CHAPTER XXII 



IRRIGATION IN HUMID REGIONS, AND SEWAGE 

 DISPOSAL 



Irrigation is generally practiced in those regions where the 

 natural rainfall is so small as to make it quite impossible to 

 grow crops without supplying water artificially. Here irri- 

 gation is an absolute necessity. In other localities, it may 

 not be a necessity, but it may be practiced profitably to sup- 

 plement rainfall, thus securing larger yields. As agriculture 

 becomes more intensive, it is to be expected that irrigation 

 of this nature will become more common. 



The regions in which the rainfall is very small are said 

 to be arid; those having sufficient rainfall to produce good 

 crops under normal conditions are said to be humid; and the 

 regions in which the rainfall is scanty or limited are said to 

 be semiarid. It is to be expected that supplementary irri- 

 gation will be practiced more in semiarid regions than in 

 humid regions. However, if a careful study be made of the 

 distribution of rainfall in many so-called humid regions, it 

 will be found that, in many years when the demand for 

 moisture is the greatest, the rainfall is insufficient. A study 

 of the rainfall at Philadelphia, by Mr. R. P. Teele, of the 

 Office of Experiment Stations, shows that, although the 

 average rainfall for that locality is large, the records indicate 

 that there were periods of drouth during 88 per cent of the 

 seasons for the 70 years covered by the investigations, which 

 dry spells caused injury to the crops that had short growing 

 periods. The investigations also showed that all crops 

 received too little water during a third of the years. 



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