Snow interception evaporation. Review of measurement techniques, processes, and models

A global warming, primarily affecting wintertime conditions at high latitudes will influence the functioning of the boreal forest. The least known term of the winter water-balance equation is evaporation of snow intercepted in forest canopies. Several investigations stress the importance of snow-interception evaporation in coniferous forests and evaporation fractions of gross precipitation as large as 0.2-0.5 have been observed by investigators in Scotland, Canada, and Japan. Evaporation rates as high as 0.56 mm h(-1) are reported. The largest differences between the rain and snow interception evaporation processes are the differences in Storage. Snow storage (both mass and duration) is often an order of magnitude larger than that for rain. Snow interception changes the canopy albedo although some studies indicate the opposite. Process knowledge is limited because of measurement difficulties but it is known that canopy closure, aerodynamic resistance (r(a)), and vapour-pressure deficit are important factors. Existing formulations of r(a) as function of storage location and age cannot fully explain observed differences in evaporation rates. Operational hydrology and weather models, and GCMs describe snow interception in a very simplified way and might benefit from incorporation of more realistic schemes.