WATER-POWERS OF ALABAMA. ]3 



ESTIMATED MONTHLY DISCHARGE, ETC. 



This table gives in the first three columns, the maximum, 

 minimum, and mean discharge for each month in cubic feet 

 per second (second feet.) Column No. 4 gives the "total 

 acre feet" flowing down the stream during each month. An 

 "acre-foot" is the amount of water that would be necessary 

 to cover one acre with a depth of one foot, which is 43,560 

 cubic feet. It furnishes a convenient unit for storage, where 

 the water is to be used for irrigation. A cubic foot is practi- 

 cally 7.48 gallons, and is usually estimated at 7.5 gallons. 

 An acre- foot is 43,560 cubic feet, or 320,851 gallons. One 

 cubic foot per second flowing for 24 hours will cover an acre 

 to a depth of 1.98 feet. It is therefore customary in round 

 numbers to state that a cubic foot per second for a day of 

 24 hours is equivalent to 2 acre feet. Now, as one inch of 

 rainfall per hour falling for 12 hours would cover one acre 

 a foot deep, it is evident that rainfall at the rate of I inch 

 per hour will produce a flow of I cubic foot per second, or 2 

 acre feet per 24 hours for each acre of watershed, no allow- 

 ance being made for evaporation or percolation. It is also 

 convenient to remember that 1,000,000 gallons in a reservoir 

 are equal to a little more than 3 acre feet (3.069). In a 

 general way it may be said that water stored in reservoirs 

 is reckoned in acre-feet for irrigation, cubic feet for water 

 power, and in millions of gallons for city watxj&eupply. 



Columns 5 and 6 give the "run-off" from the drainage 

 area. The run-off in inches and decimals of an inch is given, 

 just as rainfall is given. For instance, a run-off of 2.23 

 inches from a given drainage area, means that enough water 

 ran off during the month to have covered the entire drain- 

 age area or water-shed to a depth of 2.23 inches. This is 

 convenient in estimating the proportion of the rain-fall on 

 any drainage area that can be stored for irrigation, city 

 water supply, or other purposes. The run-off in second-feet 

 per square mile of drainage area, is obtained by dividing the 

 mean discharge for the month by the number of square miles 

 in the drainage area, and is useful in estimating the mean 

 discharge of a tributary whose drainage area is known, and 

 in comparing different drainage areas. The "run-off" is 

 not a fixed percentage of the rainfall, but is that part of the 

 rainfall which is not lost by evaporation into the air, or by 

 percolation in subterranean outlets. Being a remainder and 



