Snow compaction method for the analysis of runoff from rain on snow

A description of a water budget method used in inflow design flood studies to account for the effect of a fresh snowpack on the runoff from rain and snowmelt in the mountainous areas of the western U.S. is presented. The parameters used in the method were derived from a laboratory experiment and were checked by analysis of a large flood in California. The unique feature of the method is the evaluation of the amount of compaction (or shrinkage) in a fresh snowpack that results form the metamorphosis of the snow crystals when water is added. The snow crystals in a fresh snowpack undergo changes, as free water is added, that results in a shrinkage (or compaction) of the snowpack. Free water is retained in the snowpack until the threshold density is attained. Subsequent melting releases this free water. The procedure described, which uses a water budget based on the concept of snow compaction and a threshold density, has been a valuable aid in our design flood studies to estimate runoff from a design condition of warm rain on a relatively fresh snowpack.