Unraveling groundwater contributions to evapotranspiration in a mountain
headwaters: Using eddy covariance to constrain water and energy fluxes
in the East River Catchment
Abstract
Despite the importance of headwater catchments 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,
physiography, and atmospheric patterns in these high-elevation regions
lead to difficulty in developing spatially-distributed characterization
of ET. As the largest terrestrial water flux behind precipitation, ET
represents a significant fraction of the water budget for any watershed.
Likewise, groundwater is the largest available freshwater store and has
been shown to play a large role in the water balance, even in headwater
systems. Using an eddy covariance tower in the East River Catchment, a
Colorado River headwaters basin, this study estimates water and energy
fluxes in high-elevation, complex systems to better constrain ET
estimates and calculate overall water and energy budgets, including
losses from groundwater. The eddy covariance method is used to estimate
ET from years 2017 through 2019 at a saturated, riparian end-member
site. Owing to complexities in near surface atmospheric structure such
as stable boundary layers over snowpack and shallow terrain driven flow
from surrounding landscape features, energy flux and ET estimates were
limited to the warm season when energy closure residuals from the
eddy-covariance system were reliably less than 30 %, a threshold
commonly used in eddy covariance energy flux estimation. The resulting
ET estimations are useful for constraining water budget estimates at
this energy-limited site, which uses groundwater for up to 84 % of ET
in the summer months. We also compared East River ET magnitudes and
seasonality to two other flux towers (Niwot Ridge, CO and Valles
Caldera, NM), located in the Rocky Mountains. This data is useful for
constraining ET estimates in similar end-member locations across the
East River Catchment. Our results show that groundwater-fed ET is a
significant component of the water balance and groundwater may supply
riparian ET even during low-snow years.