CHAPTER IX 



LAND DRAINAGE AND IRRIGATION 



This chapter may be considered as a continuation of the sub- 

 ject of ^'Moisture Conservation and Control/' which was partly 

 considered in the preceding chapter. 



Land drainage may be defined as getting rid of free or excess 

 water from wet lands. This subject is a broad one, touching 

 both the field of engineering and of soil management. Because 

 of lack of space, we shall study in this chapter the relation of good 

 drainage to soil fertility, and consider the fundamental principles 

 of land drainage and irrigation. 



Too Much Water is Harmful. — It is common knowledge that 

 water standing long on a cultivated field either injures or destroys 

 the crop. This injury is largely the result of the shutting out of the 

 air and the checking of the soil activities, because of too much water. 



Other Harmful Effects of Too Much Water. — ^Wet soils are 

 cold, and this retards germination and plant growth. Too much 

 water makes fields, or portions of them, unfit for cultivation. Much 

 injury is done poorly drained soils when they are worked too wet. 

 Injury of crops by heaving (through frost action) is greater on 

 wet lands than on those having good drainage. Wet fields which 

 become dry enough to permit of late planting are less profitable 

 than when well drained; moreover, such conditions make crop 

 production uncertain (Fig. 45). It is generally recognized that 

 wet swamp lands are detrimental to pubHc health. 



Benefits of Proper Drainage. — ^Many direct benefits are to be 

 derived through proper drainage. The most important are 

 the following: 



1. Soils Warm Up Better. — ^Water requires much heat to warm 

 it, and a very great amount to evaporate it.^ A httle of the heat 

 of the sun is absorbed by the water in a wet soil, but the most of 

 it is used up in evaporation. Thus it is that a wet soil is a cold 

 soil (Fig. 46). If the same land were well drained, the heat of 

 the sun would be absorbed by the soil instead. 



1 To raise the temperature of a pound of water (1 pint) l"* F., requires 

 1 British heat unit. To evaporate a pound of water requires 966.6 heat 

 units. It requires no more heat to raise 7 pounds of dry loam one degree 

 than to warm a pound of water one degree. 



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