242 SOILS: PnOPEBTIES AND MANAGEMENT 



part of the hygroscopic water vaporizing in the order 

 named. If the loss of the surface moisture were the 

 only consideration, the problem would not be serious; 

 but the capillarity of the soil must be considered also. 

 As the films at the surface become thin a capillary move- 

 ment begins, and if the evaporation is not too rapid a 

 very great loss of water may occur in a short time. 



The evaporation from a bare soil in the Rothamsted 

 lysimeters^ averaged about seventeen inches a year, 

 with a rainfall ranging from twenty-two to forty-two 

 inches. This means that from one-third to one-half of 

 the effective rainfall was entirely lost as thermal water. 

 The necessity of checking such a loss becomes apparent, 

 especially in regions where rainfall is slight or drought 

 periods are likely to occur. As no country is free from 

 one or the other of such contingencies, the great promi- 

 nence that methods of moisture conservation hold in 

 systems of soil management is understandable. While 

 means of checking losses by leaching and run-off are 

 advocated, effective retardation of surface evaporation 

 is always particularly emphasized. 



^ Warmgton, R. Physical Properties of the Soil, p. 109. 

 Clarendon Press, Oxford. 1900. 



