126 RELATIONS OF SOIL TO WATER 



determining a greater growth, and a greater evaporation from 

 the vegetation covering the surface. Thus the growth of 

 forests tends greatly to diminish the volume of springs and 

 rivers, and the destruction of forests is often followed by 

 disastrous floods. 



We have seen that on the bare soil at Rothamsted, active 

 drainage commences in October, and lasts five months. In 

 the wheat field at Rothamsted, a running of the drain-pipes is 

 rare between March and September, owing to the drying of the 

 land by the crop. The drain-pipes do not usually commence 

 to run freely till November, and active drainage is limited to 

 four months. With crops which cover the land in autumn 

 and winter, drainage is reduced to a minimum. The farmer 

 has thus the power, by suitable cropping, of greatly diminishing 

 the autumn and winter drainage ; this power is of great value, 

 as the drainage water removes from the soil considerable 

 quantities of plant food, especially calcium salts and nitrates. 



Underground Water. The water which passes through the 

 soil accumulates in the subsoil at very varying depths, forming 

 a perfectly saturated stratum of soil or rock. The height of 

 this saturated stratum varies with the character of the soil, 

 and also with the rainfall and the season. When the height 

 of this saturated stratum reaches a certain point, a discharge 

 generally takes place in the form of springs, or as a general 

 drainage from a wider area, into the river valleys, and finally 

 into the sea. If no such discharge is possible, the soil becomes 

 saturated to the surface, and a swamp is produced. 



The position and behaviour of the underground water may be 

 very much complicated by the presence of beds of impervious 

 clay in the subsoil ; the existence of such a bed determines 

 at once the position of the underground water, which must 



