CHAPTER XII 

 THE USE OF THE RAINFALL 



RAIN falls upon the whole surface of the earth Where 

 there is much rain, the country is called humid; where 

 there is little rain, the country is called arid. Humidity 

 and aridity are conditions that depend, primarily, upon 

 the water that falls from the heavens as rain or snow, 

 although where water-dissipating factors, such as winds 

 and shallow soils, are small, a low rainfall may be more 

 effective than a high rainfall where these factors are large. 

 Growing plants require large quantities of water. Some 

 of this necessary water evaporates directly from the soil; 

 another part evaporates from the leaves of the plant; 

 some water may be lost, also, by seepage through the 

 soil. Unless there is enough water in the soil, it is impos- 

 sible for plants to thrive and to yield sufficient returns 

 to the farmer. Irrigation is the art whereby the deficiency 

 in the natural rainfall, whether large or small, is supplied 

 by water, artificially added, so that regular, abundant 

 crops may be obtained. 



140. Irrigation supplementary to rainfall. Such a 

 definition of irrigation makes it evident that the quantity 

 of water to be used hi irrigation depends on the degree of 

 the natural precipitation. The higher the annual rain- 

 fall that may be retained hi the soil for crop use, the 

 smaller the quantity of water required in irrigation. The 

 lower the annual rainfall that may be so retained, the 

 higher the irrigation requirement. This is a somewhat new 



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