Snowmelt

Snowmelt is the process by which snow and ice are transformed into liquid water. It occurs when energy input to the snowpack surpasses the latent heat of fusion, enabling melting. The dominant energy source is solar radiation, but sensible heat from the air and ground, convection, and wind also contribute. Melting typically begins near 0 degrees Celsius, though snow properties and structure influence the timing.

The snowpack energy balance includes shortwave and longwave radiation, sensible heat flux, latent heat flux, and ground heat flux. Snow albedo is high at first, reflecting much sunlight, but degrades as the snow ages and becomes granular, accelerating melt under clear, sunny conditions. Meltwater either runs off or infiltrates the soil, depending on ground conditions and precipitation events.

Hydrological effects are significant in many regions, where snowmelt provides a major portion of annual runoff. Melt timing controls streamflow peaks, groundwater recharge, and soil moisture. Rain-on-snow events or warm conditions can cause rapid runoff and floods, while cold periods can delay release and preserve water in the snowpack.

Factors influencing snowmelt include temperature, solar radiation, elevation and aspect, wind, cloud cover, and snow properties such as grain size, density, and stratification. Early-season melt may be limited by a frozen ground layer that inhibits infiltration, whereas later in spring rain can hasten melt.

Measurement and modeling use snow water equivalent (SWE) as a key metric, along with snow pillows and depth sensors. Hydrological models apply degree-day factors or energy balance approaches to forecast melt rates and streamflow.

Climate change is altering snowmelt patterns, with earlier onset, longer melt seasons, and greater variability. These shifts affect water resources, flood risk, and ecosystems, prompting adaptations in forecasting, reservoir management, and land-use planning to cope with changing melt dynamics.