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Abstract It is well known that snow provides the major input of water into Rocky Mountain watersheds. Furthermore, we know that over 90 percent of summer rainfall in such watersheds is normally intercepted by the forest canopy and litter (Reynolds and Knight 1973). Thus, a study on the hydrologic cycle in the Wyoming Rockies is quite naturally focused on the fate of snowmelt water. In this report we further evaluate the effect of forest vegetation on the movement of that water. Interception and transpiration, which contribute significantly to annual evapotranspiration, were estimated for a stand of lodgepole pine forest in the Medicine Bow Mountains of Wyoming. The results have significance to the mechanism of nutrient loss from an ecosystem.
Some ecologists have calculated the proportion of snow-water that is evapotranspired from a watershed by subtracting annual output (streamflow) from annual input (total precipitation). Such studies are very useful but they view the watershed as a homogeneous unit which is usually not accurate, they do not separate evaporation from transpiration, and sometimes the approach is not possible because of permeable bedrock. In general the watershed approach does not provide accurate information on the effect of the vegetation on water movement unless the vegetation is partially removed, e.g. by clearcutting, which is often not practical or permissible.
Another approach focuses on water movement in stands rather than watersheds. Input to stands is easily measured also, but output is difficult and calls for the estimation of transpiration. Subtracting transpiration from snow-water input provides an approximation of evaporation plus soil storage plus surface and subsurface runoff. Obviously the stand approach has limitations also, but we adopted it because 1) we did not want to treat watersheds as a homogeneous unit; 2) we wanted to evaluate the significance of forest structure, soil depth, and soil waterholding capacity on snowwater movement and nutrient leaching; and 3) we wanted to study transpiration, not evapotranspiration.
Water movement through a watershed can be described in general as in Fig. 1. The water in precipitation falls directly on other bodies of water (lakes, streams), on vegetation surfaces, on litter, or directly onto the soil where it may be transpired or move as subsurface flow to the lakes and streams. Surface flow to lakes and streams may also be important. Fig. I illustrates how the vegetation modifies water movement primarily by increasing the surface area for interception (whether by live vegetation, dead vegetation, or litter) and by allowing for transpiration. Fig. 2 from Molchanov (1960) shows how significant evapotranspiration may be in a forest; the major pathways for water movement would be considerably different in the absence of vegetation.
The effect of coniferous vegetation on snow-water is different than on rain however, primarily because of the season that they occur and the physical structure of snow compared to rain. For the most part, rain, in contrast to snowfall, occurs at a time when the trees are transpiring. Canopy interception (and subsequent evaporation) of snow is probably minimal in the Rocky Mountains because the snow is usually light and easily blown from the branches to the ground (Hoover and Leaf 1967), and because the proportion of the snowwater intercepted by the litter is small (Reynolds and Knight 19731 Evaporation of snowmelt water is minimal. By contrast, most of the rain is intercepted by the canopy or litter and promptly evaporated. (Reynolds and Knight 1973). The significance of this is that nearly all transpired water comes from snowfall and "the excess of snow-water that is not transpired or held by the soil serves as the vehicle for nutrient loss by leaching from the stand. In coniferous forests where transpiration may occur in the spring while there is still snow on the ground (Swanson 19671 Owston et al. 1972), the amount of vegetation may have a large effect on the amount of spring transpiration and, indirectly, on the rate of nutrient loss during the snowmelt or "spring-flush" period. From this line of reasoning a hypothesis emerges, namely that soil waterholding capacity and spring transpiration (while there is still snow on the ground) are important regulators of nutrient leaching from Rocky Mountain coniferous forests. Fig. 3 is a diagram which defines the type of data needed to test the hypothesis. Our research has been organized in part to quantify that diagram, thereby clarifying the effect of vegetation on both water and nutrient movement in a lodgepole pine forest ecosystem.
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