2.1 East River Region
In our study, we installed an eddy covariance tower in the approximately 85km2 East River watershed near Crested Butte, Colorado (38.922242, -106.949699). The East River is part of the headwaters of the Upper Gunnison River, which is a primary tributary to the Colorado River, provides much of the flow to the Upper Colorado River Basin, and is a site typical of high-elevation watersheds with minimal human disturbance. Due to the basin’s influence on water availability in the west, it is a U.S. Department of Energy Scientific Focus Area (DOE-SFA) where many researchers are studying earth systems processes from the bedrock to lower atmosphere to better understand the hydrologic behavior mountain watersheds (Hubbard et al., 2018). It is a semi-arid, snowmelt-dominated watershed where much of the downstream water comes from the snow stored in these mountains during the winter. The basin’s land cover is highly heterogeneous with about 38% classified as shrub/scrub, 23% as evergreen forest, 13% as barren land, 11% as deciduous forest, 7% as grassland, and the remainder split between 10 other land cover types classified by the NLCD 2011 (Yang et al., 2018) (Figure 1). The tower sits in a valley at an elevation of about 2800m with mountain peaks on either side that reach about 3700m in elevation. The East River runs through the valley next to the tower and the site is covered in snow for six months of the year and, if it is a wet year, the ground is saturated for about one to two months in early summer.
Due to the complexities of eddy covariance inherent in mountain regions, we located the tower to minimize terrain influences, provide a consistent fetch, and capture the upper-bound floodplain ET behavior (Figure 1b,c,d). The tower was oriented so that the tower would mainly capture turbulence coming from the dominant, northwestern (up-valley), wind direction (Figure 2). The valley also gives saturated, end-member observations as evidenced by the soil moisture data throughout the study period, which shows the subsurface is saturated, has standing water or is near saturation for large fractions of the year (Figure 3). Observations at this end-member site allow us to constrain the upper values of latent heat flux in the basin given that this area is going to be the area with the greatest potential ET.