54 



THE IRRIGATION AGE. 



BROUGHT BY THE POSTMAN. 



Letters from Correspondents to The Irrigation Age. 



i LONDON, OHIO. 



hditor THE IRRIGATION AGE, Chicago, 111. : 



Dear Sir The writer discussed in a former number of 

 THE IRRIGATION Ac the velocity of flow and the discharge 

 in cubic feet per second of ti!e drains under a given head 

 and diameter. It was there shown that the velocity of flow 

 in feet and the discharge in cubic feet per second, approxi- 

 mately, was 2.27664 feet for the flow and 0.4458984 of a cubic 

 foot for the discharge, per second, of our 6-inch tile drain. 



The reader was informed that many things conspired to 

 make all computations of the velocity of flow in feet and dis- 

 cfiarge in cubic feet, per second, of tile drains, mere approxi- 

 mations. Here let another be named. The internal circum- 

 ference friction of a 6-inch tile drain is half as much as the 

 circumference friction of a 12-inch tile drain, while the cross 

 section area of 6-inch drain is only one-fourth that of the 

 12-inch. Here the 6-inch has half the friction of the 12-inch 

 and only one-fourth its working capacity. 



If the reader desires to witness the fidelity and honest 

 working of our 6-inch drain during varied rainfalls let him 

 don his waterproof and hat to make believe it rains and hie 

 away with the writer to the field in which our 6-inch drain 

 of 2,640 feet length and 12 feet head is at work. Now, as 

 our 6-inch drain may be "conceity," let us figure out what 

 our drain would do with a downpour of two inches of rairi 

 per hour. To simplify our work, let us take the discharge 

 per second of our drain at 0.446 of a cubic foot. The dis- 

 charge of 0.446 of a cubic foot of water per second would 

 equal 26.76 cubic feet per minute and 1605.6 cubic feet per 

 hour. There are 43,560 superficial feet in an acre and a 

 downpour of 2 inches of rain upon these 43,560 superficial feet 

 gives us a number of cubic feet of water equal to the one- 

 sixth of 43,560, equal to 7,260 cubic feet and as one-half 

 of these 7,260 cubic feet is supposed to be taken up by the soil, 

 there remain 3,630 cubic feet per acre to be carried by a 

 drain. Our 6-inch drain is 160 rods long and must have a 

 strip one rod for an acre and on this acre, 2,640 feet long 

 and 16J4 feet or one rod wide, there fail in a downpour of 

 rain of 2 inches per hour 3,630 cubic feet of water to be 

 carried away by a drain. Can our 6-inch drain do it? No 

 Above you see our drain can only carry 1,605.6 cubic feet 

 per hour, instead of 3,630 cubic feet. This takes the conceit 

 all out of our 6-inch drain. 



Now, let us see what the diameter of a drain must be to 

 carry, as they come, these 3,630 cubic feet of water falling on 

 an acre strip 2,640 feet long and 16J4 feet wide. The dis- 

 charging capacity of tile drains are to each other as the 

 squares of their internal diameters. This gives us the pro- 

 pprtion, as 1,605.6 cubic feet of water carried per hour by 

 otir 6-inch drain are to 3,630 cubic feet to be carried, so is 

 the square of 0.5 of a foot to the square of the required 

 diameter. The square of 0.5 of a foot is 0.25 of a foot, and 

 multiplying 3,630 feet by 0.25 gives 907.5 cubic feet for a 

 product and these 907.5 cubic feet divided by 1605.6 gives a 

 quotient of 0.5645 of a foot for. the square of the diameter 

 of the required drain to discharge 3,630 cubic feet from our 

 strip 160 rods long and one rod wide. The square root of 

 <X5645 of a foot is 0.75 of a foot, or 9 inches, the required 

 diameter. 



Next let us see how wide a strip 2,640 feet long our 

 6-inch tile drain can do the work for. To get this we must 

 state as 3,630 cubic feet coming upon the acre strip per hour 

 are to 1,605.6 cubic feet discharge of our drain per hour 

 so are 16^ feet width of acre strip to width of strip our 

 drain can carry, that is 3630:1605.6: :16J4 :7.3 feet, nearly. 



Now, suppose it were necessary that our 6-inch drain, 

 2,640 feet long, should drain a strip 8 rods or 132 feet wide, 

 what continuous rainfall per hour could our 6-inch drain do 

 the work for? To answer this query we must compute how 

 deep the water would be if 1,605.6 cubic feet of water carried 

 by our drain were uniformly spread over 8 acres or 348,480, 

 the superficial feet in 8 acres. Dividing 1,605.6 cubic feet by 

 ^48,480 superficial feet gives as a quotient 0.0046074 of a 

 foot in depth of water over the 8 acres or 1.77 thirty-second 

 of an inch per hour, which is equal to 15/3 inch per 24 hours. 

