Results

The magnitude and timing of annual peak snowpack is sensitive to both air temperature and precipitation changes in Wolf Creek as shown in Figure 3. The interaction between air temperature and precipitation affecting peak SWE is evident in the curvature and slope of the contours; the interaction is complex in the alpine and shrub tundra (curved contour lines, Figure 3a & c) but less so in the forest (Figure 3e). The sensitivity of peak SWE to precipitation is somewhat higher in the high elevation alpine zone (contours have higher slope) and its sensitivity to temperature is somewhat higher in the lower elevation shrub tundra and forest zones (contours have lower slope). The peak SWE in the shrub tundra zone is very sensitive to a decrease in precipitation with warming due to precipitation phase change and suppression of blowing snow redistribution from the alpine zone under warmer air temperatures (Rasouli et al., 2014) and drops from 162 mm to 75 mm (87 mm reduction, Figure 3c) with 80% of precipitation and +5oC of warming. The sensitivity of peak SWE to increasing precipitation in the shrub tundra zone declines as the temperature warms. The peak SWE in the forested zone is slightly less sensitive to temperature than shrub tundra because unloading of intercepted snow from the canopy, where it is prone to sublimation, increases with winter air temperatures and moderates the impact of declining snowfall with rising temperature. Whether 20% additional precipitation can offset the effect of warming on snowpacks in Wolf Creek is illustrated in Figure 3 by comparing the black dot, indicating no change in air temperature or precipitation, to the white dot, indicating the degree of warming that can be offset by a 20% increase in precipitation. This is 3.5°C of warming for peak SWE in the alpine zone (Figure 3a) and 2.7°C and 3°C of warming for peak SWE in the shrub tundra (Figure 3c) and forest (Figure 3e).
There is no clear pattern to the small changes, less than six days, in the timing of peak SWE in the Wolf Creek alpine zone with air temperature and precipitation changes (Figure 3b). This is likely due to the persistently colder temperatures during winter at high elevations in the subarctic (Figure 3b). In the shrub tundra and forest, the mean annual peak SWE occurs 25 and 20 days earlier respectively with 5°C of warming and 20% reduced precipitation (Figure 3d and 3f).
In cold continental Marmot Creek, peak SWE in all zones is influenced by changes in both air temperature and precipitation but responds more strongly to temperature than in subarctic Wolf Creek (Figure 4). Peak SWE is progressively more influenced by warming temperature with declining elevation due to the influence of lapse rates on precipitation phase and other factors. Because of reduced blowing snow inputs from the alpine zone, the treeline forest zone loses the most snow (-422 mm under 5°C of warming and 20% less precipitation, Figure 4c), but because it has the highest snow accumulation, snow is still deep and its proportional change with temperature was not substantially different from the other zones. In contrast, almost all snow is lost in the forest zone, suggesting a high sensitivity of snow in Marmot Creek’s low elevation forests to warming because of the large losses of snow. The response of the peak SWE to warming and precipitation changes shows that an increase in precipitation of 20%, slightly greater than the maximum indicated by climate models, can offset the effect on peak SWE of warming in the alpine of 2.9°C (Figure 4a), in the treeline forests of 2.1°C (Figure 4c) and in the forest and forest clearing of 1.8°C (Figure 4c, g). The peak snowpack in Marmot Creek is more sensitive to warming, and so increased precipitation can offset less of a temperature increase than in Wolf Creek.
The changes in the simulated timing of peak SWE in Marmot Creek are substantial and complex. Timing responded much more to warming than to precipitation change and precipitation increases could not compensate for any degree of warming at any elevation (Figures 4b, 4d, 4f, and 4h). In the alpine, forest, and forest clearing zones, peak SWE advanced between 19 and 28 days for 2°C of warming, and between 60 and 70 days for 5°C. In contrast, the treeline forest peak SWE timing advanced only 10 and 27 days for 2 and 5°C of warming, its lower sensitivity (range of contours) due to the high snow accumulation in this zone associated with continued redistribution of snow from the alpine (Figure 4d).
In Reynolds Mountain, annual peak SWE is very sensitive to increases in air temperature and much less sensitive to changes in precipitation (Figure 5a, 5c, 5e, and 5g). The slope and curvature of the annual peak SWE contours show the sensitivity to precipitation change decreases as temperature increases. This suggests that the effects of warming on SWE cannot be easily offset by increased precipitation; a precipitation increase of +20% can offset warming up to from 1.2 to 1.5°C depending on location. The warmest and driest scenario (+5°C and -20% precipitation) caused the peak SWE decline in all zones, e.g., from 570 mm to 58 mm in the sink (Figure 5a) and from 427 mm to 39 mm in the interception zones (Figure 5e). The blowing snow sink zone lost more snow with warming and drying than other zones due to the suppression of blowing snow transport from the source zone (Figure 5a). An increase in precipitation greater than 20% would be needed in Reynolds Mountain than in Wolf Creek and Marmot Creek to offset the effect of the same warming on peak SWE.
