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New Mexico State University

Estimating Water Flow From Pipes

Guide A-104

C.H. Hohn Extension Agricultural Engineer

College of Agriculture and Home Economics, NMSU

This publication is scheduled to be updated and reissued 12/02.

In recent years, there have been increased demands upon our limited water supply by industry, cities and towns, recreation, and agriculture. So irrigators should use the available water supply more wisely than ever before.

Proper management takes the guesswork out of the application of irrigation water to the land. Times and amounts of water applied are just as important as the times and amounts of seed and fertilizer used.

Irrigation management involves measuring water at delivery points to determine exact amounts going to the field.

The standard term for rate of flow of irrigation water is cubic feet per second (cfs). A cubic foot per second of water is flowing when a cubic foot volume of water (equal to one foot wide, one foot long, and one foot high) passes a given point every second.

A cubic foot of water equals:

  • Approximately 405 gallons per minute (gpm).
  • Approximately 1 acre-inch per hour.
  • Approximately 1 acre-foot per 12 hours.
  • Approximately 2 acre-feet per 24 hours.

An acre-inch is the volume of water required to cover an acre of land one inch deep. An acre-foot is the volume of water required to cover an acre one foot deep.

The following tables will be helpful to the irrigator in determining the amount of water being applied to the land with nothing more than a carpenter's rule:

pipe with measurements pipe with measurements pipe with measurements
DISCHARGE FROM PIPES FLOWING FULL
in gallons per minute
with
vertical drop "Y" = 13 inches
 
Pipe size
Horizontal distance "X"
Inside Area
diameter (Sq. In.)  12" 14" 16" 18" 20" 22" 24" 26" 28" 30" 32" 34" 36"
2" 3.14 38 44 50 57 63 69 75 82 88 94 100 107 113
2.5" 4.91 59 69 79 88 98 108 118 128 137 147 157 167 177
3" 7.07 85 99 113 127 141 156 170 184 198 212 226 240 255
4" 12.57 151 176 201 226 251 277 302 327 352 377 402 427 453
5" 19.64 236 275 314 354 393 432 471 511 550 589 628 668 707
6" 28.27 339 396 452 509 565 622 678 735 792 848 905 961 1013
7" 38.48 462 539 616 693 770 847 924 1000 1077 1154 1231 1308 1385
8" 50.27 603 704 804 905 1005 1106 1206 1307 1408 1508 1609 1709 1810
9" 63.62 763 891 1018 1145 1272 1400 1527 1654 1781 1909 2036 2163 2290
10" 78.54 942 1100 1257 1414 1571 1728 1885 2042 2199 2356 2513 2670 2827
11" 95.03 1140 1330 1520 1711 1901 2091 2281 2471 2661 2851 3041 3231 3421
12" 113.10 1357 1583 1809 2036 2262 2488 2714 2941 3167 3393 3619 3845 4072
 
Q=3.61 AX
          Y		 Where: A=Cross sectional area of discharge pipe in square inches
X= Horizontal distances in inches
Y= Vertical distances in inches
 
AN APPROXIMATE METHOD OF ESTIMATING
DISCHARGE FROM PIPES FLOWING
PARTIALLY FULL
pipe with measurements


Rate of flow in gallons per minute
Y
Inside diameter of pipe "D" in inches
D 4 6 8 10 12
0.1 142 334 579 912 1310
0.2 128 302 524 825 1185
0.3 112 264 457 720 1034
0.4 94 222 384 605 868
0.5 75 176 305 480 689
0.6 55 130 226 355 510
0.7 37 88 152 240 345
0.8 21 49 85 134 194
0.9 8 17 30 52 74
1.0 0 0 0 0 0
 

ESTIMATING FLOW FROM VERTICAL PIPE OR CASING

The approximate flow from vertical pipes or casings can be determined by measuring the maximum height (h) in inches to which the water jet rises above the pipe, and inside diameter of the pipe (d) in inches.

The flow in gallons per minute is given in the following table for different sizes of standard pipe and for different heights of the water jets.

pipe with measurements

Height (H)
inches
Nominal Diameter (D) of Standard Pipe (in inches)
2 3 4 5 6 7 8 10 12 14 16
2 29 65 113 180 269 352 460 797 1148 1583 2042
2.5 32 72 126 202 291 396 517 898 1293 1760 2299
3 35 77 135 217 311 425 569 950 1416 1928 2518
3.5 38 85 149 238 341 465 626 1055 1530 2083 2720
4 41 92 161 252 369 503 687 1115 1636 2227 2908
4.5 44 98 172 270 396 540 733 1200 1735 2362 3085
5 47 104 182 286 420 575 779 1270 1829 2489 3251
5.5 49 109 192 301 444 606 825 1332 1918 2611 3410
6 52 115 202 316 469 638 872 1391 2003 2727 3562
6.5 54 121 211 331 490 667 913 1448 2085 2838 3707
7 57 126 219 345 509 700 949 1503 2164 2945 3847
8 61 135 236 370 548 751 1025 1606 2313 3149 4113
9 65 144 251 396 585 802 1091 1704 2454 3340 4362
10 69 153 265 418 621 850 1150 1796 2586 3520 4598
12 76 169 294 463 685 933 1259 1967 2833 3856 5037
14 83 184 319 502 740 1020 1360 2125 3060 4165 5440
16 89 197 342 540 796 1090 1454 2272 3272 4453 5816
18 95 209 364 575 845 1160 1542 2410 3470 4723 6169
20 101 221 386 607 890 1225 1626 2540 3658 4979 6503
25 113 249 433 680 998 1375 1818 2840 4090 5566 7270
30 124 273 476 746 1095 1505 1991 3111 4480 6098 7964
35 134 298 516 810 1175 1630 2151 3360 4839 6586 8602
40 144 318 551 865 1270 1745 2299 3592 5173 7041 9196
 

For other pipe sizes and heights of jets. Use the formulae:

Gal. per min. = 5.68 CD2(H).5
Cu. ft. per sec. = 0.0126 CD2(H).5

Where D = inside pipe diameter in inches
      H = jet height in inches (Note: (H).5= the square root of H)
      C = a constant varying from 0.87 to 0.97 for pipes of 2 to 6 inches in diameter and heights of 6 to 224 inches.
Printed November 1997
Electronic Distribution December 1997