THE IRRIGATION AGE. 



POINT OF GREATEST VELOCITY. 



The most casual observation will show that this is 

 far from being the case, even in straight channels. 

 The water" flows faster at the center than at the sides, 

 and generally faster at the surface than at the 

 bottom. The point of greatest velocity is usually 

 near the middle of the stream and a little below the 

 surface. This being the case, if a division board is 

 placed so as to be, for instance, one-tenth of the dis- 

 tance across the stream, the amount of water which 

 is separated by it will, as a rule, be less than one- 

 tenth of the amount of water flowing in the ditch. 

 In order to make the division exact, it is necessary to 

 make the current uniform across the whole section, 

 and to prevent all cross-currents and eddies. It is 

 desirable, then, to make the division at a point where 

 the channel is straight and uniform for some distance 

 above the point of division. This portion of the 

 ditch should be uniform in cross-section, and with 

 bottom and sides as even as possible. The more 

 even they are the better will the division be. We 

 have not yet arrived, in this country, to the point 

 where the users are willing to take sufficient pains in 

 arranging the conditions for the division. In Spain, 

 where such means for division have existed for cen- 

 turies, and date back to the time of the Moors, it is 

 customary to take special pains with the channel for 

 some distance above the point of division, even going 

 so far as to pave the sides and bottom of the ditch 

 for several hundred feet. This gives a much more 

 satisfactory division than where the sides are irregu- 

 lar, and are covered with grass and other growth. 



ONE METHOD OF DIVISION. 



As before stated, even under these conditions the 

 division is not exact when based on this principle, 

 unless the flow of water can be made uniform across 

 the whole channel. The nearer this can be done, the 

 more fair will the division be. Where there is fall 

 enough to the ditch to allow a drop of some inches to 

 a foot or more, it is possible to take special means 

 for making the flow uniform. Thus, in such cases, 

 a board placed across the channel so as to hold the 

 water back and deaden its flow, making the water 

 pass over from an approximately still pool and mak- 

 ing the channels below the division board alike in all 

 respects as to drop and fall, the division may be 

 much more exact. In one of the most satisfactory 

 forms of divisors this method of stilling the water 

 has been used, and the partition boards are brought 

 so that the place of division is on the crest of a board 



which holds the water back in this manner. Then 

 below this point, as the water passes away to the dif- 

 ferent channels, it again drops over another board, 

 so that each channel has practically the same condi- 

 tions as to discharge. 



Except by making some arrangement for mak- 

 ing the flow of water uniform, the division will be, to 

 some extent, inexact; but it may be satisfactory. 

 The most perfect method of making the flow uniform 

 would be a screen, using either one or more, as the 

 case might be; but, practically, this is impossible,, 

 except in the case of waters which carry little or no 

 sediment, and no floating material to clog up the 

 screen, which almost all the waters we have to deal 

 with in irrigation do. 



WHERE PROPORTION IS VARIED. 



In some cases of division, where there are a limited 

 number of users on the same ditch, it is desirable to 

 arrange it so that each one may vary the proportion 

 of water which he takes at different times, either be- 

 cause he does not need his share or because he trades 

 water with his neighbors. This may be effected in 

 the case of the ordinary form of divisor by making 

 the division board a movable one so that its distance 

 from the side of the box may be varied and the pro- 

 portion it takes of the whole stream vary in approxi- 

 mately the same ratio. In some forms of divisors,, 

 where the stream is small and the partitions are fixed 

 by placing a strip across the channel at the tops of 

 the boards and a corresponding strip at the bottom, 

 over which the water flows, then a series of narrow 

 boards may be used. These may be placed vertically 

 and will be held in position by the two pieces across 

 the channel. If desired, then one, or anyone of the 

 various openings, may be entirely closed or reduced 

 to as small opening as desired. It is convenient to be 

 able to do this, because it is rarely the case when a 

 user wants the amount of water to which he is entitled 

 to flow constantly, and he can handle it to much better 

 advantage if he has a larger quantity for a small por- 

 tion of the time. This he can do by exchanging 

 water with some of his neighbors. 



It is hardly necessary to call attention to the neces- 

 sity of avoiding placing the division boxes on curves 

 in the ditch, where this principle of dividing is used. In 

 such case the heaviest current is thrown to one side, 

 and in consequence the bulk of the water is thrown 

 to that side. If the division is made on that side, then 

 a larger quantity of water is separated than the dis- 

 tance of the board would indicate, and less if the: 

 board is on the inner part of the curve. 



