Dry Farming and Irrigation 165 



thousand acres. Besides this, it gives water power 



capable of generating electricity enough to do the work 



of nearly ten thousand horses. The largest dam in 



the West is the Elephant Butte Dam across the Rio The dams 



Grande River. It irrigates parts of Texas and New 



Mexico. The highest, though not the largest, dam in 4 rr ? M ' 



the world is the one at Arrow Rock, across the Boise 



River, in Idaho. The largest masonry dam in the world 



is the Assuan Dam in Egypt across the Nile River. It TheAssuan 



is one hundred and twelve feet high in the highest place 



and is more than a mile long. 



The principal ways of using irrigation water are sprin- Four ways 

 kling, furrow irrigation, level check flooding, and hillside f n irri ^ ai ~ 

 flooding. 



Gardens and other small tracts are often watered by Sprinkling 

 sprinkling with a garden hose. Sometimes a system 

 of overhead pipes is run above the area to be watered, 

 and the gardener has but to turn on the water. In 

 very hot weather, if plants are sprinkled during the 

 middle of the day the leaves may be scalded. The late 

 afternoon is a better time to sprinkle. 



The chief objection to sprinkling is that usually not 

 enough water is applied at once. Whatever method of 

 irrigation is used, the water should flow until the ground 

 is thoroughly soaked to a depth of two feet or more. 

 Then it should not be necessary to repeat the irrigation 

 for at least ten days or two weeks. If the surface of The effects 

 the ground is kept wet, and the soil below is allowed ^nddeep 

 to become dry, the roots will spread out near the surface, wetting 



compared 



Thoroughly wetting the ground to a good depth and 

 allowing it to become dry on top encourages the roots to 



