Snow hydrology is a scientific study in the field of hydrology which focuses on the composition, dispersion, and movement of snow and ice. Studies of snow hydrology predate the Anno Domini era, although major breakthroughs were not made until the mid eighteenth century. Snowfall, accumulation and melt are important hydrological processes in watersheds at high altitudes or latitudes. In many western states in United States, snow melt accounts for a large percentage of the spring runoff that serves as water supply to reservoirs, urban populations and agricultural activities.