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

A global warming, primarily affecting wintertime conditions at high latitud es will influence the functioning of the boreal forest. The least known ter m of the winter water-balance equation is evaporation of snow intercepted i n forest canopies. Several investigations stress the importance of snow-int erception evaporation in coniferous forests and evaporation fractions of gr oss precipitation as large as 0.2-0.5 have been observed by investigators i n Scotland, Canada, and Japan. Evaporation rates as high as 0.56 mm h(-1) a re reported. The largest differences between the rain and snow interception evaporation processes are the differences in Storage. Snow storage (both m ass 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 difficul ties but it is known that canopy closure, aerodynamic resistance (r(a)), an d vapour-pressure deficit are important factors. Existing formulations of r (a) as function of storage location and age cannot fully explain observed d ifferences in evaporation rates. Operational hydrology and weather models, and GCMs describe snow interception in a very simplified way and might bene fit from incorporation of more realistic schemes.