Unravelling groundwater contributions to evapotranspiration and constraining water fluxes in a high‐elevation catchment

Abstract

Despite the importance of headwater catchments for the water supply of the western United States, 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, we estimated 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. We used the eddy covariance method 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. These data are 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.

Evaluation and Optimization of Snow Albedo Scheme in Noah‐MP Land Surface Model Using In Situ Spectral Observations in the Colorado Rockies

“…The second model parameter transferability assessment site is referred to as the “East River” pumphouse eddy‐covariance/energy balance (EC/EB) study site, located approximately 6.5 km northeast of the town of Crested Butte (14.3 km east‐northeast of the Irwin site) at an elevation of 2,762 m (9,061 ft.). The East River tower was installed as part of the Department of Energy East River Watershed Science Focus Area program and was designed to characterize land‐atmosphere exchanges of radiation, energy and water from a high‐mountain riparian area (Ryken et al., 2021). As is typical of many EC/EB tower installations, a single four‐way radiometer (Kipp and Zonen CNR‐4) was installed at the top of the EC/EB tower at a height of 6 m above the ground surface.…”

Resolving streamflow diel fluctuations in a small agricultural catchment with an integrated surface‐subsurface hydrological model

“…For example, the cumulated ET over the simulation period achieved about 280 × 10 3 m 3 of water when accounting for oxygen stress in a catchment of 2.67 km 2 but it increased by about 100 × 10 3 m 3 of water without accounting for oxygen stress. These figures highlighted the crucial significance of ET for water budget calculations (Knight et al, 1981; Ryken et al, 2022; Trenberth et al, 2007).…”

“…The European Alps are experiencing lusher vegetation but drier streams due to decreases in precipitation (P) and increases in evapotranspiration (ET) (Mastrotheodoros et al, 2020). As P and ET can be the largest components of the water balance (Ryken et al, 2022), the crucial role of ET in driving low flows during meteorological droughts has long been recognized (Bosch & Hewlett, 1982; Czikowsky & Fitzjarrald, 2004; Haslinger et al, 2014). Indeed, during the recent summer droughts, ET contributed to the depletion of soil water storage in Europe (Teuling et al, 2013) as well as in the United States (Floriancic et al, 2021).…”

Constraining Bedrock Groundwater Residence Times in a Mountain System With Environmental Tracer Observations and Bayesian Uncertainty Quantification

“…Upwelling of bedrock groundwater to high‐conductivity zones at the bottom of the hillslope has been simulated numerically (W. P. Gardner et al., 2020). Groundwater discharge to these reservoirs has been shown to contribute to catchment evapotranspiration fluxes (e.g., Ryken et al., 2022) and streamflow generation processes (e.g., Hale et al., 2016). It has further been suggested that groundwater reservoirs characterized with premodern residence times can produce hydrologic and ecosystem resilience during drought conditions (Meyers et al., 2021; Singleton & Moran, 2010).…”