3. RESULTS
The annual snowmelt pulse typically began in early May, peaked in early
June, and had subsided by early July (Figure 2). Seasonal increases in
concentrations of DOC and NO3-N appeared to precede the
melt pulse by several weeks, typically peaking in April to May and then,
especially in the case of NO3-N, already declining well
before Q peaked in June. Similar temporal patterns were observed in the
historic data collected by the LTER (Supporting Figure 1).
There was considerable
interannual variation in the timing and magnitude of the snowmelt pulse
among the four study years (Table 1). For example, the centroid of the
pulse in 2019 was over 3 weeks later than in 2018, and the water yield
for 2019 was over 50% higher than 2018. The historical LTER records
showed similar interannual variation (Supporting Table 1).
The annual flux estimates from downsampled sensor data were not
substantially different from those made using the whole dataset (Table
2), never off by more than ±3% in any year. This indicates that weekly
concentration sampling is likely sufficient for estimating annual
fluxes, and that values calculated using the 2018-2021 NEON sensor data
can be accurately compared with historical LTER values. We noted that
this was not the case if Q was downsampled to a discrete dailymeasurement rather than a daily average . Because of large
diel variation in Q, the time of day chosen for the Q measurement has
the potential to substantially impact annual flux estimates (by up to ±
30% or more!). Because it exerts such a strong influence,
high-frequency Q measurements appear essential for accurately estimating
fluxes.
Despite concentrations which were already declining, maximum export of
DOC and NO3-N (mass flux shown as gray lines in Figure
2) occurred concurrent with the peak of the melt pulse. Over the 4 years
of NEON sensor data, the CVQ was 2.47 while
CVDOC was 0.47 and CVNO3-N was 0.61.
This made Q variation the dominant control on flux. Flux was highly
unequal in time, with a GDOC of 0.87 and
GNO3-N of 0.82. Seventy nine percent of the DOC export
and 71% of the NO3-N export occurred within just 10%
of the time; the peak weeks of snowmelt.
Globally, fitted C-Q relationships suggest DOC was slightly enriched
(Log-Log slope = 0.14), though still in the range (slope < ±
0.2; Godsey et al., 2009) which would be considered relatively
chemostatic. NO3-N was almost perfectly chemostatic
(Log-Log slope = 0.01). In contrast, C-Q relationships for major
conservative solutes (Supporting Figure 2) exhibited moderate dilution
responses, though with slopes still far from a value of -1 which would
signify a perfect dilution response of a fixed mass flux of solute.
Coefficients of variation for conservative solutes (e.g.
CVNa = 0.64, CVCa = 0.46) were also much
less than CVQ , and using SpC as a surrogate for a
continuous conservative ion concentration produces a
GSpC of 0.75. Thus, despite the weak dilution response,
the majority of conservative ion export also occurs during the melt
pulse.
The seasonal increase in DOC and NO3-N concentrations
which preceded the melt pulse resulted in noticeable clockwise C-Q
hysteresis for both solutes over annual time scales (dashed lines in
Figure 3a&b). Within these larger annual hysteresis loops are smaller
loops generated by individual precipitation events. These include spring
ROS, summer rainfall when no snow is present, and autumn/early-winter
snow that quickly melts without becoming snowpack. These events produce
consistent enrichment of DOC and NO3-N, with fitted
LogC-LogQ slopes ranging from 0.15 to 0.35 across individual events.
These slopes for individual events are distinctly steeper than those
observed at seasonal time scales. Concentration variations typically lag
Q during such events (Figure 3c-f), producing counterclockwise C-Q
hysteresis loops within the larger clockwise seasonal loop (solid line
in Figure 3a&b).
Discharge exhibited significant diel variation during the melt pulse,
with peak daily values often 50% higher than the minima for the same
day (Figure 4a). These daily pulses, presumably driven by greater
daytime melting, typically peak in the late afternoon. DOC and
NO3-N, as well as SpC, DO and Temperature also exhibited
varying degrees of diurnal variation (Figures 4 & 5). SpC was
especially interesting in that during the peak of the melt pulse it
showed extremely modest diel variation (<0.2 µS
cm-1) despite large diel variation in Q
(>100 L s-1). During base flow however,
when diel Q variation was orders of magnitude less, diel Spc variation
was considerably greater (1-2 µS cm-1).