendle and King (1985) concluded that the increases in snoiv pack accumulation observed to occur in the openings, and thought by Hoover and Leaf (1965) to be due to reclistribution, actually reflected a reduction in what would have been an interception loss. Troendle and King (1985) concluded the 23 percent average increase in accumulatioii observed to coccur in the openings equated to an average 9 percent increase in snow pack ac;cumulation on the entire watershed. Other watershed experiments (e.g. Troendle and King 1987), plot studiies (Troendle and Meiman 1984,1986) and process summaries (e.g. Meiman. 1987) further (iocumented that partial cutting could result in increased stream flow through a reduction in ET that included a reduction in winter interception losses. These observations were consistent wnth conclusions already reached by Wilm and Dunford (1948) and others 40 yeass earlier. Intemeption theory replaced redistribntion theory as the basis for interpreting the effect: that forest remi)val had on snow pack accumulation. The WRENSS hydrology model (Troendle and Leaf 1980) was inrtially programmed for general use by Swanson and Bernier (1986) and used by Swanson (1987) to simulate the effect of forest management on water supply from subalpine forests. In the earlier report on "Estimating Additional Water Yield From Changes in Ma.nagement of National Forests in the North Platte Basin", Troendle and Tfankervis (20001) modified the operation of the Swanson and Bernier (1986) program to refle;ct the changes in process interpretation that had occurred since the WRENSS hydrologic niodel was initially developed (see Troendle and Nankervis 2000 for operational details). As noted above, in the original WRENSS Hydrologic model the precipitation input to the model was adjusted to reflect differential accumulations in the forest and open using the "Rho" redistribution function (see Figure II1.6, Troendle and Leaf 1980). The Rho function, as originally defined, represented the integated influence of two separate wind driven actions on snow pack accumulatioin in forest openings. First, small openings in the forest canopy were believed to change the af;rodynamics in the canopy and result in greater deposition of snow in the openings at the expense of the surrounding forest. As much as 35 percent more water equivalent could be deposited in a 5 tree height (5-H) wide opening as the result of redistribution. As the size of the opening inci•eased beyond 5-H, the efficiency of the opening to retain the trapped snow decreased as a result of increase;d scour. As the opening size approached 14-H, scour eliminated the increase in accumulation and the creation of openings larger than 14-H could result in a net loss in snow pack accumulation. The RHO function integrated two processes: differential accumulation and scour. Differential deposition, or redistribution, did not result in a change in total precipitation entering the: system while scour represented a loss in precipitation to the system. Snow scoure:d from the opeiungs is evaporated back to the atmosphere or transported out of the watershed. Ci.inent thinking concludes that the increased deposition in the openings is the result of a reduction in interception loss and not due to redistribution. However, in larger openings, losses due to scour can and cio occur as defined in the original Rho function, though roughness can be used in large openin€;s to reduce scour loss (Troendle and Meiman 1884). In the hydrologic simulatic>ns performed ELs part of the initial North Platte analysis (Troendle and Nankervis 2000) estimat:es of precipitati.on were mechanically increased to reflect the appropriate