THE CONTEOL OF SOIL-MOISTURE 



203 



ing, and evaporation as to maintain optimum moisture condi- 

 tions in the soil at all times. Such control should result in 

 a proper and economical utilization of soil-water by the plant. 

 107. Run-ofif losses. — In regions of heavy rainfall or 

 in areas where the land is sloping or rather impervious to 

 water, a considerable amount of moisture received as rain is 

 likely to be lost by running away over the surface. Under 



TR.f\M3Plf^fiTiON 



KVA PORATI ON 



J FE^oiJnSrt:^ 



Fig. 36.— Diagram illustrating the various ways by wMcli water may be 

 lost from a soil. 



such conditions two considerations are important: (1) the loss 

 of water that might otherwise be of use to plants; and (2) the 

 erosion that usually occurs when much water escapes in this 

 manner. Of the two, the latter is generally the more impor- 

 tant. The amount of run-offl varies with the rainfall and its 

 distribution, the slope, the character of the soil, and the vege- 

 tative covering. In some regions loss by run-off may rise as 

 high as 50 per cent, of the rainfall, while in arid sections it 

 is of course very low, unless the rainfall is of the torrential 

 type as in the arid Southwest. 



