Snowmelt Trends
Snowmelt Trends
Embedded Files
Mid-winter Melting
A warm winter is defined as one in which the mean monthly temperature exceeds 32°F for any one of the three core winter months (December–February).
During warmer winter seasons, sunlight alongside warmer, drier air can cause snow that does fall to melt rather than stay preserved in the snowpack.
Rain-on-snow: warm rain can melt cold snow, but will also percolate (filters gradually) through air pockets creating liquid water within and below the snowpack, increasing the risk of wet and glide avalanches.
Glide Avalanches are when the entire snowpack slowly moves as a unit on the ground. This is typically caused by meltwater lubricating the snow-ground interface. (Learn more: see Tremper, 2008)
Wet Avalanches are caused when snow loses its strength after becoming saturated with water. (Learn more: see Tremper, 2008)
Effects of rain-on-snow: rain creates runnels in the snow.
Image Credit: Avalanche Canada
Glide Avalanche
Image Credit: Utah Avalanche Center
Earlier springs
Snowmelt is happening 2-3 weeks earlier than is typically expected.
Earlier spring causes earlier peak runoff which leads to longer, drier summers.
Dust-on-snow: Atmospheric dust - from nearby deserts experiencing drought and fossil fuel soot – is blown onto the snow surface during both winter and spring.
These dust particles decrease the albedo (amount of light reflected off snow), absorb more sunlight, warm the snow, and ultimately lead to increased melting. (Learn More: see Reynolds et. al., 2020)
Examples of Dust-on-Snow
Photo Credit: Outside Magazine, Climate.gov, and National Geographic
Contrary to the intuitive belief that warming temperatures mean more rapid snowmelt, an earlier spring actually means slower melting.
This is because the days are shorter and the angle of the sun is not as high as it would be during late spring/early summer.
Impacts on Water Availability
Impacts on Water Availability
Snowmelt provides 75% of the water supply to 60 million people in the West.
An earlier, slower snowmelt reduces streamflow, which has multiple implications for water availability and management.
This graph shows the relationship between streamflow and snowmelt rate
Image Credit: Wu, et. al, 2018
When snow melts more slowly, runoff is less efficient; this is because it reduces the amount of moisture being pushed deep into the soil, where it is less likely to evaporate.
Conversely, rapid snowmelt creates pressure that pushes moisture deep into the ground.
When moisture doesn’t get pushed deep into the ground, it lingers at the subsurface where soil and vegetation absorb the water instead.
This leads to higher levels of evapotranspiration (the process by which water is transferred from the land to the atmosphere by soil, plants, and water) and therefore less water in the stream. (Learn more: see Wu, et. al, 2018)
Warming air temperatures also lead to increased evapotranspiration due to a Vapor Pressure Deficit (VPD).
VPD describes how much water the air DOES hold, versus how much water the air CAN hold.
Since warmer air can hold more moisture, it creates a larger VPD, which increases the air’s potential to pull more water from the ground, consequently decreasing streamflow. Learn more about VPD!
Less water absorbed deep into the ground also means less groundwater recharge.
Implications for Water Management
Implications for Water Management
Historically, April 1st holds the maximum Snow Water Equivalent (SWE) used to predict streamflow; however, mid-winter melts and earlier spring melting are making this number unreliable, especially for entire watersheds.
Lake Powell at low levels
Image Credit: AZCentral.com
How can we make water usage decisions if we cannot accurately predict how much water we will have?
Reservoirs and dams in the US were built decades ago during a different climate.
Now with earlier snowmelt, reservoirs reach capacity sooner and are forced to release meltwater earlier– often before cities, farmers, and ecosystems need it.
With less runoff from snow overall, the reservoirs don’t return to capacity and cannot provide adequate water supply throughout the summer as water demands increase.
Page updated
Google Sites
Report abuse