128 LAND DRAINAGE AND IRRIGATION 



under construction, bringing under cultivation millions of acres. 

 According to the Thirteenth Census, there are in all about 

 14,000,000 acres of irrigated land in the continental United 

 States (Fig. 63). 



Irrigation or flooding proves beneficial and necessary in growing 

 certain crops, such as rice and cranberries. 



On irrigated lands flooding is extensively used to moisten 

 fields preparatory to plowing; and flooding, instead of tillage tools, 

 is frequently employed to firm a seed bed prior to planting. 



In some sections irrigation is extensively practiced, supple- 

 menting a good and evenly distributed rainfall, to make highly 

 fertilized lands produce the highest possible yields. This is espe- 

 cially true in densely populated countries, like China, for example. 



In some places irrigation is the means whereby fertilizing 

 material is carried to the land sewage from great cities like Paris 

 and Edinburgh, for example. In several countries the water of 

 rivers is used to fertilize meadows, and, in most cases, to supply 

 needed water at the same time. 



How Irrigation Water is Secured. Before any area of land can 

 be irrigated a source of water must be -provided. The most 

 common sources are rivers. In such cases, a portion of the river 

 water is diverted and conducted by means of canals, conduits and 

 huge pipe lines to the area to be reclaimed, where it is turned from 

 the main canal to branch canals which carry the water to the farms 

 to be irrigated, to which it is delivered by still smaller branches. 

 It is interesting to read about or to see some of the wonderful 

 engineering feats accomplished in some of the great western 

 irrigation projects (Fig. 64). 



Other sources of irrigation water are lakes, large reservoirs 

 made by constructing dams across gorges or narrow valleys, and 

 flowing wells. Water may also be secured by pumping it from 

 wells, streams and canals. 



How Irrigation Water is Applied. There are four ways in which 

 irrigation water may be applied to land by flooding, through 

 the use of furrows, by spraying, and by subsurface irrigation. 

 Flooding and the furrow method are the two approved methods 

 of irrigation in irrigated sections. Whichever method is used 

 depends upon the character of the soil, the lay of the land, the kind 

 of crops, the water supply, and the "head," or the volume of water 

 supplied to the unit of time. Under some systems of irrigation 

 management, farmers are given large streams of water for short 



