“To show you how to measure miner’s inches and to determine a supply of water when you do not know it, in case you should ever want to.”
“Then you won’t need to build a reservoir?” said Ted.
“Why not? This simply proves that the brook has a natural flow of about 37 miner’s inches.” Then taking out another table, he read: “One miner’s inch equals .02 cubic feet per second; 1.2 cubic feet per minute; 72 cubic feet per hour. Now an acre-inch of water, or water to cover the surface of an acre of ground to the depth of 1 inch, equals 3630 cubic feet, which 1 miner’s inch will supply in approximately 50 hours. The average amount of water for one irrigation of ordinary ground, that is not sunbaked, is 2.3 inches. With a little calculation you can determine how long it would take your flow of 37 miner’s inches to give you 2.3 acre-inches.”
“May we take that table, Andy?” asked Ted. “I’ll copy it tonight. We never could remember it, and when it is time to irrigate, we shall want to know how long it will require.”
“But what has all this to do with the reservoir?” Phil inquired, as the agent handed the table to his brother.
“Just this. On the reverse of the table you will find the miner’s inch represented in gallons; 27,152 gallons are required for an acre-inch. When you get the west side cleared, you will have about 60 acres. Now 1 cubic foot of water equals 7.48 gallons. To get the required depth of 2.3 acre-inches for irrigating, you must have 8349 cubic feet of water, or practically 62,450 gallons per acre, or 3,747,000 gallons for the 60 acres. As your reservoir will have a depth of only 10 feet, you will have a million gallons, which will allow you to irrigate only about a quarter of your land at one time. But, of course, it will be years before you will have the entire 60 acres under cultivation, considering all you have on the east side, and by that time you may be in a position to double the size of your reservoir. In irrigating, the more laterals you can use at one time the better, and the more water you have the more you can use. Now we’ll lay out the course for the ditch with our grader.”
Placing the shorter leg at the spot where the head gate to control the supply from the reservoir was to be, he told Ted to swing the longer leg until Phil should announce that the bubble was in the centre of the spirit level. When this had been done, the agent marked the second spot, then placed the shorter leg on it, and continued the operation until they had traversed all of the section to be irrigated, the contour, as the course is called, being nearly diagonal.
“Tomorrow we’ll plow a furrow connecting those grade marks and then construct a ditch,” said Andy, when the grading had been finished, “or rather begin it.”
“Where do the laterals come in?” queried Phil.
“They run from your farm, or main, ditch. For grain, they are usually 75 feet apart; for alfalfa, 90, and about 1300 feet long and they will run here at a grade of from one-half to three-fourths of an inch to the rod.”