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.