What Is Snowmelt, and Why Is It Important?

Snowmelt—water released from snow cover when air temperatures rise above freezing, thereby melting snow—may not be something most people give much thought. But, in truth, it's just as essential as rain for recharging groundwater and supplying drinks of freshwater to plants, animals, and us humans.

Anywhere that has snow days and has them melted away by the sun experiences snowmelt to some degree. But snowmelt largely refers to the significant, seasonal thawing of snowpack from mountains and high elevations in the western, northeastern, and upper Midwest regions of the United States, typically from April (the end of the snow season) through July.

According to a study in Nature Climate Change, snowmelt contributes more than 50% of runoff across nearly one third of the global land area, including in the western United States. Climate change, however, is limiting how much water from cold-season stores is available for use in the year ahead.

Snowmelt in the Water Cycle

Snowmelt is a vital component of Earth's water cycle—the process through which water recycles itself by moving through the atmosphere, land, and waterways. In cold climates, precipitation builds up as snow, ice, and glaciers. Once air temperatures begin to warm above 32 degrees F, however, this snow and ice melts into liquid water and becomes runoff (water that "runs off" of the land surface). This runoff then flows downhill into lakes, rivers, and oceans. Some of the meltwater also soaks into the ground (infiltration). The water closest to the surface contributes to things like irrigation of agricultural crops. Any water that isn't taken up by plants' roots seeps deeper into the earth and becomes groundwater, which is where nearly half of Americans get their drinking water from.

How much water is released by snowmelt varies depending on the properties of the snow. As a general rule, 10-12 inches of snowfall produces one inch of liquid water. However, if snow is more "powdery" and dry, it could take double that amount, say 20 inches, to equal one inch of water. On the other hand, it might only take 5 inches of heavy, wet snow to produce this much.

Spring Snowmelt and Flooding

Ordinarily, snowmelt is a gradual process, melting at rates in the neighborhood of several tenths of an inch per day. But if temperatures warm too quickly, melting snow can produce runoff faster than ground surfaces can soak it up, thereby triggering springtime flooding. If the meltwater is traveling fast enough as it rushes downhill, the sheer force of it can carry mud and trees in its currents, leading to landslides and debris flows.

Heavy downpours, which have increased in all but one U.S. region as a result of our warming climate, can also contribute to snowmelt-related flooding, landslides, and debris flows. When rains fall onto an existing snowpack in what's called a "rain-on-snow" event, they're unable to soak through the hardened snow's surface layers, and so, they become runoff almost instantaneously.

Snowmelt Decline Due to Climate Change

In addition to giving snowmelt events a more destructive edge, climate change is lessening states' ability to depend on snowmelt for their water supply. 

For one, warmer winter temperatures have caused a decline in total snowfall in some parts of the country. (Warmer temperatures mean more precipitation falls as rain instead of snow.) What's more, winters over the most recent 30 years have been 15 days shorter than those of the preceding 30 years, meaning there's a smaller window of opportunity for snow to even occur.

Earth's 2.2-degree F warmer atmosphere is also shifting the timing of snowmelt events. According to NOAA's Climate.gov, spring snow cover is disappearing earlier in the year than it did 50 years ago. For example, declines in June snow-covered area of 5 to 25% are common across North America.

In addition to providing less water for drinking and for the generation of hydroelectric power, such changes could affect food production by way of agricultural river basins that depend on snowmelt to water their crops. The Colorado River Basin, for example, which currently gets 38% of its water for irrigation from snowmelt, can expect to squeeze no more than 23% from snowmelt under a 7-degree F warming scenario.