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.