| Bowen ratio measurements of water vapor flux were obtained at the U.S. Sheep Experiment Station near Dubois, Idaho. Three different treatments were established: controlled site, dominated mainly by shrubs and some grasses, a burned site, dominated by grasses and forbs, and a grazed site, dominated by shrubs. Management practices of burning and grazing were applied separately on the 2 sites. Burning occurred only once, in the fall of 1995. The first grazing occurred in the spring of 1997 and continued every spring and late fall from 1997 through 1999. During all 3 years of study, the controlled site had the greatest ET values, followed by the grazed site. The least water vapor flux values were observed on the burned site, presumably due to the ability of shrubs to better access water in deeper soil layers. ET values decreased during the summer at all 3 sites during all 3 study years. However, shrubs remained physiologically active and exhibited significant ET rates throughout the summer, while grasses at the burned site at the end of the summer senesced. Critical factors which governed water vapor flux exchange at all three sites during 3-year study were available energy, canopy resistance, surface conductance and leaf area index (LAI).; The effects of intermittent precipitation events on water vapor flux were determined for the above-mentioned treatments. Significant responses to precipitation events were observed, although the magnitude of the response varied with season and among the 3 sites. The greatest seasonal response of water vapor flux to precipitation events was observed at the controlled site, followed by the grazed and burned sites. This may be due to shrubs developing deep root systems and utilizing water from deep soil depths. Variations in precipitation appeared to affect ET of the three communities differently.; The use of the Shuttleworth-Wallace (SW) Two-Component Model was useful in understanding the main parameters governing water vapor flux exchange among the three study sites. After the model was applied, computed ET results at all three sites during the 3-year study were similar to measured ET results obtained by Bowen ratio (BR) system. The SW Model showed sensitivity to soil moisture, LAI, and canopy and surface resistances, with less sensitivity to mean boundary layer and aerodynamic resistances. |
