Eddy Covariance in a Mountain Headwaters: Evapotranspiration Estimates in a Tale of Two Limitations

Abstract:

Despite the importance of headwater basins for western United States’ water supply, these regions are often poorly understood, particularly with respect to quantitative understanding of evapotranspiration (ET) fluxes. Heterogeneity of land cover, topography, and atmospheric patterns in these high-elevation regions lead to difficulty in developing spatially distributed characterization of ET. As a significant fraction of the water budget, ET contributes to overall water and energy availability in the basin and understanding the limitations on ET also leads to better estimations of ET in these complex regions. Using an eddy covariance tower in the East River Basin, a Colorado River headwaters basin, along with sap flow meters and meteorological stations, this study improves the quantification of water and energy fluxes in high-elevation, complex systems to better constrain ET estimates and calculate overall water and energy budgets, along with the factors limiting ET. The eddy covariance method estimates ET from years 2017 through 2019 at a saturated, riparian end-member site. During the late spring, summer, and early fall months, evidenced by a less than 30% energy balance closure error in these months (within the range of closure error reported at other riparian locations), we conclude that the eddy covariance method is useful in high-elevation, complex areas such as the East River Basin and helps bound regional ET estimates. East River ET estimations are useful for constraining water budget estimates at this energy-limited site, which uses groundwater for up to 76% of ET in the summer months. The flux tower data is useful for constraining ET estimates in similar end-member locations; however, to better constrain ET estimates across the entire East River basin, data from sap flow meters and meteorological stations is used to scale ET across higher elevations. This data results in estimates of ET across the elevational gradient of the basin, which are useful for understanding the shifts in the basin between energy-limitations and moisture-limitations on ET. This study helps constrain both the energy and water budgets, as well as the underlying hydrological limitations, in locations that are underrepresented by observations and where indirect estimates of ET may perform poorly.