 So you can see our drain can carry away the half of a rain- 

 fall of Z 2 /3 inches in 24 hours. 



Suppose it is and has been raining at above rate and our 

 drain has fully got its harness on, let us go to its outfall 



and see how it is working there. Just see ! The tile is run- 

 ning not more than half full, just as might be expected. The 

 acceleration of velocity of flow in our tiie has so attentuated 

 the volume of flow as to reduce it one-half. See that barrel 

 of molasses with its faucet 4 feet above the floor. The 

 merchant sets his measure under" and turns the faucet and a 

 stream of molasses issues nearly half the thickness of your 

 wrist. But see ! At the measure the acceleration of gravity 

 has attentuated the stream to the size of a goose quill. The 

 cause is the same. Suppose we have a field of 80 acres, 

 twice as long as wide, of comparatively level land, with a 

 serpentine swale running somewhat centrally through it* the 

 long way. What size tile and how deep should it be laid 

 in the swale? Since it would have to become a main to 

 laterals and sub-laterals in the swale the tile should be laid 

 to a depth of not less than 6 feet and deeper where the cut- 

 ting across the tapering end of a swell would materially 

 straighten the ditch and if the laterals are let in at about 

 uniform distances, the first 40 rods at outlet of main should 

 have 12-inch tile, the next 40 rods 10-inch, next 40 8-inch, anu 

 upper 40 could do with 6-inch or less, this supposing no 

 water comes from the land above. If water did come on our 

 field from the land above it would have to be provided for 

 according to its volume. There are riparian water rights. 

 You are above. You can not divert the natural flow of a 

 stream of water onto the land below if the owner objects. 

 If below, you must take the water as it comes if the owner 

 above so desires. You have your remedy. If the land above 

 is benefited it must assist the land below in drainage. Par- 

 don digression. 



All laterals should be let into Ys in the main on side 

 and never into Ts. The flow of water in a lateral let into 

 a main by a T tends to expend its force of flow on the inner 

 opposite surface of the main and the flow of the volume 

 of water in the main is impeded by having to overcome the 

 inertia of and set in motion at right angle to its former 

 course the volume of water entering it from the lateral. 



The lateral entering our main should start at a depth of 

 not less than 4 feet, increasing to 5 feet or more at the main. 



Once more in regard to deep under drainage. Some 

 farmers advocate the laying of tile at a depth of 20 to 24 

 inches for the reason, they say, that in a downpour of rain 

 the water can reach the tile and away :>o much quicker than 

 if laid so deep. That the water could reach the tile quicker 

 is true, but with little head to force it on its way, compared 

 with the deeply laid drain. The advantage of tile drainage 

 laid to a depth of 4, 5 and 6 feet over drainage to a depth of 

 20 to 24 inches does not end with the readiness with which 

 the water can reach the drain. Whether hot or cold, rain or 

 shine, snow pr blow, day or night, the deeply laid drain is 

 ever at work, so long as any water remains in the soil its 

 capillary pores can not hold. 



The air entering this deeply drained soil aerates it, 

 slakes it like lime and makes it porous and it becomes a vast 

 sponge, ready to drink up a downpour of rain before any 

 water can reach the drain to put it to work. The air, com- 

 posed of oxygen and nitrogen, partners in the chemical lab- 

 oratory of Nature, entering a deeply drained soil, even of 

 hard pan, thus freed from superabundant moisture, aerates 

 it and produces in the soil those conditions necessary to 

 plant growth. Is this all ? Nay, verily. There can be no 

 all to a thing, illimitable. Many of our spils are alkaline. 

 With these as bases, acids form salts. Many salts are 

 deleterious to plant growth. These salts and water have a 

 great avidity, each for the other. Superabundant water enter- 

 ing a deeply drained soil absorbs these salts and in solution 

 hies away with them adown the drain, brook, creek and 

 river to Mother Ocean's salt storehouse. So alkaline lands 

 by deep under-drainage in time become salt neutral. 



J. ARNETT. 



NEW YORK CITY, N. Y., Nov. 7, 1904. 

 EDITOR THE IRRIGATION AGE, Chicago, 111. 



My Dear Sir Having a deep personal interest for many 

 years in irrigation, and being called upon now and then to 

 write upon the subject, I am impelled on reading your anni- 

 versary number to say that I have never been able to find any 

 publication but THE IRRIGATION AGE that would keep me thor- 

 oughly informed regarding irrigation matters. I am always 

 glad to see THE AGE and only wish it came more often. 



Very truly, CHARLES C. JOHNSON. 



