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THE IRRIGATION AGE. 



The Art of Irrigation 



THE IRRIGATING HEAD 



By T. S. VAN DYKE 



Having your ground level enough and levees strong 

 you turn on the water and are quickly plunged into 

 despair by the fact that it is not spreading fast enough 

 to cover the check in half a day. You have merely 

 made the natural mistake of most beginners in having 

 the head of water too small, or, what is the same thing, 

 having the check too large for it. Some turn in too 

 large a head, wash out the entrance gate with the eddy, 

 cut a big hole in the ground and wash the silt over the 

 ground to make it bake worse, break a levee which goes 

 so quickly that it takes the next check perhaps or a 

 whole line of them before the water can be shut off. 

 Too small a head is just as bad in loss of. time and 

 waste of water also, for if it takes too long to 'cover 

 the part of the check that first receives it more water 

 soaks in there than it needs, while the rest does not 



To measure this accurately enough when running 

 requires a weir which you will find so well described in 

 other books on irrigation or hydraulics that I shall 

 pass it by for want of space, it being only one of the 

 instruments of irrigation and one that you may not 

 have to use at all. This weir is essential for large 

 quantities of water, but for small ones is generally a 

 nuisance. The miners in early days of California 

 adopted a pressure measurement which for small quan- 

 tities is accurate enough and is often used to measure 

 as high as two second feet or over. Thus, if a hole an 

 inch square be made in the side of a box and the water 

 inside be kept at a level of four inches above the center 

 of the hole the discharge will be practically one-fiftieth 

 of a second foot. This will be about nine gallons a 

 minute, about thirteen thousand in twenty-four hours, 

 will just fill a 12-foot cube in twenty-four hours which 

 makes 1,728 cubic feet a day. 



This is the common measurement for small quan- 

 tities in California, Arizona and some other states, but 

 in others it is under a pressure of six inches or even 

 seven. In all cases it is called the miner's inch. But 

 you know not what it is until the pressure is given. 

 It is used with a hole one, two or three inches high, 



Hot Outlet of the waste way of the Tri-State Canal. The floor of this part of the waste way is four feet below the bottom of 

 the canal. When water is turned from the canal into the river all silt is carried away, thus preventing the wash filling the canal. 



get enough. This slow feeding is a constant source of 

 annoyance and loss and to avoid it properly you must 

 understand the measurement of water, a subject puz- 

 zling to some, but simple enough to one who will take 

 a little time to study it. 



The standard measure in the United States is a 

 cubic foot a second, or "second foot," as it is more 

 commonly called. If you had an aqueduct one foot 

 deep and one foot wide inside with the water moving 

 at an average velocity of one foot a second this would 

 give you a second foot. But the surface velocity is 

 not the average velocity, because of the friction. For 

 the smoothest flume the surface velocity in the center 

 should be 25 to 35 per cent more than one foot a second 

 and for rough ones often twice that. By dropping a 

 handful of oat meal into a flume carrying water you 

 will see the whole thing as some of the particles float 

 and others sink slowly to the bottom. Even a flume of 

 glass will show eddies all through it in this way and 

 every one of them means some loss of velocity. 



But you don't have to be accurate about this. You 

 need only remember that a second foot is seven and a 

 half gallons a second, or a kerosene can, once and a 

 half full, and that it will equal an inch of rain an 

 hour on one acre or two feet a day. 



and many inches long with the pressure kept even on 

 the center, the discharge being regulated by a slide 

 running in from one end. The difference in pressures 

 makes great confusion and this is not the best measure- 

 ment, though you should know what it it. The four- 

 inch pressure has the advantage of being one about 

 fiftieth of a second foot, which equals half an inch 

 (rain measure), in twenty-four hours on one acre. 



The acre foot is a more satisfactory expression for 

 the quality of water used in irrigation, being the amount 

 necessary to cover an acre a foot deep. An acre inch 

 is also used the same way though a rain inch or a rain 

 foot is simpler as every one understands rain measure. 

 For this reason I shall use the rain inch in this book 

 wherever possible. 



Nbw if you could just lay down a blanket of water 

 an inch thick on an acre perfectly flat with levees 

 around it to keep it from running off before it could 

 soak in you might be happy, for a while at least. But 

 this is exactly what you cannot do. If the soil is as 

 porous as it should be for good results it will be quite 

 sure to cut badly if you rush in water enough to cover a 

 check in a few minutes. And even then you are likely 

 to get into the ground an inch and a quarter or more 

 on the side near point of delivery to three-fourths of 



