268 SOILS: PROPERTIES AND MANAGEMENT 



about by drainage operations or by methods that will 

 increase the water-holding capacity of the soil. The 

 former is really a matter of engineering technique and 

 will be treated in a separate chapter. The latter is a 

 function of the soil itself and must be specifically con- 

 sidered at this point. 



The necessity of giving attention to losses due to 

 run-off and leaching varies with climatic conditions. 

 In very humid regions these losses are of grave importance, 

 while in arid regions they are insignificant as compared 

 with losses by evaporation. For example, in England 

 the losses by percolation and run-off in many cases are 

 as high as 60 per cent of the rainfall. In the Mississippi 

 River basin the loss is 50 per cent, in the Missouri it is 

 about 20, while in the Great Basin it drops to zero. This 

 does not indicate that drainage is not practiced in the 

 last-named region, however, for, owing to over-irrigation, 

 seepage, and other conditions, drainage operations often 

 become as important as in humid climates. 



The quantity of water entering a soil is determined 

 almost entirely by the physical condition of the soil. 

 If the soil is loose and open, the water enters readily and 

 little is lost over the surface as run-off. If, on the other 

 hand, the soil is compact, impervious, and hard, most 

 of the rainfall runs away, and not only is there a serious 

 loss of water, but considerable erosion may also result. 

 The first step in checking run-off losses, therefore, is 

 strictly physical in nature. As the water that has entered 

 the soil moves downward it is continually being changed 

 to capillary water in its passage. If the capillary capacity 

 of the soil is high, a greater percentage of this rain water 

 becomes capillary and a less percentage is left to be carried 

 away as gravitational water. The secret in the control 



