Guide A-104
C.H. Hohn, Extension Agricultural Engineer
College of Agriculture, Consumer and Environmental Sciences New Mexico State University (Print Friendly PDF) 

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:

Discharge From Pipes Flowing Full

Table 1. In Gallons Per Minute with Vertical Drop “Y” = 13 Inches

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

Table 2. Rate of flow in gallons per minute

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.

Table 3. Flow in gallons per minute for different sizes of standard pipe and for different heights of the water jets.

For other pipe sizes and heights of jets. Use the formulae:
Gal. per min. = 5.68 CD² √H
Cu. ft. per sec. = 0.0126 CD² √H

Where
    D = inside pipe diameter in inches.
    H = jet height in inches.
    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.

To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at pubs.nmsu.edu.

Contents of publications may be freely reproduced for educational purposes. All other rights reserved. For permission to use publications for other purposes, contact pubs@nmsu.edu or the authors listed on the publication.

New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.

Reprinted October 1997, Las Cruces, NM.