3.3 Comparisons to East River Basin and Rocky Mountain Headwaters
To estimate the amount of water leaving the system through latent heat flux, we calculated evapotranspiration using the latent heat from the tower and the latent heat of vaporization. We compared our eddy flux estimates with those from another flux tower in the central Rocky Mountain region, operational since 1998, located at the Niwot Ridge Ameriflux site (US-NR1) in Colorado about 174km northeast of the East River Basin (Burns et al., 2015; Monson et al., 2002) (Figure 7). The daily ET values at the East River site compare well with those at Niwot Ridge with both locations having obvious seasonal cycles with ET increasing as the snow melts in the spring, reaching a peak in summer, and decreasing as both water and energy availability decrease in the fall and winter. ET is greatest during the summer of 2018, followed by summer 2017, then 2019 in the East River location. Both locations are high-mountain environments, though the East River has more heterogeneous land cover, as Niwot Ridge is characterized by an evergreen needleleaf forest, and the East River has greater water availability throughout the year as the flux tower sits in a saturated valley 300m lower than the Niwot Ridge tower.
We also compared the East River data to the Valles Caldera AmeriFlux tower (US-Vcp) (Table 5) located in southern Rocky Mountain region in north-central New Mexico in the Jemez River Basin. This site is characterized by Ponderosa Pine and Gambel Oak and sits below the flux tower at an elevation of 2500m (Litvak, 2007). Comparison of the East River, Valles Caldera, and Niwot Ridge flux towers allows for scaling estimates of ET across the rest of the East River basin as these three locations represent locations typical of Rocky Mountain headwaters catchments as well as a variety of elevations and land covers similar to those found at various locations inside the East River basin, which is mostly characterized by shrub/scrub, evergreen forest, barren land, and deciduous forest.
The Valles Caldera has the greatest ET both annually and seasonally across all three years of study. Valles Caldera receives the least amount of cumulative precipitation across the three years, but this location has the greatest energy input shown in the largest temperatures leading to larger ET values. East River ET is the next largest for summer (June-August) ET in all years except 2017 where Niwot Ridge is slightly larger; however, Niwot Ridge annual ET is greater than East River ET in 2018 and 2019 while East River annual ET is greater in 2017. Niwot Ridge had the greatest annual ET in 2019 while the summer ET was greatest in 2017. In 2019, Niwot Ridge had lower overall summer values, but they remained consistent from April to September making 2019 the year with the largest annual ET at Niwot Ridge, whereas 2017 experienced larger summer values with lower values in the surrounding months making 2017 the year with the largest summer ET. Both the East River and Valles Caldera experienced the greatest annual and summer ET during the same years (2018 and 2019, respectively).
The maximum summer ET values seem to correspond to the years with greatest precipitation at the two sites outside of the East River. Both Valles Caldera and Niwot Ridge seem to be moisture limited sites with the amount of precipitation and available water dictating the maximum ET in the dry summer months, whereas the East River site is more dependent on energy as it is located in a saturated end-member location and variations in precipitation have less effect on ET than variations in energy.