at the growing period, i.e. about 12 inches per month for a 

 little over 2 months. 



207. Irrigation of paddy fields. Suppose one wishes to 

 provide for the irrigation of paddy fields. What provision of 

 water should be made? It is enough if rice plants have \ an 

 inch of water at their base for 72 days, i.e. if they have 36 

 inches of rainfall during the three months of vigorous growth 

 from July to September. An acre (4840 sqr. yards) would 

 thus require 4840 cub. yds. of water. An allowance of 

 2,160 cub. yds. may be made for evaporation and percolation, 

 and the total maximum requirements per acre may be thus 

 put down at 7,000 cub. yds. for the 72 days. Now there 

 are 640 acres in a square mile. If a square mile of rice 

 fields has to be provided with the maximum quantity of 

 water (for a season of severe drought), and the water in the 

 canal runs at the rate of i mile an hour, a vent of only about 

 9 sq. feet is required. To provide means of irrigation for 

 any considerable agricultural area by means of tanks and 

 wells is not feasible. 



208. Drainage. We have said over-irrigation or irriga- 

 tion with water surcharged with soluble salts, results in an 

 accumulation of these salts in the soil which gradually render 

 it barren. Where canal irrigation is provided the means 

 of correcting the evils of irrigation should be also provided. 

 This consists in having drainage channels. Drainage would 

 make usar land fertile. A land which is drained, readily 

 parts with its soluble salts. Irrigation canals should be built 

 with a fall of i foot per mile and the drainage channels 

 should have a fall of 2 feet to the mile, and the drains 

 empty themselves finally into a canal, stream, or river 

 farther down where the level is 6 to 8 feet below the level 

 of the highest portion of the channel where the particular 

 irrigation section begins. Drainage and irrigation channels 

 should be simultaneously provided wherever water other 