The response of the timing of annual peak SWE is much more sensitive to warming than to precipitation change in all zones in Reynolds Mountain (Figures 5b, 5d, 5f, and 5h). The timing changes in Reynolds Mountain are the largest of the three basins with the change in peak SWE date being between 50 and 70 days earlier for the maximum 5oC warming. Additional precipitation of 20% can only offset the effect of 0.5°C of warming on peak SWE date (Figures 5b, 5d, 5f, and 5h).
The rate of change in the simulated snowpacks can be estimated in relation to temperature. Peak SWE reduction per degree of warming is 8% in Wolf Creek, 10% in Marmot Creek, and 17% in Reynolds Mountain (Table 1). The loss of snowpack with warming is reflected in the reduction in the snowcover duration of 11 days in Wolf Creek, 18 days in Marmot Creek, and 30 days in Reynolds Mountain per degree of warming (Table 1). The duration of snowmelt declines between 0 and 9 days per degree of warming in all basins, much less than the snowcover duration, and smaller than the advance in the timing of snow disappearance which ranges from 7 (Wolf Creek) to 13 (Marmot Creek) to 21 (Reynolds Mountain) days per degree of warming. Snow melts more slowly as the melt season advances in some of these simulations, which partly offsets the impact of the decrease in peak snowpack on snowmelt period duration.
In Wolf Creek, as in the other basins, the distribution of hourly simulations of SWE widens if precipitation increases and narrows if precipitation decreases (Figure 6). In the alpine, the accompanying warming shifts the distribution to the left and causes additional narrowing (Figure 6a). If the precipitation increase is large (+20%); a warming of up to 3.5°C can be offset for all hourly SWE simulations in all three zones in Wolf Creek (mean annual temperature exceeds 1.4°C). In contrast to higher sensitivity of peak SWE in the forest zones, the distributions of different snowpack regimes show that the forest changes the least under warming and changes in precipitation because of cold sub-canopy winter temperatures and reduction in sublimation losses from intercepted snow with warming (Figure 6c). An increase in precipitation can offset the impacts of some warming and affects high and medium values of SWE the most and low SWEs only slightly (Figure 6). The warming impacts SWE in early winter during the initiation dates of snow accumulation and early spring during snow depletion more than peak snowpacks in all of the snow regimes (Table 1). The snowpack regime in Wolf Creek is more sensitive to changes in precipitation than to warming because of its consistently very cold winters. Each of the five distributions for the warming and changed precipitation scenarios in each zone are significantly different (p-value ≤ 0.05) to the snowpack distribution in the base period (0°C warming, 100% precipitation) based on the Kolmogorov-Smirnov (K–S) test.
In Marmot Creek, the distribution of hourly simulations of SWE is wider if precipitation increases and much narrower if temperature warms by more than 2°C (Figure 7). Precipitation increases of 20% can offset effect of a warming up to 2°C on snowpack regime in each zone (Figure 7) but cannot offset warming of more than 2°C. The distributions of different snowpack regimes show that a very shallow snowpack (SWE < 100 mm) is expected in the forest (Figure 7c) and forest clearings (Figure 7d) under the extreme case of 5°C warming and 20% decrease in precipitation. Warming impacts peak more than shallower snowpacks in all zones. An increase in precipitation can offset the impacts of warming by increasing SWE, especially high values of SWE (Figure 7). In general, the snowpack regime in Marmot Creek is equally sensitive to warming and changes in precipitation (Figure 4). Each of the five distributions for the warming and changed precipitation scenarios in each zone are significantly different (p-value ≤ 0.05) to the modelled snowpack distribution in the base period based on the Kolmogorov-Smirnov (K–S) test.
In Reynolds Mountain, the distribution of hourly simulations of SWE is wider if precipitation increases, and much narrower if temperature warms by more than 1°C (Figure 8). Increasing precipitation offsets less of the warming impact in Reynolds Mountain than in Wolf Creek or in Marmot Creek; a precipitation increase of 20% can only offset the impact of a 1°C warming (mean annual temperature exceeds 6°C). An additional 20% precipitation cannot offset warming of 2°C or more. The different snowpack regimes are sensitive to impacts of 5°C warming and 20% decrease in precipitation (Figure 8) and simulated maximum SWE values drop below 240 mm from the base case of over 800 mm in the source and sink HRUs (Figure 8a), interception (Figure 8c), and sheltered regime (Figure 8d). Of particular interest is that high values of SWE (>500 mm) do not occur. Each of the five distributions for the warming and changed precipitation scenarios in each zone are significantly different (p-value ≤ 0.05) to the snowpack distribution in the base period based on the Kolmogorov-Smirnov (K–S) test.
