Declines in western water towers

Mountains act as natural water towers, influencing the water cycle by storing snow until the melt season when downstream demand is high. We calculate and track historic changes in the degree to which mountainous snowpacks delay the timing and amount of water availability relative to precipitation.
Published in Earth & Environment

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A group of snow hydrologists measure various snow characteristics near the Continental Divide of Colorado. 80% of the downstream water in this area comes from snowmelt, emphasizing the importance of this mountainous area to act as a water tower.

Mountainous snowpacks as natural reservoirs:

In western North America and across the globe, mountainous regions are often relied on for delayed water availability, as the accumulated winter season snowpack stores water until periods of snowmelt. For that reason, these areas are referred to as natural water towers, since the storage of water in the snowpack acts to align the timing of snowmelt with increased atmospheric demand and increased downstream natural and societal water use. In short, mountains naturally storage water until we (humans, plants, animals) need it. The timing of snowmelt can be delayed between 0-6 months (or longer) after snowfall occurs. This temporal delay, relative differences in the amount of snowfall and snowmelt, and changes in the timing and magnitude of water inputs (snowfall) and water availability (snowmelt) are expected to influence the hydrologic cycle of a given area.

Some internal motivation behind this work originated from conversations regarding snowfall events where snowmelt occurs shortly thereafter, prompting the question: how different are relatively rapid snowfall-snowmelt events from a rainfall event through the lens of a watershed? Typically, the snow-related variables considered for quantifying downstream runoff volumes or ratios are snow water equivalent (SWE) or snow fraction (the amount of annual precipitation which fall as snow). However, neither of these variables considers the fundamental influence that snow water storage has on water availability. Rainfall was further included in this analysis to address the perspective that areas without summer rainfall inherently behave more like water towers, since late season water is only available through snowmelt. In this way, delayed water availability via snowmelt in areas without summer or fall season rain is more critical or necessary for downstream subsistence.

In response to these snow hydrology queries, our analysis takes a first step in quantifying the ability of numerous mountainous regions and snowpacks to act as a natural reservoir. Further, we evaluate the historic change in the ability of mountain snowpacks to store water as snow.

The Snow Storage Index and declines through time:

We evaluate snow water storage across western North America, which includes twelve major mountainous areas. To do so, we introduce a Snow Storage Index, which mathematically captures both the timing and magnitude differences between daily precipitation and surface water inputs. Surface water inputs are sum of rainfall and snowmelt into terrestrial systems, such as the soil surface. In this way, our definition of surface water inputs considers water to the terrestrial system from the atmosphere via rainfall or snowmelt only. Significant delays in surface water input generation are thus caused only by differences in snowfall and snowmelt timing. As such, larger Snow Storage Index values are typically a result of significant fall and winter season snowfall, followed by significant delays in the timing of snowmelt from snowfall and thus in highly seasonal snowmelt. Alternatively, a lower Snow Storage Index value is the result of a shorter period between less seasonal precipitation and less seasonal snowmelt (see illustrations below).

Monthly temporal differences and relative magnitudes of snowfall and snowmelt in an area with a high Snow Storage Index (SSI, thus storing more water as snow for a longer duration) and a low SSI.

 We found that long-term average high Snow Storage Index values (closer to 1) were most concentrated in the North Cascades, Cascades, Columbia Mountains/Northern Rockies, Canadian Rockies, Sierra Nevada and the Middle Rockies. Lower Snow Storage Index values (closer to 0) existed in the Middle Rockies, Wasatch/Uintas, and Southern Rockies. From 1950-2013, the Snow Storage Index declined in all mountain ranges, with > 25% of the total area experiencing a significant decline. Declines in snow water storage were related to primarily earlier snowmelt timing, with secondary decreases in winter season snowfall.

This perspective on snowfall, snowmelt, and seasonal snow water storage targets an unaddressed gap in hydrology by evaluating how future changes in climate may modify local and regional water availability through changes in the timing of water inputs to the terrestrial system relative to the timing of precipitation.

The link to the full publication can be found here.

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