4.3 Projections in a changing climate
Over the past decades, the western United States has experienced
declining snowfall (Kunkel et al., 2009; Harpold et al., 2012) and
earlier melt pulses (Clow, 2010), a trend that is projected to continue
(e.g., Siirila-Woodburn et al., 2021). Based on observations that solute
fluxes are strongly tied to the magnitude and timing of the melt pulse,
this will potentially result in decreased carbon and nitrogen export and
greater retention within the catchment. While regionally, the western
United States is expected to become dryer, some areas may actually see
increased precipitation as a result of local conditions. The leeward
side of the central Rocky Mountains appears to potentially be one such
case (Kunkel et al., 2009). Indeed, from 1951 to 1994, Niwot Ridge LTER
experienced a statistically significant (p < 0.01) increase in
annual precipitation of 7.5 mm yr-1 (Williams et al.,
1996b). However, this trend appears to have reversed, or at the very
least stalled in the more recent SNOTEL data from 1995 to 2021 (slope =
-3.9 mm yr-1; p = 0.19).
In addition to the overall volume, the form of precipitation (i.e. snow
versus rain) may also change. With warmer temperatures, a greater
fraction of precipitation would be expected to fall as rain rather than
snow. As illustrated by the sensor data, rainfall events are clearly
capable of rapidly mobilizing and transporting solutes, perhaps even
more effectively than snowmelt. This suggests that a shift from snow to
rain may not result in as large a decline in solute export as otherwise
expected. Warmer temperature could also result in more frequent
“mini-melt” periods. Modeling studies (Jennings and Molotch, 2020)
suggest only a 3° C increase is required for much of Como Creek
watershed to experience significant melting throughout the winter. More
precipitation falling as rain rather than snow, combined with more
frequent periods of winter melt would alter the timing of export such
that it is more evenly distributed and less condensed into a singular
seasonal pulse
Several additional sources of very large uncertainty also remain. In
addition to changing precipitation, changing catchment characteristics
may also influence stream hydrology. Warmer temperatures will likely
result in a general upward migration of the tree line, though
temperature is far from the only controlling factor (Bueno de Mesquita
et al., 2018). The fraction which is forested has been demonstrated to
exert substantial control on rates of evapotranspiration and water yield
in nearby catchments (Sueker et al., 2001). It is possible that a shift
to an earlier melt pulse, when rates of evapotranspiration are lower,
could at least partially offset these losses (Barnhardt et al., 2020).
Recent modeling work in Como Creek (Barnhart et al., 2021) has also
suggested that expansion of forested area in response to warming
temperatures may actually increase streamflow by decreasing snow
wind-scour. In short, there is large uncertainty in how temperature
driven changes to catchment vegetation will interact to affect catchment
hydrology, including runoff.
Another large unknown is how biogeochemical processing of carbon and
nitrogen within the catchment will respond to changing temperatures and
snow cover. Biogeochemical cycles consist of multiple interconnected and
sometimes reciprocal pathways. With the N cycle for example, atmospheric
fixation, mineralization of organic nitrogen, nitrification and
denitrification are all sensitive to temperature and soil moisture
(e.g., Fisk and Schmidt, 1995; Osborne et al., 2016; Chen et al., 2020;
Maslov and Maslova, 2021). Somewhat counterintuitively, reduction in the
depth and duration of the insulating snowpack may actually result in
colder subnivean temperatures and reduced biological activity (Williams
et al., 1998). Snow cover has been identified as a critical control on
subnivean microbial processing (Brooks et al., 1996), and by extension,
soil water chemistry (Lewis and Grant, 1980). Williams (et al., 1998)
projects that increased snow cover will enhance net soil nitrification
and result in greater NO3-N availability, while a
reduction in snowfall will have the opposite effect and result in
greater retention. This is strongly supported by the observation that
solute flux appears most closely associated with persistence of the
snowpack later into the spring (which results in higher discharge but
also higher concentrations). While the total snowfall, maximum snowpack
depth, and its duration are certainly linked, they are not perfectly
correlated. An unseasonably warm spring can quickly melt a deeper than
average snowpack, as we observed in 2020. This provides a third example
of how changing temperatures over the coming decades can alter the
solute export, even if total annual precipitation remains unchanged.