268 



SOIL PHYSICS AND MANAGEMENT 



those of uniform texture. (3) Different crops require different 

 amounts of water. Forage crops, especially alfalfa, require more 

 water than cereals. (4) A fertile soil requires less water than a 

 run-down soil. (5) The amount of water required depends to some 

 extent upon the amount of water applied and the means taken to 

 conserve it. 



Duty of Water in Different Countries. Irrigation is prac- 

 ticed on all continents. The duty of water in Egypt is 115 acres 

 for cotton and other dry crops and 60 acres for rice. This is for an 

 irrigation period of 75 days. In southern Africa, where the annual 



A - % l . 



Fia. 120. Rectangular weir. 



rainfall is from 20 to 35 inches, the duty of water is, for vegetables 

 100 to 180 acres; for cereals 140 to 200 acres; for sugar cane 50 to 

 75 acres. In India the duty from June to October is 80 to 170 

 acres, while from November to March it is 90 to 200 acres. Under 

 some canals 160 acres have been adopted as the normal duty. 



In Europe the duty is somewhat higher than in most countries, 

 because of higher rainfall. The average for Spain is 172 acres, 

 while that for France, Spain and Italy is 239 acres. Investigations 

 in North America show that the duty of water is about 100 acres 

 for an irrigation sea-son of 90 days. 



Measurement and Distribution of Water. Since water is a 

 thing of such great value in irrigation, its measurement becomes a 



