DUTY AND DIVISION OF WATER 339 



water, and make it unlikely that the duty as above given 

 represents the most economical use of water. When the 

 Nile overflows, water is conducted into large basins and 

 allowed to stand there until the silt carried by the river 

 water is deposited and the soil itself has become thor- 

 oughly saturated with water. Afterwards the surface 

 water is allowed to flow back into the Nile. This makes it 

 certain that plants do not use all the water actually applied 

 to the soil. Moreover, the above results represent the 

 gross duty of water in Egypt. Few studies have been 

 made of the net duty, but since the gross duty varies 

 little from that obtained in other sections of the world, 

 it is likely that the net duty is not greatly different from 

 that obtained in other parts of the world. 



Some investigations have been made also on the duty 

 of water hi southern Africa. According to Mawson, the 

 duty of water in South Africa, under an annual rainfall 

 of 20 to 35 niches, is, for grain, 115 acres; for vegetables, 

 100 to 180 acres; for cereal crops, 140 to 200 acres; for 

 sugar-cane, 50 to 70 acres; for fruit trees, 200 to 300 acres. 

 In the Cape Colony, the duty of water has been found to 

 vary from 150 to 285 acres, although two crops were raised 

 on the land. In the Transvaal, not quite 24 acre-inches of 

 water are applied to land for the production of crops. 



201. Duty of water in Asia. In Asia, as in Egypt, 

 irrigation has been practised from before written history. 

 The best example of Asiatic irrigation is India. More 

 systematic irrigation work has probably been done re- 

 cently in India than hi any other part of the world, 

 unless it be the recent irrigation progress hi western 

 United States and western Canada. 



The rainfall of India varies greatly, from the highest 

 hi the world to a condition of extreme aridity. Over the 



