4.1 Drivers of inter-annual variability
Inter-annual variability in the timing and magnitude of the melt pulse appears somewhat, though imperfectly correlated with metrics of precipitation and snowpack. This is likely due to the complex nature of how precipitation translates to runoff in this catchment, and the difficulty in characterizing something as nuanced as snowpack and a melt pulse with singular metrics. Annual WY was only weakly correlated with annual precipitation (Figure 6a), and constituted only around 30-40% of the latter. This difference is the result of sublimation and evapotranspiration (ET), and is consistent with findings in adjacent catchments (Sueker et al., 2001). Maximum snowpack depth was not correlated with annual precipitation (not shown, p = 0.71). This can partially be explained by remembering that around a third of the annual precipitation falls as rain, and does not contribute to the snowpack (and may even accelerate melting of the snowpack). This fraction also changes from year to year. For example, 2020 experienced relatively heavy snowfall followed by a relatively dry summer, with 76% of the annual precipitation falling as snow. In contrast, the reverse was true in 2021 and this fraction was only 58%. Even then, the maximum snowpack is not a perfect reflection of cumulative snowfall; the region is famous for its Chinook Winds, and some fraction of the snowpack may sublimate.
Timing also appears critically important. For example, 2020 saw rapid early snowpack accumulation, and late season storms pushed the maximum depth to near-record levels in late April (Figure 7b). However, it began to rapidly melt and was gone by mid-May. In contrast, 2021 saw near record low snowpack early in the season and a lower maximum depth, but it ultimately persisted into June. Persistence of the snowpack later into the spring appears to be an important factor in generating a larger melt pulse and a higher annual water yield (Figure 7c). Thus, while annual WY shows some correlation with maximum snowpack depth (Figure 6b), the correlation with the date the snowpack remains above 10 cm is stronger (Figure 6c). Persistence of the snowpack later into the spring appears a critical factor in generating larger melt pulses. This may be because a greater volume is melting and becoming runoff, versus losses earlier in the season which are more to sublimation.
These larger melt pulses in turn seem to transport a larger mass of DOC (Figure 6e,f). This correlation was less apparent for NO3-N mass flux in the years with high frequency data. This potentially has to do with differences in the seasonality of NO3-N vs DOC concentrations, with the latter peaking several weeks earlier than the former, and thus less temporally aligned with the Q peak. It is also worth noting that NO3-N is only one component of dissolved nitrogen. Grab samples revealed concentrations of NH4-N often comparable to NO3-N, and by difference suggest the largest component of TDN may be dissolved organic nitrogen (DON). TOC and TN values were almost identical to DOC and TDN, suggesting minimal particulates. Interestingly, TDN concentrations appeared to peak later in the spring than NO3-N, closer to the annual peak Q. The NEON grab sample dataset is admittedly short (and COVID-19 mitigation protocols further limited grab sampling in much of 2020), but historic samples collected by the LTER seem to confirm this (Supplemental Figure 1c). Flux of TDN is likely to show greater alignment with Q, and respond more strongly to inter-annual variability in the melt pulse (Figure 6f).
Looking at extremes from the historical record (Supplemental Table 1), 2011 had the highest annual precipitation, the deepest maximum snowpack, the latest centroid of the melt pulse, and nearly the highest annual water yield. Unsurprisingly, this resulted in the greatest annual export of DOC and TDN. Conversely, 2012 had the lowest annual precipitation, the shallowest maximum snowpack, the earliest centroid of the melt pulse, and the lowest annual water yield. Again unsurprisingly, this resulted in the lowest annual export of DOC, and nearly the lowest of TDN. Similar to what we observed in the high frequency data, years with below average maximum snowpack (e.g., 2010) can still produce above average annual water yield and in turn above average solute fluxes, but only when the snowpack persists later into the spring.