Annual runoff

The map shows the annual runoff in millimetres. Annual runoff varies from year to year owing to variations in precipitation, evaporation, and natural storage. The map presents the long-term average of these annual variations. The annual runoff is smallest in southern Saskatchewan and southeastern Alberta with values lower than 25 millimetres. The greatest annual runoff occurs on the west coast of British Columbia where values reach over 3200 millimetres. In the east of the country, the values are around 1000 to 1400 millimetres on the coast of Newfoundland and decrease as one goes west to reach values lower than 200 millimetres in Manitoba. The map has been compiled using actual and synthesized data from a number of sources. In most parts of the country, isolines of mean annual runoff are shown at 100-millimetre intervals, but for mountainous terrain, particularly in British Columbia, such an interval is not possible on a map of this scale. A few representatives isolines are shown, to indicate the trend of runoff variation. In addition, the location of the isolines for much of the northern part of the country is approximate because of the scarcity of streamflow records in this area. The map is so constructed as to represent the annual runoff at its place of origin, that is, where it first collects in stream channels as streamflow. The map does not represent annual runoff at its point of actual gauging in the river channel. The method of presenting annual runoff is therefore comparable to that of the hydrologic parameters in this atlas; for example an average depth of water over an area.