Snowpack: not the water supply metric it used to be Email
Written by Lyn Dean   
Tuesday, July 13, 2021 03:00 AM

Colorado Green NOW

Most who have lived in Colorado for at least a few years are accustomed to hearing the frequent media reports on snowpack measurements during the winter and spring. As a water-challenged state dependent on snowfall stored in high-elevation mountains and valleys during the winter months, our lives, livelihoods and the natural environment depend on the gradual melting of winter snowpack to meet all the competing water needs. According to the USGS (U.S. Geological Survey), about 75% of water supply in the western states is derived from the melting snowpack.

NRCS always working in the background
The Natural Resources Conservation Service (NRCS), continuously collects data about snowfall in the western U.S. and has done so for decades. Data are collected using both automated snow stations in high-elevation mountain watersheds in the west and also through manual snow monitoring in mountain meadows protected from the wind. The availability of historical data is not lost on those whose job it is to plan, manage, advise and further study water resources in western states. They understand the complexity of the numerous factors.

“Drought is not just a matter of snowpack,” said Greg Baker, Aurora Water, on a recent webinar hosted by Colorado WaterWise. “It’s In early 2021, researchers at the University of Colorado Boulder (CU Boulder) published a new study1 that took a closer look at decades of snow records from western North America—all western U.S. states, and north to western Canada and Alaska. They found some important statistically significant trends about water availability in this vast mountainous region, which of course includes Colorado.

Snowpack, snowmelt and snow drought
This new CU Boulder study, led by Keith Musselman, and some other earlier studies, emphasize the changes and variability of mountain water storage in the form of snowpack. They also point to the complexity and uncertainty of predicting snowpack. As a result, the term ‘snow drought’ is now becoming more widely used to explain some of the uncertainties and challenges. NOAA (National Oceanic and Atmospheric Administration) says snow drought can occur in two ways: 1) below-normal cold-season snow; or 2) reduced snow accumulation—despite near-normal precipitation—because of warmer temperatures that cause early snowmelt and/or precipitation to fall as rain rather than snow.

Water seasons
Why are warmer temperatures a problem if we receive normal precipitation? Rain, snow or snowmelt, it’s all water. It’s not so simple. For eons, the storage of snow in the winter at high elevation has acted as ‘natural reservoir’ providing a gradual melt in spring and summer, allowing water to flow into rivers and streams throughout the hotter and drier summer months (though summer Colorado monsoonal rains often provide some much-needed moisture).

Typically, water resource management in mountain-based watershed regions has depended on the assumption of two separate ‘water seasons:’ snow accumulation and snowmelt. These seasons are traditionally agreed to be October through March for snow accumulation, and the snowmelt season begins April 1. Snowpack—total accumulation—would inform water management for the coming year.

Snowpack is the quantity upon which snow water equivalent (SWE) is calculated. Yet, if snowmelt occurs before the date of maximum snow accumulation and blurs the distinction between the snow accumulation and snowmelt seasons, then the ability to accurately forecast streamflow and drought is compromised.

Uncovering relationships
This CU research study was the first to analyze snowmelt trends during traditional snow accumulation months. The researchers analyzed data recorded at the 1,000+ automated snow stations in western North America. Now that records extend back a sufficiently long time (40+ years), data from stations facilitated a long-term melt analysis.

The challenge for studies monitoring snowpack and therefore SWE is that snowpack depends more on long term precipitation trends, making it difficult to assess the impact of warming on snowpack. By contrast, snowmelt would seem more sensitive to warmer temperatures than precipitation trends.

Winter melting and snowpack have a complicated relationship.

The CU researchers found a way to relate snowpack and melting. They calculated the cumulative average daily melt. Then they also developed a new metric using the data “to characterize the mobilization of snow water resources during what is typically considered to be the accumulation period before spring melt and use it to assess the historical snowpack response to climate variability,” they wrote.

Melt and snowpack trends
The analysis found that 42% of stations spread throughout the western North American region had increased melt before April 1, which was at least three times the trend of a lower annual snowpack. In other words, early melting is increasing at a faster rate than snowpack is decreasing. Melt has increased in all cold-season snow-accumulation months but most notably in March and November.

As noted, relating snowpack changes to temperature is more complicated because snowpack is more influenced by precipitation. It’s difficult to tease apart the influence of temperature versus precipitation on snowpack. But for winter snowmelt, based on their findings the researchers state that “seasonal temperature reliably controls snowmelt.”

Musselman, who was also lead author of another study2 on snowmelt in 2017, suggested that the impact of earlier snowmelt on water supplies and ecosystems could be significant. Snowpack melting during the winter occurs more slowly, resulting in more water being absorbed into the soil, and not contributing to streamflow. This means even if snowpack remains the same, streamflow in watersheds will decline and reduce availability of water resources.

What does it all mean?
Hydrology in mountainous regions is changing, affecting the amount and timing of streamflow. Snow is melting earlier. Snowpack is generally declining but at a slower rate than melt is increasing. And, snowpack is harder to predict due to its dependency on precipitation—which is a fickle factor at the mercy of variability in storm tracks that are also changing.

According to Jeff Lukas at Lukas Climate, Boulder, speaking at a recent Colorado WaterWise webinar, warmer temperatures promoting increased and earlier melting are creating drier conditions with more sun, and will eventually result in soil moisture decline.

Studies about the impact of warming on snow conditions are showing us the current state of our water resources. They are not presented to be depressing but as a reminder that we must play our part and respond as best as we can as good stewards of the planet in all that we do. In the green industry, we are well-positioned to continue to make a positive impact through educating and making recommendations to clients on preserving and conserving resources.

1 Musselman, K.N., Addor, N., Vano, J.A. et al. Winter melt trends portend widespread declines in snow water resources. Nat. Clim. Chang. 11, 418–424 (2021).
2 Musselman, K., Clark, M., Liu, C. et al. Slower snowmelt in a warmer world. Nature Clim Change 7, 214–219 (2017).

This article originally appeared in the July/August 2021 issue of Colorado Green magazine.

Read more in this issue of Colorado Green NOW:
Video: Establishment permits in Colorado Springs
Denver seeks experts for code amendment committees
Roller coaster week in H-2B legislation

Outdoor improvements, lawn & garden purchases grew in 2020