THE CONTROL OF SOIL MOISTURE 



271 



RECORDS FROM ROTHAMSTED (1870-1878) 1 



A rough calculation may be made which will show the 

 apportionment of the yearly rainfall in a humid region of 

 the temperate zone between the three forms of losses 

 run-off and percolation, evaporation, and transpiration. 

 The percolation under a rainfall, say, of 28 inches, as 

 shown by the Rothamsted work, is roughly 14 inches, or 

 50 per cent. The water requirement of an ordinary 

 crop is about 7 inches. This leaves a loss of 7 inches 

 to be credited to evaporation. In other words, one- 

 half the rainfall goes as run-off and percolation, while 

 the other half is divided about equally between the plant 

 and loss by evaporation. While run-off and percolation 

 may be checked to some extent, not enough conservation 

 can occur in this direction to tide a crop over a period 

 of drought. Paramount attention should therefore be 

 directed toward the checking of losses by evaporation, 

 since moisture thus saved means just that amount added 

 to the water available for crop use. It should be remem- 

 bered that over a large proportion of cultivated lands 



1 Warington, R. Physical Properties of the Soil, p. 109. 

 1900. 



