EIS Arapahoe & Roosevelt National Forest, Pawnee National Grassland

Home Browse Search

Aspect - Each combination of vegetation regime and treatment was modeled for each aspect. Because aspect is a discrete rather than a continuos variable, a a weighted mean average caanot be calculated. The model is also very sensitive to aspect. We modeled water yield for each aspect and then calculated a weighted mean water yield based on the area of a vegetation type occupying each aspect (thie inforniation can be obtained from GIS and RIS databases). Precipitation - We digitized the isohyetal lines from the "Colorado Average Annual Precipitation Map, 1951-1980" (Colorado Climate Center, 1984) into our GIS. We then used GIS to determine the area of each vegetation type within each precipitation zone (e.g. 16-20 inches, 20-25 inches) and detennined a mean weighted annual precipitation as described above for aspect. The model requires monthly precipitation. To determine this, we gathered monthly precipitation data from.SNOTEL sites within or adjacent to the Forest, converted each monthly value to a percentage of annual precipitation and then averaged the values. We then used the monthly distributions and the weighted mean annual precipitation for each of the vegetation types to calculate monthly precipitation input for the model. Chuck Troendle suggested that rather than calculating a mean weighted annual precipitation for each vegetation regime, WRENSS should be run for every precipitation regime for a given timber type and the resulting water yield should be weighted by area. He suggested calculating water yields using this method for one or two timber types and comparing the results to those provided by the alternate method. When we did this for west slope and east slope lodgepole pine, we found that predicted water yield increased by 2* for the west side and decreased by S!k for the east side. We concluded that any changes `_.:7° ?'i::E:z_°g_=_ater tirecisicn did zot warrant the significantly greater data input which would be required. The WRENSS procedure adjusts precipitation and evapotranspiration based on the windward width of the harvested opening in relationship to the surrounding forest canopy. Changes in aerodynamics above the canopy cause increased snow deposition into the harvested areas. The model predicts that deposition will increase with increased opening size until the windward width is equal to approximately five tree heights. Deposition then begins to decrease until it returns to pretreatment conditions at approximately 13-14 tree heights in width. For openings larger than 24 tree heights, WRENSS predicts a reduction in snow retention as snow scour increases (see WRENSS Fig. III-6). This adjustment can be made in the model by adjusting the "Windward Width" variable. Troendle recommended another approach. He said that more recent research in both Canada and the U.S. indicated that if sufficient roughness were left in harvest units to retain snow, harvest units of any size would accumulate additional snowpack, and that the amount of increased snow retention was primarily a function of aspect. He suggested manualiy increasing the post-harvest precipitation by 20%r for south aspects, 30% for east-west aspects, and 401r for north aspects. Because the model does not allow for the adjustment of post-harvest precipitation (except through the indirect adjustment of windward width), this adjustment requires that the model be run twice for each vegetation regime.