Snow Water Equivalent




In many applications (e.g. flood forecasting, agriculture, reservoir management), the amount of water held in the winter snowpack (snow water equivalent or SWE) is of primary importance. SWE essentially represents the depth of water (mm) that would be produced if the snow melted. Weekly or bi-monthly measurements of SWE are commonly made by provincial water resource agencies and hydroelectric companies. These measurements are most often collected by manually extracting a snow core and measuring the depth and mass of the snow sample along pre-determined transects (or snow surveys) to monitor the amount of water stored in the snowpack prior to melt. From 1955 to 1985, Environment Canada published an annual summary of Canadian snow survey data that have been digitized to create a national snow course data set covering the early 1960's to 2003 (MSC, 2000). At peak levels in the early 1980's, there were over 1,700 snow courses operating in Canada, but this number declined to around 800 in the early 1990's. Unfortunately, there are relatively few snow courses with consistent long periods of data for analysing historical variation in SWE.

SWE can also be derived from passive microwave satellite measurements (e.g., Goodison et al., 1986) and has been shown to provide SWE values to within +/- 20 mm of in-situ measurements in the Canadian Prairies (Figure 1; Derksen et al., 2003). Extending satellite estimates of SWE to other environments has proven to be challenging, but validated SWE algorithms have now been developed for the Canadian boreal forest and tundra environments (Derksen, 2008; Derksen et al., 2010). SWE estimates can also be derived from snow depth information as snow density tends to follow a fairly well-defined seasonal evolution that can be estimated from available snow course data (Brown et al., 2003).

Interactive SWE Data of Canadian Prairies
Figure 1: Interactive map of SWE data for the Canadian Prairies, presented in pentads (5-day periods) for years 1978-2007 and in weeks (7-day periods) for years 2010-2012. The coloured contours show variations in SWE derived from passive microwave satellite data. Data are available only during the winter months. SWE anomalies have been calculated relative to the 1978-2007 average. SWE anomaly data is not available for years 2010-12. Data courtesy of Environment Canada, Chris Derksen, current as of 24 August 2012.

Interactive SWE Data of Northern Hemisphere
Figure 2: Interactive map of SWE for Canada and the Northern Hemisphere. The coloured contours show variations in SWE derived from passive microwave satellite data. Data are available only during the winter months, and data for some months is missing, especially early and late winter during certain years. SWE anomalies have been calculated relative to the 1979-2013 average. Erroneous map data, such as Asia appearing covered with water, appear during some months (especially October and May), but this does not affect the SWE estimates. Data courtesy of the European Space Agency's GlobSnow project, Finnish Meteorological Institute, current as of 10 January 2014.


References
Snow Links

Interactive tools on this page were built by CCIN staff.