Intuitively, the shape of a mountain basin - and in particular how much area is occupied at different elevation bands - would be an important factor in how sensitive the basin is to climatic change. Top-heavy basins, with a higher proportion of area at high elevations, would be less sensitive to warming. Bottom-heavy basins, with more area at lower elevations, would probably be more sensitive. Exactly how sensitive, and what role does the geometry play? Well, those are the questions we try to answer in our new paper published in Frontiers last month.
It turns out that mountain basins in western Canada fall into 3 different shape categories when we look at their hypsometry (how much area there is at different elevations in the basin; Figure 1 below). And if we strip away information about slope and aspect, give them all the exact same climate inputs and elevation range, and only keep the differences in hypsometry in play, the basins respond very differently to simple increases in temperature using a robust, physics-based snow melt model. In this paper we use CRHM.
Figure 1: Fifty shades of basin hypsometry, clustered into three main categories: top-heavy (purple), middle (green), and bottom heavy (yellow).
Snow Accumulation
For snow accumulation, we assume a linear gradient of snow accumulation: in general, snow accumulation increases with elevation. As the climate warms, we expect this gradient to change: snow at the highest elevations will remain roughly the same, and accumulations will decrease at lower elevations. We simulate this in the model by shifting the elevation where all precipitation falls as rain upwards in response to warming, and we recalculate the gradient (Figure 1). Bottom-heavy basins store more of their snow at the lower elevations, so this leads to big decreases in the total snow volume: on average the bottom-heavy basins could see a 15% decrease for +2C scenarios, and 35% decrease for +4C scenarios. Top-heavy basins could see decreases of 8% and 23% for the same scenarios.
Figure 2: Snowpack volume responses for the three different hypsometry types and two warming scenarios.
Snow Melt
To model snowmelt, all basins are assumed to have the same climate: we use a synthetic temperature curve, some noise, and standard lapse rates to generate reasonable estimates of observed temperatures in the region. As our model is process-based we also need wind speed, but we simplify again here (because wind is notoriously difficult to measure and model) and use a constant wind speed. Shortwave and longwave radiation are calculated using CRHM routines and assumed values for cloudiness and atmospheric transmissivity. We are working with a real model and throwing some reasonable numbers at it, as we are focused here on the geometry of the basin itself.
One would reasonably expect snow melt to start earlier with warmer temperatures, and our snowmelt model shows that this is true. Though your mileage may vary depending on the basin geometry: bottom-heavy basins show melt onset beginning nearly a month early with +4C of warming. Somewhat counterintuitively, we also show that average melt rates decline under warming scenarios (Figure 3)! While melt starts earlier at the lower elevations of the basins, the duration of the melt season advances less rapidly - resulting in lower average melt rates. This also agrees with the “slower slowmelt in a warming world” hypothesis put forward Musselman et al. (2017): as solar radiation is the main driver of snowmelt, an earlier start to the melt season means lower energy is available for melt.
Figure 3: Average daily snowmelt rates for three different basin geometries (purple = top-heavy, yellow = bottom-heavy) and temperature increases.
While slower melt rates might be seen as a positive from a flood forecasting perspective, changes in the timing and magnitude of snowmelt-driven streamflow and the loss of snowpack volumes will result in substantial changes in local hydrology. And again, the basin hypsometry plays an important role: bottom-heavy basins show the greatest sensitivity to a warming climate, and top-heavy basins are less sensitive. The loss of mid-summer snowmelt will create challenges for ecosystems and water managers alike, and increased monitoring of snowpacks at all elevations, through stations and remote sensing observations, will help us adapt to a future that is already here.
Oh: and where do these bottom-heavy basins tend to be located? On the eastern slopes of the Canadian Rockies. The water towers of the prairies.
Figure 4: Study area map, with the location of top-heavy (purple), intermediate (green), and bottom-heavy (yellow) mountain basins.
If you want to play with the code and data used to generate all the figures in the paper, point thy browser in this direction: https://doi.org/10.20383/102.0318.
JMS \m/