THE ART OF IRRIGATION. 



75 



streams to soak the ground evenly. If 

 you can get a run of only two days or a 

 day and a half, you will need a larger 

 head. Under some ditches you may not 

 be able to get a head for more than 

 twenty-four hours. Then you will need 

 a still larger one, for you will have to use 

 larger streams in the furrows and send 

 them through more quickly to ensure full 

 soaking of the lower end. If the run is so 

 short that you have to use streams so large 

 that they will run muddy or wash and cut 

 too much, then you are at the point where 

 it may pay you best to flood. For to do 

 good furrow work the streams should run 

 clear or very nearly so. If too muddy, 

 they will puddle the furrows and check 

 seepage, while the cutting of the soil and 

 washing off of fertilizers in end-waste are 

 disadvantages easily obviated in flooding 

 and not sufficiently compensated by the 

 other advantages of furrows. 



The nature of the crop will make a 

 difference in the amount of head and the 

 length of time you need it. Corn, for in- 

 stance, will not need so deep a soaking of 

 the ground as trees, although if you have 

 plenty of water you are not likely to hurt 

 it if the water is warm. So for many 

 kinds of vegetables a run of two or three 

 hours will do, and five or six will be 

 enough for most any. Young trees will 

 not need the whole ground soaked so that 

 a smaller number of furrows may do, 

 while such things as olives, that need little 

 water, will do very well even when old and 

 in full bearing without the centers be- 

 tween the rows being wet if a reasonable 

 amount is run near the tree. In the East 

 and other places where the rainfall is 

 nearly sufficient, good enough work for 

 most trees could be done with one furrow 

 on each side, while two on each side should 

 be enough for any. In such countries the 

 shortage of moisture is generally in the 

 top soil, caused by too long a delay in the 

 rain at the time when the fruit most needs 

 it. Consequently such deep soaking is 

 not needed as in the very dry countries 

 and a much shorter run of water generally 

 will suffice. It is impossible to go into 

 detail in these matters. From the analo- 

 gies of a few cases the reader must work 

 out other cases for himself; but for the 

 dry countries twenty inches for twenty- 

 four hours for ten acres is little more than 

 enough for any kind of vines or trees, 



alfalfa or other field crop irrigated with 

 furrows and not enough for most trees 

 when old and in full bearing. 



The number of streams into which to 

 divide the irrigating head will also vary 

 with the crop and the character of the 

 soil as well as the size of the head and the 

 length of the run. But if the soil stands 

 the test described in Chapter VII of hold- 

 ing up small streams, the water is quite 

 certain to soak well on each side. Hence, 

 if you can give a long run, the smallest and 

 most shallow rooted vegetation is likely to 

 be wet enough at eighteen inches from the 

 furrow and even on a ridge several inches 

 above the level of the water eighteen 

 inches away. On many soils a long run 

 of water will not wet farther than this; 

 and you need rarely feel any alarm at see- 

 ing the top of the ridge between the 

 furrows remain dry long after the water 

 has been running. If the soil will carry 

 a stream of a gallon a minute at the rate 

 of a yard a minute, the water will generally 

 work up to the roots in time, no matter 

 how high you make the ridge. 



You may therefore feel quite safe in 

 placing these little streams a yard apart 

 for almost any kind of orchard or field 

 crops. For many things, such as corn, 

 four feet will do; that is, two feet from 

 each row. On the other hand some garden 

 stuff like strawberries that are great 

 drinkers may do better with the streams 

 two feet apart or even less. A few trials 

 on a small scale will settle these questions 

 and you should make them before you 

 proceed farther in arranging the ground. 



Suppose you have found that the right 

 distance for your ground and crop is three 

 feet apart for the streams. Ten acres are 

 two hundred and twenty yards square. 

 At a yard apart you would then have two 

 hundred and eighteen streams. If the 

 head of water were thirty inches measured 

 under four inch pressure, as before de- 

 scribed, which is about the average re- 

 quired for this kind of work, each stream 

 would be a little over one-seventh of an 

 inch or about one and one-third gallons a 

 minute. Such a stream would need from 

 fifteen to twenty-four hours to cross a 

 square ten acre tract, or six hundred and 

 sixty feet. It might do it in less or take 

 even more according to porosity of the soil 

 and the care with which the furrows have 

 been made. At first it seems ridiculous to 



