CHAPTER XII. 



THE WATER OF SOILS. Continued. 



SURFACE, HYDROSTATIC AND GROUND WATER ; PERCOLATION. 



SINCE all the water of soils and plants is directly or indi- 

 rectly derived from the rainfall (including therein snow and 

 hail), some general points regarding this factor require first 

 consideration. While it is not the object of this work to dis- 

 cuss climatology in detail, yet the times of the year and the 

 manner in which precipitation comes, acts upon and is disposed 

 of in the soil under different climatic conditions, must of neces- 

 sity form an essential part of its subject matter. 



Amount of rainfall. The rain falling in the course of a 

 year is usually stated in the form of " inches " (or centime- 

 ters), implying the height of the water column that would be 

 shown at the end of the year had it all been allowed to accu- 

 mulate; or, the sum of all the successive rains (including 

 snow) observed during the year. Since this amount ranges 

 all the way from nothing, or a mere fraction of an inch (as in 

 portions of the Andes, and of the great African and Asian 

 deserts) to as much as 600 inches or fifty feet (Cherapundji 

 in eastern India), the adaptation of agricultural practice to the 

 maintenance of the proper moisture-supply to crops is largely 

 a local question, oftentimes of not inconsiderable difficulty. 

 This is especially the case where torrential rains, yielding sev- 

 eral inches of rain in a few hours, alternate with light, soaking 

 rainfall, as is very commonly the case in the interior of con- 

 tinents, and more especially in the United States east of the 

 Rocky Mountains. Westward of the same the rainfall de- 

 creases so rapidly that at or about the one-hundredth meridian 

 (the longitude of Bismark and Pierre, Dakota, and Dodge City, 

 Kansas) we already reach the annual average of 20 inches, 

 which is commonly assumed to be the limit below which crops 

 cannot safely be grown without irrigation. The " cloud- 



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