Page 8 



BETTER FRUIT 



December, ipip 



Information on the Measurement of Irrigation Water 



By 0. W. Israelsen of the Utah Agricultural Experiment Station 



THE economical use of water in 

 irrigation depends primarily on 

 water measurement. That signi- 

 ficant advantages, public and private, 

 attend the measurement of water de- ,.in,n,nn Wr.i.. xi,. commonly- 



livered to individual irrigators has 

 long been recognized in older irrigated 

 coimtries. The rapidly increasing 

 utilization of Utah's available water 

 supply, the consequent increase in its 

 value, and the tendency on the newer 

 canal systems to base the annual irri- 

 gation charges on the amount of water 

 used make an understanding of the 

 methods of water measurement an ab- 

 solute necessity. Furthermore, many 

 irrigators now realize that the vast 

 store of information concerning the 

 relations of water, soils and plants 

 that has been accumulated in years 

 past cannot be utilized in practice 

 without the measurement of water. 



As a result of the growing apprecia- 

 tion of the value of water measure- 

 ment, there is frequent inquiry as to 

 materials and methods used in measur- 

 ing water under different conditions. 

 To facilitate the answering of such 

 inquiries, and otherwise to spread in- 

 formation concerning water measure- 

 ment, this circular is prepared.* 



•No attempt is made to present an exhaus- 

 tive discussion of tlie subject. The material 

 is simply a compilation from State and 

 Government publications on water measure- 

 ment. An attempt is made to meet the needs 

 of practical irrigators and canal company 

 officers and hence technical language is avoid- 

 ed where possible. 



Units of Water Measurement. 

 The units of water measurement 

 naturally fall into two classes: first, 

 those expressing a specific volume of 

 water at rest, and second, those ex- 

 pressing a rate of flow. 



Water at Rest. — The commonly-used 

 units of volume of water at rest are 

 the gallon, the cubic foot, the acre- 

 inch and the acre-foot. 



Flowing Water. — The 



The necessary 



applied are given, 

 time is to be found. 



To do this proceed as follows: 3 

 c.f.s.=3 acre-inches per hour. The to- 

 tal number of acre-inches needed is 50, 

 that is, 5 on each of 10 acres. Since 

 each hour brings 3, the time necessary 

 is 50-^3=16 2-3 hours. Ans. 



2. Smith has a pump which dis- 

 charges 900 g.p.m. If he spends 30 

 hrs. in irrigating a 10-acre orchard, 

 what average depth in inches does he 

 apply? 



used units of rate of flow are gallons 

 per minute, miner's inches, cubic feet 

 per second and acre-inches per hour. 



Convenient Relations. 



Some convenient relations between 

 the units of flow above given follow: 



First: One cubic foot per second (c. 

 f. s.) or (sec.-ft.)=450 gal. per min. 

 (g. p. m.) (approximately) because 

 there are nearly 7.5 gal. in one cu. ft. 

 and 60 sec. in one min. and therefore 

 7.5x60=450 g. p. m. 



Second: One cubic foot per second 

 (c. f. s.):=one acre-inch per hour, (ap- 

 proximately). Since there are 60x60 

 =3600 sec. in 1 hr., one c. f. s. will 

 give 3600 cu. ft. in 1 hr. and there are 

 1-12 of 43560 cu. ft.=3630 cu. ft. in 

 one acre-inch. One c. f. s. therefore 

 equals one acre-inch per hour (ap- 

 proximately).* 



*(It will be noted that saying that 1 c.f.s. 

 is equal to 450 g.p.m. is true within % of 1 

 per cent and that 1 c.f.s.=l acre-inch per hour 

 is true within 5/6 of 1 per cent, both of which 

 are amply accurate for practical purposes.) 



"fiiird: One cubic foot per second ^'^- ^- "^^"^ notch and bulkhead in weir pond. 



(c. f. s.)^50 Utah miner's inches. 



Use of Convenient Relations. — Ex- 

 amples of how to use the above rela- 

 tions are given below: 



1. An irrigator is entitled to 3 c. f. s. 

 to irrigate a 10-acre alfalfa tract. How 

 long will it take him to apply 5 acre- 

 inches per acre? Note that the quan- 

 tity of flow, the area to be irrigated 

 and the average depth of water to be 



Plan of weir box. 



Note that the quantity of flow, the 

 time run and the area of land covered 

 are given, and that the depth of water 

 applied may be found as follows: 900 

 g.p.m. = 2 c.f.s. = 2 acre-inches per 

 hour. Therefore, in 30 hrs., 60 ac.-in. 

 will be supplied, and this amount 

 spread uniformly over 10 acres will 

 cover it to a depth of 6 inches. Ans. 



Weirs Having End Contractions, t 

 Descriptions of the rectangular, the 

 trapezoidal! and the 90-degree triang- 

 ular notch weirs, with "complete con- 

 tractions," free fall and sharp crests 

 are accompanied by tables for each 

 weir. The quantity of water passing 

 over either weir can be determined by 

 use of the proper table. The depth of 



tA weir having end contractions is one in 

 which the length of the weir crest is so much 

 less than the width of the water channel that 

 the water filaments are completely deflected 

 in flowing from the sides of the channel past 

 the sides of the weir. 



§This weir is also called the Cipolletti weir 

 after the engineer who designed it. 



water flowing over the weir must be 

 determined and if the rectangular or 

 trapezoidal weir is used the length of 

 weir crest must be known. For ex- 

 ample, if the length of trapezoidal 

 weir crest is 2 feet, that is the bottom 

 width of the notch, and the head or 

 depth of water over the weir crest is 

 yi foot or six inches, the discharge 

 will be 2.37 cubic feet per second. 



The following definitions taken from 

 Farmers' Bulletin 813, U. S. Depart- 

 ment of Agriculture, by V. M. Cone, 

 will make clear the above and other 

 terms used in connection with weirs. 



