240 Irrigation and Drainage 



so satisfactory as one which expresses the depth of 

 water to be applied to a unit area, and the acre -inch 

 for English -speaking people, or the hectare -centimeter 

 for those who use the metric system, should become 

 universal. Rainfall is now universally measured in 

 units of depth, and, as irrigation is intended to make 

 good deficiencies of rainfall, it would simplify matters 

 greatly if the irrigator could call for the depth of water 

 he desired. 



An acre-inch is enough water to cover 1 acre 1 inch 

 deep; and 10 acre -inches of water is enough to cover 1 

 acre 10 inches deep, or 10 acres 1 inch deep. As an 

 acre contains 43,560 square feet, 12 acre-inches is equal 

 to 43,560 cubic feet of water, and 1 acre -inch equals 

 one-twelfth of this amount, or 3,630 cubic feet. As 

 there are 1,728 cubic inches in a cubic foot, and 231 

 cubic inches in a gallon, 1 cubic foot equals 7.48+ 

 gallons, and 1 acre-inch equals 27,150 gallons. 



As 1 cubic foot of water at 60 F. weighs 62.367 

 pounds, 1 acre -inch equals 226,392 pounds, or 113.2 

 tons of 2,000 pounds. 



Another measure frequently used in the gauging of 

 streams, and also used as an irrigation unit, is the 

 second-foot, which means a discharge or flow of water 

 equal in volume to 1 cubic foot per second of time; 

 and a stream having the volume of 1 second -foot would 

 supply an acre -inch in 3,630 seconds, or in 30 seconds 

 more than one hour. In 24 hours, a stream of 1 

 second-foot would supply 23.8 acre -inches, and would 

 cover 7.93 acres of land with water 3 inches deep. 



Still another unit in common use in the western 



