Snow and ice

The occurrence of precipitation in the form of snow as opposed to rain typically causes a change in how a drainage basin responds to the input of water. The reason for the modified hydrological response is that snow is held in cold storage on a basin for an extended period of time before it enters the runoff process. There is such a vast difference in the physical properties of snow and other natural surfaces that the occurrence of snow on a drainage basin can cause significant changes in the energy and water budgets. As an example, the relatively high albedo of snow reflects a much higher percentage of incoming solar shortwave radiation than snow-free surfaces (80% for relatively new snow as opposed to roughly 15% for snow-free vegetation). Snow may cover up to 53% of the land surface in the northern hemisphere (Foster and Rango, 1982) and up to 44% of the world’s land areas at any one time. On a drainage basin basis, the snow cover can vary significantly by elevation, time of year, or from year to year. The Rio Grande basin at Del Norte, Colorado is 3419 km2 in area and ranges from 2432 m a.s.l. at the streamgage up to 4215 m a.s.l. at the highest point in the basin. Figure 11.1 compares the snow cover depletion curves obtained from Landsat data in 1977 and 1979 in elevation zones A (780 km2; 2432-2926 m), B (1284 km2; 2926-3353 m), and C (1355 km2; 3353-4215 m) in the Rio Grande basin. In a period of two years from April 10, 1977 to April 10, 1979, a great difference in seasonal snow cover extent was experienced. Landsat data show that 49.5% or 1693 km2 were covered by snow on April 10, 1977. Only two years later in 1979, 100% or 3419 km2 were snow covered on April 10.