1. Background
Mountain headwater regions provide much of the world’s water (Chang, Foster, Hall, Rango, & Hartline, 1982; Ikeda et al., 2010) and in the western United States, the Colorado River provides water for 30 million people and 4 million acres of irrigated agriculture (Reclamation, 2012). However, many western watersheds lose more water to the atmosphere through evaporation and transpiration (ET) than is discharged into rivers and streams (Knight, Fahey, & Running, 1981; Trenberth, Smith, Qian, Dai, & Fasullo, 2007). In the Western U.S., ET is often the second largest component of the water balance, after precipitation, and is also associated with a watershed’s energy balance making accurate estimations of this flux vitally important to overall water availability predictions (Healy, Winter, LaBaugh, & Franke, 2007) as well as regional energy budgets. A recent study by Milly and Dunne (2020) estimates that the mean annual discharge of the Colorado River is decreasing due to increased evapotranspiration while another study shows similar trends in the Alps with an air temperature increase causing an increase in ET leading to a decrease in runoff consistent with a 3% precipitation decrease (Mastrotheodoros et al., 2020). Both studies reveal the importance of ET flux to overall water availability. However, ET is challenging to quantify in headwaters regions as these regions are often marked by complexities such as heterogeneous land cover, large topographic gradients, and spatially varying atmospheric patterns. Due to these complexities, few studies have tested the effectiveness of measuring ET in complex headwaters regions (Flerchinger, Marks, Reba, Yu, & Seyfried, 2010).
Eddy covariance is a commonly used technique for quantifying latent heat—equivalent to ET multiplied by the latent heat of vaporization of water at a given temperature— and often used interchangeably with ET in this paper (Ceperley et al., 2017; Hirschi, Michel, Lehner, & Seneviratne 2017; Maes, Gentine, Verhoest, & Miralles, 2019; Mamadou et al., 2014). This method computes the flux as the covariance between deviation from the mean of the vertical wind speed and deviation from the mean of the concentration multiplied by the air density (Burba, 2013). This method is effective in flat terrain with homogenous land cover due to the assumptions that must be made using this technique, including the lack of surface heterogeneity, adequate fetch, and turbulent fluxes (Burba & Anderson, 2006). This then leads to difficulty in predicting how these water and energy fluxes, and overall water availability, will change with a changing climate.
Despite the challenges outlined above, some studies have successfully used the eddy covariance in high-elevation environments. Burns, Blanken, Turnipseed, Hu, & Monson (2015) studied how changes in precipitation affect the overall surface energy budget in a high-elevation forest while Frank, Massman, Ewers, Huckaby, & Negrón (2014) studied how disturbances, such as bark beetles, shift the energy balance components in the Rocky Mountains. Complimentary studies have also used eddy covariance in western riparian areas leading to helpful end-member ET values, which range from 10 to 30% surface energy balance closure error (Nagler et al., 2005; Scott et al., 2004; Scott et al., 2008; Wilson et al., 2002). Through the measurement of these fluxes and thus better estimations of the water and energy balances, these studies allow for a better understanding of energy fluxes in mountain environments and result in data sets that can be used to check model performance.
In this study, we improve the quantification of water and energy fluxes in high-elevation, complex systems, we estimate groundwater contributions to ET, and we compare our results to other sites in the Rocky Mountain region. Our study site, a basin that contributes to the Upper Colorado River Basin and eventually the Colorado River, provides an archetype of other Rocky Mountain basins. Our study period includes multiple water years characterized by both wet and dry conditions resulting in ET estimations useful for application across the entire basin and to other headwaters regions. This study leads to greater understanding of water availability in headwaters systems, which can then be used for ecological and hydrological applications and downstream water planning.