PERCOLATION 91 



varies in a contrary manner. The season of active drainage 

 through a bare uncropped loam is shown by the figures in the 

 table to commence in October and to continue till the end of 

 February. In a climate having a severe winter, as Canada, 

 this heavy winter drainage will not occur, but in its place 

 there will be a large amount of drainage in April when a 

 thaw occurs. During the spring thaw in such climates large 

 quantities of snow water will however flow away over the 

 surface, when the inclination allows of it, the frozen con- 

 dition of the soil hindering percolation. The amount of 

 annual percolation is thus considerably diminished in a severe 

 winter climate. 



It will be noticed that the deep and shallow soils both 

 deliver the same amounts of drainage at the end of winter, 

 the deepest soil continuing draining longest. In summer 

 and autumn the shallow soil yields the most drainage, the 

 supply of water to its surface by capillary action, and con- 

 sequently the amount lost by evaporation, being somewhat 

 smaller than in the case of the deeper soil. 



The amount of water passing through a soil is so largely 

 influenced by the rate of evaporation from its surface, that 

 it will be necessary to return to the subject again when the 

 conditions of evaporation have been considered (p. 1 25). 



