4. Discussion
To better understand the variability in precipitation and ET across the
basin over the three water years, we calculated the water balance at the
East River valley location using the equation,
\(residual=P-\text{ET}-Q-S\), Eq. 4
where ET is total evapotranspiration, P is precipitation,Q is the runoff at the outlet of the East River basin co-located
with the flux tower (Carroll & Williams, 2019), and ∆S is the
change in soil moisture in the top 30cm of soil (Table 6). For water
year 2017, ET is only available for April 15 through September 30. For
the remaining water years, ET is available for the full water year with
some interspersed data gaps. Using the daily ET values, we also
gap-filled the ET data by taking the linear average ET of the days
surrounding the missing data to get an understanding of the range in ET
at the end-member site. Q is calculated for the full water years
based on a basin area of 85km2. P represents
the three full water years and we used values from both the Billy Barr
meteorological station as well as from the Schofield SNOTEL station
located at the northern edge of the watershed at approximately 3260m in
elevation and 13.4km northwest of the flux tower and discharge outlet
(Natural Resources Conservation Service). We also calculated an
effective precipitation at each station to account for sublimation from
the canopy, surface, and blowing snow, which is not captured by the flux
tower. We left the summer precipitation unchanged and removed 28% of
precipitation in the winter based on the percentages of winter
sublimation values in open, forested, and alpine sites by Sextone et al.
(2018). ∆S is equal to the values found in Section 3.2 (Table 4).
While the flux tower is useful in estimating ET in complex, headwaters
regions, it is an uncertain variable in the water balance. When ET is
gap-filled, we see an increase in ET of 17%, 22%, and 13% for water
years 2017, 2018, and 2019, respectively. This increase in ET has
significant impacts on the residual of the water balance, increasing the
residual when the system is losing water and decreasing the residual
when the system is gaining.
This variability can also be seen in the precipitation. In 2017, the
Billy Barr station records precipitation that is approximately half of
what is registered at the Schofield station while 2019 has a difference
of 555mm of precipitation. This variability could have large
consequences as it alters the amount of water available in the system
depending on the station used for water balance calculations (Table 7).
This variability in precipitation and ET is evident in the residuals of
the calculated water balance for the basin (Table 7). When using Billy
Barr precipitation in 2017, the system is losing water. However, when we
use Schofield precipitation that same year, the system is gaining water
for both the original and gap-filled ET values. Though water years 2018
and 2019 are both losing water, the difference in the amount of water
lost is large between using Billy Barr precipitation and Schofield
precipitation regardless of ET estimations. The difference in residuals
when using the original ET as opposed to the gap-filled ET is 66.13mm,
98.48mm, and 54.79mm for water years 2017, 2018, and 2019, respectively
at both Billy Barr and Schofield. This increase in ET using the
gap-filled values leads to greater water usage and a decline in the
residual resulting in less water available per year.
The variability in residuals due to variability in precipitation and ET
has large implications for water balance estimations and water use in
the East River basin. If the water balance shifts by 55mm to 100mm per
year based on ET estimations, these estimations become vitally important
for water availability estimations. We also only have ET estimations at
one location in the basin, which is not very near either of the
precipitation gages. The precipitation data is equally important as it
can swing the basin from a gaining system when using one set of data to
a losing system when using data from another location as seen in the
residuals from 2017, which was considered a wet year for this basin.
This variability makes the water balance difficult to estimate across
the entire East River basin without access to more data highlighting the
need for meteorological stations for precipitation and eddy covariance
towers for ET estimations to constrain the variability in these fluxes.