Moisture derived from precipitation is disposed of in two ways. Either it is evaporated and transpired back to the atmosphere in the locality in which it fell or it runs away in the streams and eventually finds its way back to the ocean. The ability of the atmosphere to take up moisture is, of course, directly related to temperature. If at any season of the year the supply of moisture is greater than the capacity of the atmosphere to absorb it, there is a surplus to feed the streams. On the other hand, deficiencies occur when the climatic need for water, that is the evaporating power of the atmosphere, is greater than the supply. If the deficiency is great and prolonged then we call it a drought. In Canada, as a rule, surplus water comes during the winter when temperatures are very low. Much of it accumulates as snow which does not melt until spring thus not only completely saturating the soil but giving rise to floods as well. On the other hand during the summer season the climatic need is much greater than the rainfall, the soil dries out and a moisture deficiency results.
Many investigators have tried to measure climatic moisture relations. The most recent attempt and the one which seems most adapted to Canadian conditions is that of the American climatologist, Dr. C. W. Thornthwaite. Using his formulae Mrs. M. Sanderson has drawn up the maps reproduced herewith.
The southeastern seaboard of Nova Scotia and Newfoundland has a surplus of more than 25 inches per year while the west coast has more than 50 inches. These, of course, are the areas of greatest rainfall. On the other hand the Prairie Provinces and Northwest Territories have a surplus of less than two inches and some small areas actually have none at all.
Very large areas in the east have less than two inches while the same is true for the west coast and high mountain areas. The southern prairies and the interior of British Columbia have annual deficiencies which may amount to more than eight inches.
Southwestern Ontario, for instance, has a deficiency of four inches and a surplus of ten. These, of course, occur at different seasons giving that area a pronounced wetand-dry climate. The same is true in varying degree for other parts of Canada. While zero isopleths have not been drawn on the maps and, in fact, would be very hard to locate, it is evident that somewhere in western Ontario, or eastern Manitoba, very low deficiencies cancel out equally low surpluses. This is a very significant zone which separates the humid east coast climate from that of the interior. In the Cordilleran region a similar zone, which is even more difficult to indicate on a map, separates the interior from the moist west coast.