3.2 Summer Energy and Water Flux Comparisons
Most of the annual ET occurs over the summer months (Table 3), and
closer errors are lowest during these times, so we then compared
datasets of summers (June-August) 2017, 2018, and 2019. We averaged
latent heat, total net radiation (Rn-G), air temperature, and dewpoint
depression for each summer using the same dates for all variables across
each summer. We calculated ET (latent heat flux multiplied by the latent
heat of vaporization) for all three summers along with the total ET for
the water year (water year 2017 is shortened and begins when we
installed the tower in April 2017). This table includes the total
precipitation for each water year, which was obtained from a
precipitation gage located within the East River basin at an elevation
approximately 200m higher and 6.2km northwest of the flux tower (Billy
Barr station). Average latent heat flux is greatest during the summer
2018 with summer 2019 having the lowest average latent heat flux (Table
3). The table shows that the differences between summers of available
energy, ET, latent heat, and air temperature are much less across the
three summers than the differences in precipitation. 2019 had the
greatest precipitation with the year of lowest precipitation, 2018,
being 400mm less. Total summer ET does not follow the same patterns with
the largest ET value occurring during the lowest precipitation year.
Similar to the findings of Scott et al. (2008), ET is not as variable as
precipitation over the three study years. While precipitation varies by
30% across the three years, ET varies by only 14% suggesting that
precipitation is not the primary driver (or limiter) of ET in this
system and confirming that this site is generally a saturated end-member
site, which is not water-limited for ET.
We then compared these summers on a time series (Figure 6). All three
summers have similar monthly patterns and magnitudes of ET showing
overall site consistency across three water years despite large
differences in precipitation (Figure 6b). However, ET is at times
inversely correlated to soil moisture at both 5cm and 25cm (Figure
6c,d). For example, figure 6d shows soil moisture at 5cm and we see that
May and June respond to snowmelt with the highest average soil moisture
in 2019. However, ET for May and June of 2019 is the lowest of the three
summers. Available energy also does not correlate perfectly with the
patterns seen in ET across the three years (Figure 6a). The years with
greatest average monthly available energy do not always result in years
with the greatest average monthly ET. This data, along with the data
shown in table 3, seems to suggest that ET and soil moisture are, at
times, inversely correlated indicating that water for ET comes not just
from shallow soil moisture, but also from other sources such as deeper
groundwater or ponded water.
The drying of soil moisture throughout the summer can also be observed
in the May through September groundwater use values (Table 4). This is a
critical time for vegetation as the spring snowmelt recharges soil
moisture and groundwater and this plant water source is drained by
vegetation until the availability of summer rainfall. There is
substantial evidence that plants also access groundwater, particularly
during this time in mountain systems (Bearup, Maxwell, Clow, & McCray,
2014). Using an equation from Scott et al. (2008) (Equation 3), we can
estimate how much ET comes from groundwater over the summer for years
2017, 2018, and 2019 as follows:
\(\text{ET}_{\text{gw}}=ET-(P-S)\), Eq. 3
where ETgw is ET from groundwater, ET is
total evapotranspiration, P is precipitation, and ∆S is
the change in soil moisture in the top 30cm of soil from May to
September. Runoff is assumed to be negligible since we are calculating
ETgw over the area of the eddy flux tower footprint.
Positive ETgw indicates ET is greater than precipitation
and soil moisture change resulting in ET drawing from older groundwater.
The ETgw values for all three summers are positive
indicating that groundwater supplies a fraction of ET regardless of
precipitation. ET drew most heavily from groundwater in the summer of
2017 (76.2%) closely followed by 2019 (75.5%), with the summer of 2018
having the least amount of groundwater use. While 2018 had the least
precipitation annually (Table 3), much of the annual precipitation
occurred during the summer months leading to less groundwater use than
the other two summers showing a reliance on rainwater for summer 2018
rather than a reliance on groundwater due to snowmelt. These summers
offer insight into the variability of ET groundwater use across water
years. Though the magnitude of precipitation is crucial for ET, the
timing of precipitation across the year dictates whether ET needs to
draw from groundwater as in 2017 and 2019 when ET seems to use water
from snowmelt, or whether ET coincides with summer rain as in the case
of 2018. This shift in water sources is suggested by simulations (Bearup
et al., 2014; Kollet & Maxwell, 2008; Maxwell & Condon, 2016; Maxwell,
Condon, Danesh-Yazdi, & Bearup, 2019) and corroborated by shallow
groundwater observations on a hill slope adjacent to the tower site that
confirm that vegetation would have access to groundwater along riparian
flood plain (Tokunaga et al., 2019).
Without access to groundwater, ET values would decrease substantially in
2017 and 2019 as the only contributing water, we estimate, would be from
precipitation and soil moisture resulting in 71.82mm and 65.21mm of ET
for summers of 2017 and 2019, respectively. These ET values are what we
might expect as a low end-member at higher elevations on ridge tops
where land-surface energy processes may be disconnected from groundwater
(Kollet & Maxwell, 2008; Maxwell & Kollet, 2008), and may not have as
much access to groundwater as the flux tower location, which sits in a
convergent zone. While subject to uncertainty, these estimates indicate
that groundwater may increase ET values by up to 76% making it critical
to better constrain these higher elevation water fluxes. As soil
structure and soil moisture conditions vary across the East River basin
it is important to provide additional observations of ET to constrain
this variability.