Changes in the distribution of SWE in each headwater basin show that the zones in Wolf Creek (Figure 6) and the treeline forest in Marmot Creek (Figure 7b) are the least sensitive to air temperature and precipitation changes. Each of the regimes in Reynolds Mountain (Figure 8) are sensitive in terms of the absolute magnitude of snow loss.
The sensitivity of five main characteristics of basin snow regimes to warming and change in precipitation averaged over Wolf Creek shows that both changes in precipitation and warming affect the magnitude of the peak SWE (Figure 9a). Precipitation increases of 20% can offset a 3°C temperature increase in Wolf Creek peak SWE. Delay in the initiation of snow accumulation is sensitive to warming rates above 3°C regardless of precipitation changes (Figure 9b). The snow-free date advances from late-June (June 28) in the recent climate to early June (June 11) with a warming of 2°C (Figure 9c, Table 2). The snow-free date is also sensitive to warming and almost insensitive to precipitation changes (Figure 9c). The snow season duration in Wolf Creek is also driven by warming and not by precipitation changes (Figure 9d). The snowmelt period, the timing difference between peak SWE and the snow-free date, is sensitive to warming and almost insensitive to precipitation changes (Figure 9e).
In Marmot Creek, the peak SWE drops from 220 mm to 92 mm under a warming of 5°C and decreasing precipitation (20%), (Figure 9f, Table 2). The start of snow accumulation is not affected to a large amount by either warming or precipitation (Figure 9g), but increased temperatures have a large effect on the end date (Figure 9h) and snow season duration (Figure 9i) but this is reduced with increased precipitation. The duration of the melt season also is not affected (Figure 9j). In contrast to Wolf Creek, the initiation date of snow accumulation is sensitive to precipitation changes and would advance if warming rates are below 2°C and precipitation increases. The snow-free date advances from early June in the recent climate to late May with a warming of 2°C (Figure 9h). Similar to the ablation period, snow accumulation start date is sensitive to precipitation changes and to a lesser extent to warming. With concomitant warming (5°C) and decreasing precipitation, the snow-free date across the basin advances by 77 days to late March (Figure 9h). As shown in Figure 9, the snow-free date is sensitive to warming and insensitive to precipitation changes in Marmot Creek and snow season length is affected by both warming and precipitation changes. Similar to Wolf Creek, the combination of air temperature increasing by at least 2°C and precipitation increasing by less than 20% results in declining peak SWE and deviation from the historical ranges of snowpack in Marmot Creek.
In Reynolds Mountain, warming of 5°C and decreasing precipitation of 20%, the mean annual peak SWE decreases from 390 mm to 47 mm (Figure 9k, Table 2), snow accumulation starts later (Figure 9l) and ends earlier (Figure 9m). The duration of the snow season (Figure 9n) and duration of the melt period snow season (Figure 9o) become much shorter than in present climate (Table 2). A 1°C warming advances the timing of peak SWE by approximately 15 days (Table 1). The magnitude of peak SWE is more sensitive to temperature than precipitation (Figure 9k); the timing of the snow regime sensitive to temperature and less so to precipitation (Figure 9 l-o).
The peak SWE is 136 mm in Wolf Creek, 220 mm in Marmot Creek, and 390 mm in Reynolds Mountain; Wolf Creek and Reynolds peak SWE occur in early March, and in Marmot Creek it occurs in late April (Table 2). With a 20% decline in precipitation and a warming of 5°C peak SWE declines to 61 mm (55% decrease) in Wolf Creek, to 92 mm (58%) in Marmot Creek, and to 47 mm (88% decrease) in Reynolds Mountain. With a 20% increase in precipitation and no warming and peak SWE increases to 169 mm (24%) in Wolf Creek, to 281 mm in Marmot Creek (28%), and to 486 mm (25%) in Reynolds Mountain. With 5°C warming and no changes in precipitation, the onset of snow accumulation is delayed 17 days in Wolf Creek, 23 days in Marmot Creek, and 42 days in Reynolds Mountain and the end of winter comes earlier by 37 days in Wolf Creek, 67 days in Marmot Creek, and 104 days in Reynolds Mountain. When compared to no changes (Table 2), a 20% increase in precipitation would lengthen the snowcover duration by 5 to 20 days.
