Discussion
Air temperatures in late winter and spring have a key role in
determining the sensitivity of snowpack in mountain basins to warming
(Lettenmaier and Gan, 1990; Stewart et al., 2004; McCabe and Clark,
2005). The results presented above demonstrate and quantify the
sensitivity of annual peak snowpack and its timing, and of annual
runoff, to air temperature and precipitation changes and their
interaction in the three basins. Sensitivity of annual peak snowpack
timing to air temperature and precipitation changes in the three basins
shows that the sensitivity of peak SWE timing to precipitation changes
is greater in the colder climate conditions: Reynolds Mountain responds
to warming only (Figure 5), Marmot Creek responds to warming and to a
lesser extent to precipitation (Figure 4); and Wolf Creek responds to a
complex interaction of warming and precipitation change (Figure 3). The
potential for precipitation to counteract the effect of warming on the
magnitude of the annual peak snowpack, becomes smaller moving from
northern latitudes to southern latitudes. Therefore, regional responses
to warming and changes to precipitation must be considered (Bower et
al., 2004), particularly when evaluating future mountain hydrology
(Roche et al., 2018; Sultana and Choi, 2018). This is because the
snowpack is shallow and warm at the beginning and end of the season;
shallow warm snow ripens and melts faster than deep cold snow as it
requires less energy input to overcome cold content and fill its liquid
water holding capacity (Colbeck, 1976).
Simulations of future conditions for snow regimes in Reynolds Mountain
are in accord with the SWE magnitude and timing trajectories of the past
50 years (Nayak et al., 2010). Higher rates of warming and increased
precipitation are projected by RCMs in the northern latitudes (Mearns et
al., 2007). Latitudinal change in the role of precipitation increase in
offsetting the effect of warming on cold regions hydrology implies that,
even though northern latitudes will warm more (Graversen et al., 2008),
they will also have more precipitation. Therefore, the precipitation
increase may offset the impact of warming on snow and hydrological
regimes in northern basins. It is also expected that the response of
hydrological processes in different latitudes to the same climatic
change will differ.
Although the snow regime in Marmot Creek (Figure 4) is as sensitive as
in Wolf Creek (Figure 3) to warming and a decrease in precipitation, its
runoff regime is less sensitive than the runoff regime in Wolf Creek
(Figure 9). These results are consistent with findings and projections
for other mountain areas (Sultana and Choi, 2018; Roche et al., 2018;
Jennings and Moltoch, 2019; López-Moreno et al., 2020). The relatively
lower sensitivity of the forest peak snow in Wolf Creek (Figure 3e) is
because of the increased unloading of intercepted snow at warmer air
temperatures that counteracts the reduced snowfall (Pomeroy et al.,
2015). The higher resiliency of the Marmot Creek snowpack is due to
smaller changes at high elevations and in the blowing snow sink zone of
the treeline forest in which a deep snowpack is deposited that remains
until mid-summer (MacDonald et al., 2010; Harder et al., 2015; Rasouli
et al., 2019a). High elevation and high latitude basins are more
resilient to warming because their temperatures are currently well below
that required to shift precipitation phase (Bavay et al., 2013; Jennings
and Moltoch, 2019). The snowpack lasts longer on the ground at high
elevations in Marmot Creek, which moderates the impact of snow loss at
low elevations on runoff (Rasouli et al., 2019a; López-Moreno et al.,
2020). A high elevation band with air temperatures similar to that in
low elevations in Wolf Creek and a rainy environment in the spring and
early summer peak runoff period (Pomeroy et al., 2016) explain why the
drop in peak snow accumulation is not reflected by a proportional drop
in annual runoff in Marmot Creek. This highlights the role of the
spatial redistribution of snow on heterogeneous hydrological responses
at different elevations in Marmot Creek. The snow and runoff regimes are
the most sensitive to warming in Reynolds Mountain because of the (i)
higher annual mean air temperature, (ii) near-freezing air temperatures
in winter, and (iii) fewer number of days with freezing temperatures
(120 days a year, Rasouli et al., 2019a). Rasouli et al. (2019a) found
that under a moderate warming and increased precipitation of 7% and 2%
in Marmot Creek and Reynolds Mountain, respectively, the annual runoff
remained unchanged due to the offsetting effect of increased
precipitation on increased evapotranspiration and offsetting effect of
decreased sublimation on reduced snowfall (Rasouli et al., 2019a). Less
sensitivity of annual runoff to warming relative to snowpacks suggests
that warming mountain snowpacks can be decoupled from hydrological
regimes (López-Moreno et al., 2020). Snowpack regimes in Reynolds
Mountain are more sensitive to warming than to changes in precipitation,
similar to the Cascade Mountains of Oregon, USA (Sproles et al., 2013).
Estimated snowpack reduction per degree increase of temperature is 8%
in Wolf Creek, 10% in Marmot Creek, and 17% in Reynolds Mountain
(Table 2). In Wolf Creek these results are similar to reductions
observed in the Svalbard Archipelago (\(\sim 79\)° N, López-Moreno et
al., 2016). Snowpack loss in Marmot Creek is in the range of 11–20%
reduction reported for the Pyrenees (López-Moreno et al., 2013, 2014)
and comparable to a 15% reduction reported for the Swiss Alps (Beniston
et al., 2003). Snow loss per degree of warming in Reynolds Mountain is
similar to a 20% reduction reported for the Washington Cascades (Casola
et al., 2009). The results here are consistent with other basins with
similar climates and that climate change affects snowpack in mountain
basins across the globe with large reductions at mid-latitudes and
relatively small reductions at high latitudes (Roche et al., 2018;
Sultana and Choi, 2018).
Under a severe warming of 5°C and a 20% increased precipitation, annual
runoff increases in all three basins (Table 2) because of the increasing
importance of rain in warmer climates, suggesting that precipitation
increase has a primary role in changing annual total runoff and there is
a large shift in the runoff mechanism from being snowmelt-driven to
rainfall-driven. This shift may result in reduced streamflow (Berghuis
et al., 2014) if precipitation does not increase (Table 2). It might
also alter forest vegetation over time by making it more prone to
wildfire and disease.