268 



THE IRRIGATION AGE. 



IRRIGATION WATER MEASURING DEVICES 



*By California Agents of Irrigation Investigations, Office of Experiment Stations, U. S. Department of Agriculture 



Fig. 16 Photograph of submerged orifice used by the United States Reclamation Service. 



This is the fourth 

 of a series of 

 articles p r c pared 

 from a bulletin 

 issued by the Col- 

 lege of Agricul- 

 ture of the Uni- 

 versity of Cali- 

 fornia, at Berke- 

 ley. The articles 

 are illustrated with 

 photographs and 

 drawings of the 

 various structures 

 and devices used 

 in compiling the 

 data-Tut EDITOR. 



The measure- 

 ment of water 

 through orifices 

 has long been 

 common in irri- , 



g a t i o n practice, 



and various forms of orifices have been developed. 

 The essential condition in the use of an orifice, 

 eliminating the question of form, is that the water 

 on the up-stream side of the orifice shall completely 

 submerge it. If, when in use, the surface of the 

 water on the lower side of the orifi.ce is below the 

 bottom thereof, the orifice is said to have a free 

 discharge. If the surface of the water on the 

 lower side of the orifice is above the top of the 

 orifice, completely submerging it, it is classed as 

 a submerged orifice. Except in the case of the 

 miner's inch box, which is really but a form of 

 orifice with free discharge, use of the orifice in irri- 

 gation practice is confined to the submerged form. 



Submerged orifices, as used, can be divided 

 into two general types, viz. : those with orifices of 

 fixed dimensions (Figs. 15 and 16) and those built 

 so that the height of the opening may be varied 

 (Figs 17 and 18). Orifices with fixed dimensions 

 are usually made with sharp edges similar to the 

 crest of a weir. The most usual type of the second 

 class is the simple head gate (Figs. 17 and 18), 

 which is also used as a submerged orifice, the height 

 of opening and the loss of head being adjusted to 

 the amount which it is desired to turn out and to 

 the loss of head available. Of these two types, the 

 sharp-edged orifice with fixed dimensions is much 

 the more accurate. 



The submerged orifice with fixed dimensions 



is used for meas- 

 urement only, the 

 fixing of the size 

 of the opening 

 preventing its use 

 as a head gate. 

 The experiments 

 which have been 

 made by hydrau- 

 lic engineers to 

 determine the co- 

 efficient of dis- 

 charge for the 

 standard sharp- 

 edged orifice ap- 

 proach in accuracy 

 and number those 

 that have been 

 made for sharp- 

 edged weirs. 

 These experiments 

 have shown the 

 coefficients to vary slightly with the size of the 

 orifice. For the sizes used in the measurement of 

 individual deliveries of irrigation water this varia- 

 tion may be overlooked and one formula used for 

 all sizes. 



In order that the known formula for the dis- 

 charge through such orifices shall apply, certain 

 standard conditions must be observed in the con- 

 struction and use of these orifices. The edges of 

 the orifice must be sharp and definite in shape. It 

 is preferable to use a thin metal plate, as this is 

 not subject to wear and change. The edges of the 

 orifice should not be too near to the sides of the 

 box on either the upper or lower sides ; a distance 

 equal to twice the least dimension of the orifice is 

 sufficient. The orifice should be vertical with the 

 top and bottom edges level. The ditch above the 

 orifice should be sufficiently large so that the 

 velocity of approach will be small, as is necessary 

 in the case of a weir. Corrections can be made in 

 the computations for any velocity of approach but 

 such corrections are more or less uncertain. 



The principal sources of error in measurements 

 with this type of orifice are due to errors in the 

 gage readings to determine the difference in the 

 elevation of the water on the two sides, this being 

 the head or pressure that forces the water through 

 the orifice. As these orifices are generally used 

 where there is but little loss of head available, the 



*The installation of the measuring devices described in 

 this series of articles has been carried out chiefly by S. H. 

 Beckett and R. D. Robertson, irrigation engineers, assisted by 

 Roy Wray. The tests of the devices have been made under 

 the immediate direction of S. T. Harding, irrigation engineer, 

 in charge of irrigation investigations in Montana, temporarily 

 on duty in California, who has also prepared the reports of 

 the tests. The weir tables have been prepared by Wells A. 

 Hotchkiss. The drawings and diagrams have been prepared 

 by Stephen C. Whipple. scientific assistant. F. L. Bixby, irri- 

 gation engineer, in charge of irrigation investigations in New 

 Mexco, temporarily on duty in California, assisted in design- 

 ing the general plan of installation. The full study has been 

 olanned and. in eeneral. sunervised. and the data has been ar- 



ranged for publication by Frank Adams, irrigation manager. 



The installation of the Davis field laboratory, and the 

 testing of the devices have been jointly paid for from funds 

 contributed by the state engineering department of California, 

 the office of experiment stations of the United States depart- 

 ment of agriculture, and the California agricultural experiment 

 station. Cooperation with the state engineering department 

 of California has been effected through agreement between 

 that department and the office of experiment stations, the irri- 

 gation investigations at Davis having formerly been carried 

 on by those two agencies without financial aid from the Cali- 

 fornia agricultural experiment station. 



(The next article will deal with mechanical devices that 

 measure and register the total How of water.} 