The simulations show that changes in snow regime in these mountain basins also result in moderated changes in mean annual runoff. Unlike peak SWE, mean annual runoff is more sensitive to changes in precipitation than air temperature (Figure 10). The near vertical lines in Figure 10a & b indicate that changes in mean annual runoff are driven predominately by precipitation in Wolf Creek and Marmot Creek while in Reynolds Mountain temperature more strongly impacts runoff. A 1°C warming in Wolf Creek resulted in a 5% decrease in the annual runoff (Table 1); total decreases rise to ~14% for a 5°C warming (171 to 147 mm, Table 2, Figure 10a). The most extreme scenario of climate warming and decreased precipitation caused larger declines in runoff, but if precipitation increases there is strong compensation. For instance, if precipitation increases by 20% then annual runoff increases by 35 mm (from 171 to 206 mm) with 5°C of warming. Mean annual runoff is more sensitive than snow regime to precipitation change in Wolf Creek. Similarly, in Marmot Creek, a 5°C increase in air temperature results in a 4% decrease in the mean annual runoff (402 to 384 mm Table 2, Figure 10b). The combination of 5°C of warming and 20% decreased precipitation reduces mean annual runoff by 34% (135 mm from 402 to 267 mm, Table 2, Figure 10b). In Reynolds Mountain, mean annual runoff has a stronger temperature sensitivity than Wolf Creek or Marmot Creek (Figure 10). A 5°C increase in temperature results in a 29% (371 to 263 mm, Table 2) decrease in the mean annual runoff. The combination of 5°C of warming and 20% decrease in precipitation reduces annual runoff by 43%, (371 to 161 mm, Table 2).
Changes in mean annual runoff (Figure 10) contrasts with the change in mean annual peak SWE (Figures 3-5) in that mean annual runoff is more sensitive to precipitation than temperature. The sensitivity of annual runoff to temperature increase in Reynolds Mountain is because of the longer snow-free season and an increased growing season and energy flux for evapotranspiration with increasing temperature (Figure 5) whilst runoff responds to both precipitation change and warming (Figure 10c). In contrast to the sensitivity of snowpack to warming in Reynolds Mountain, annual runoff is less sensitive, and the impact of warming on annual runoff can be partly offset by an increase in precipitation in Reynolds Mountain.
Annual runoff changes are given in Table 2 under different scenarios of warming and changes in precipitation. Annual runoff responds strongly to precipitation changes in Wolf Creek and Marmot Creek, and to both warming and precipitation changes in Reynolds Mountain. The annual runoff is the most resilient to warming in Marmot Creek and most sensitive to warming in Reynolds Mountain. Under 5°C and a 20% increased precipitation, annual runoff increases from 171 mm to 206 mm (20%) in Wolf Creek and increases from 402 mm to 518 mm (29%) in Marmot Creek and from 371 mm to 415 mm (12%) in Reynolds Mountain (Table 1). This shows that increased precipitation with warming increases the runoff in Marmot Creek more than the other two basins. This is due to the very cold alpine snowpack at Marmot Creek which is relatively unaffected by warming and the warm snowpacks at Reynolds Mountain which become ephemeral with warming.
From this sensitivity analysis, the amount of additional precipitation needed to offset the effect of increased temperature on peak SWE, and annual runoff under future climate can be estimated. The largest increase in precipitation projected by RCPs and NARCCAP RCM–GCMs is 34% for Wolf Creek, 18% for Marmot Creek, and 16% for Reynolds Mountain. In Wolf Creek, when warming is limited to 1°C, increased precipitation of 4% is able to offset the effect of warming on peak SWE (Figure 11a); but, with warming of 5°C, an increase in precipitation of 34%, the amount expected from RCPs and NARCCAP, would be required to offset the effect of warming. In Marmot Creek, the effect of a 1°C warming on peak SWE can be offset by an 8% increase in precipitation; however, the effect of a 5°C warming on peak SWE would require precipitation increases that are greater than expected from RCP scenarios and NARCCAP simulations. In Reynolds Mountain, the impact of a 1°C warming on peak SWE can be offset by a 16% increase in precipitation, but the offset required for more than 2°C warming exceeds the projected maximum precipitation increases.
Annual runoff is less sensitive than peak snowpack to warming and smaller precipitation increases are required to offset the effects of warming simulated here. These differences are due to differences in the fraction of snow converted to rainfall in each basin under a warmer climate. The additional precipitation needed to offset the impact of warming on runoff varies with elevation range, precipitation regime and latitude; offsetting the effect of warming of 5°C on annual runoff would require precipitation increases of 8% in Wolf Creek (Figure 11a), 3% in Marmot Creek (Figure 11b), and 14% in Reynolds Mountain (Figure 11c).