THE IRRIGATION AGE. 



245 



IRRIGATION WATER MEASURING DEVICES 



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



Fig. 12 Measuring water with a small Cipolletti weir. 



'T' HIS is the third of a 



scries of articles 

 prepared from a bulletin 

 issued by the College of 

 Agriculture of the Uni- 

 versity of California, at 

 Berkeley. The articles 

 arc illustrated zcith pho- 

 tographs and drawings 

 of the various structures 

 and devices used in com- 

 piling the data. THE 

 EDITOR. 



In sections where 

 the irrigated lands have 

 a considerable slope, so 

 that water can very eas- 

 ily be led from the supply ditches or 

 laterals to the land without having 

 to check the water nearly as high as 

 the ditch banks, some form of weir 

 is the most common type of measur- 

 ing device. 



No attempt will be made to pre- 

 sent a broad and full discussion of 

 weirs, only enough being given to 

 enable the farmer who is unfamiliar 

 with water measurements to under- 

 stand their use in irrigation. The 

 weir tables that are given are those 

 that are generally used in irrigation 

 practice and are therefore based on 

 the well-known formulas. Engineers 

 have recognized that these formulas 

 do not apply throughout the wide 

 range of conditions met in the field 

 and for that reason numerous engi- 

 neers have made experiments de- 

 signed to correct the formulas for 

 the conditions to which the Francis 

 and other formulas do not properly apply. 



Taken singly, a weir, like other non-mechanical 

 meters, measures the rate of flow and does not indi- 

 cate the total quantity delivered. In conjunction with 

 a water register, which graphically records the depth 

 of water passing over the weir, or in conjunction with 

 such a device as the Hanna meter (Fig. 25), which 

 may be arranged to read directly in acre-feet, measure- 

 ment by means of a weir gives entirely satisfactory 

 results. Where conditions permit its use the 

 weir is thus far the generally accepted device 



for measuring lateral 

 diversions from main 

 canals. It is also an ac- 

 cepted standard device 

 for testing the rate of 



flow from 



pumping 



plants, just as it has 

 been the standard device 

 in the tests that have 

 been made of the vari- 

 ous devices installed at 

 Davis. Small movable 

 weirs are convenient for 

 use by farmers for meas- 

 uring the water carried 

 in their individual ditch- 

 es or discharged by pumping plants. 

 Three types of weirs are chiefly 

 in use in irrigation practice; viz.: 

 the Cipolletti weir, the weir extend- 

 ing the entire distance across the 

 ditch or flume carrying the water 

 measured, known as the weir with- 

 out end contractions, and the rec- 

 tangular weir that does not extend 

 entirely across the ditch or flume, 

 known as the rectangular weir with 

 end contractions. The first two only 

 of these are installed at Davis and 

 described in this article. 



Briefly, a weir is merely a board 

 or other crest set across a stream 

 or other water channel and over 

 which the water carried is made to 

 flow. If the velocity of the water 

 directly above the weir, known as 

 the velocity of approach, is very 

 small and due only to the falling of 

 the water over the weir crest, the quantity of water 

 passing depends entirely on the depth of the wa- 

 ter over the crest and the length of the crest. In 

 the case of the rectangular weir with end contrac- 

 tions the discharge is not proportional to the length of 

 the weir crest. In fact, the discharge is not precisely 

 proportional to the length in the case of the weir with- 

 out end contractions, but is so nearly so as to involve 

 no error of consequence by assuming it to be. As tables, 

 have been prepared that show the quantity passing over 

 both a Cipolletti weir and a weir without end contrac- 



Fig. 25 A Hanna meter. 



*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 Wra}'. 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 

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

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



planned and, in general, supervised, 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. Co-operation 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. 



