Measuring and Conserving Irrigation Water

Finding Flow Rate

It is sometimes difficult to estimate flow rates for surface irrigation systems, but several methods are possible.

• Ditch or pipeline: Measure of low rates with one of the flow measuring devices described later in this publication.
• Furrow systems: Measure flow with one of the portable measuring devices available. Or catch the flow to a single furrow in a bucket of known capacity, and measure the time it takes to fill the bucket. If you know the total flow into a furrow system, you can divide this amount by the number of furrows to find the flow rate into each furrow.
• Siphon tubes: Look up the flow rate in a siphon tube head-discharge chart.

Photo: NCAT

To calculate flow rate for most sprinkler systems, you’ll need to know the average gpm per sprinkler. For center pivots, flow varies along the length of lateral pipe, and you’ll need to know the gpm for the entire pivot.

If your sprinkler system is relatively new and the nozzles have little wear, you can use the total design gpm of your system for a flow estimate. Be aware, however, that design numbers do not take into account increased flow (sometimes significant) due to nozzle wear and pressure variations. Divide the design gpm by the number of operating sprinklers on your sprinkler system to fi nd the average gpm per sprinkler. For pivots, use the design gpm for the entire pivot.

A more accurate way to fi nd the flow rate for  your sprinkler system—especially if it’s an older system—is to conduct a simple bucket test. (See Appendix.)

Finding Irrigated Area

For most surface irrigation systems, the irrigated area is simply the entire area of the field. For furrow systems, you can use the length and spacing of the furrows to estimate irrigated area. (See example below.)

For most sprinkler systems, fi nd the area in square feet watered by one sprinkler, usually described as the distance, in feet, between sprinklers on the line multiplied by the distance, in feet, between mainline riser valves. Typical areas are 30 feet by 50 feet or 40 feet by 60 feet. (See Figure 1 for an example.) For pivots, use the entire swept area in acres and multiply by 43,560. (The conversion of acres to square feet.)

Calculating Net Water Application per Set

To calculate net water application in inches per set for any irrigation system, use the following general formula:

Wheel line, hand line, end tow: Use the average flow rate (gpm) for one nozzle.

Wild flood: Use total area flooded and total flow.

Center pivot system: Use the total pivot flow

Graded furrows: Use furrow length and spacing.

Stationary big gun sprinkler: Use flow per sprinkler.

Travelling big gun sprinkler: Use the following formula:

Determining the Correct Set Time

Use the same basic formula above to determine how long it takes to apply a desired
amount of water by inserting the amount of water you want to apply for “net water application.”

For surface irrigation systems, it may be easier to use the following extremely handy formula:

Measuring Flows

You can measure f lowing water in an open channel or pipeline using one of the methods below. You can also measure water with a simple bucket test at the point where sprinklers apply it to the field (See Appendix).

Measuring Open Channel Flow

Open channel methods generally rely on a structure such as a weir, flume, or orifice installed in the channel. These structures typically include one or more staff gages
(also spelled gauges), installed either on the structure itself or in an adjacent stilling well, to determine water depth. (A staff gage is simply a vertical staff—resembling an ordinary yardstick—marked with numbers that indicate water depth.) Using a table for the specific size and type of structure, you look up the measured depth and convert it to a flow rate. More expensive, but more accurate, electronic measuring devices are now available to offer continuous flow measurement and recording.

Most open channel flow measuring devices work only under limited conditions. A structure suitable for a river or large canal will not be the best choice for a small ditch. Only a few of the more common methods to measure flows on smaller canals, ditches, and farm turnouts are considered here. A comprehensive guide is the U.S. Bureau of Reclamation’s Water Measurement Manual.

To ensure accuracy, the measuring site must meet a number of conditions:

• The ditch or canal must have a shallow grade with a relatively straight upstream segment and uniform cross-section, little turbulence, and quiet flow.
• Weirs require more slope than flumes or submerged orifices.
• The location must not cause sediment loading, debris buildup, or flooding of surrounding areas.

In selecting a water measurement structure, consider cost, accuracy, ease of measurement, construction and maintenance requirements, and state or local water laws and regulations.

Weirs are easy to construct, install, and use, but require enough ditch slope so that water can fall freely from the structure to the downstream water surface. In the case of rectangular and trapezoidal weirs, water flows through a sharp-edged rectangular or trapezoidal notch. In the case of v-notch weirs, water flows through a sharp-edged (usually) 90 degree-angled notch. This weir is especially good at handling a wide range of flows.

Figure 2. V-notch weir

Flumes are more complex structures than weirs, and include a constricted throat section that requires careful construction and installation. Flumes are used where ditch and canal grades are relatively flat. They can still be relatively accurate even when submerged.

Parshall flumes—one of the most common types—require only about a quarter of
the ditch grade needed for weirs and can accommodate a wide range of flows. Cutthroat flumes are a “throatless” variation on the Parshall flume, resulting in simplified construction.

Figure 3. Parshall Flume

Submerged orifices are often used where ditch slope is insufficient for weirs. They generally cost less than weirs and can fit into limited spaces, but are susceptible to trash build-up. Water flowing through an orifice is discharged below the downstream water surface. For this device to be accurate, it must be submerged. The meter gate, a type of submerged orifice, can be used for farm turnouts. The gate can be used to measure flow, closed to shut off flow, or positioned at various settings to reduce or increase flow.

Figure 4. Ramp Flume

Choosing, installing, and using weirs, flumes, and submerged orifices is not difficult. In order to get accurate results, though, you need to install these devices properly, maintain them regularly, use proper measuring techniques, and use the right calibration curves and tables. Your local NRCS, Extension, or soil and water conservation district office may be able to assist you.

Measuring Pipeline Flow

Flow in a pipeline can be measured very accurately if the correct measuring device is installed and used properly. These devices need periodic calibration and maintenance, and water must be relatively (in some cases, very) clean. Accurate measurement requires full pipe flow.

Pipeline flow is measured by either intrusive devices (located inside the pipe or inserted through the pipe wall) or external devices. Intrusive flow meters include Venturi, nozzle, and orifice plate meters that measure flow through a constriction within the pipe. None have moving parts. They require little maintenance in clean water and are installed directly in the pipeline.

Propeller meters use a multi-blade propeller positioned inside the pipe. Propeller meters can pass some debris, but even moderate amounts can foul the blades.

Pitot tubes are inserted into the side of a pipe. They require drilling a hole through the pipe (allowing the insertion of the tube), making them less convenient and common than propeller meters.

Non-intrusive (external) flow meters send ultrasonic or acoustic waves through a pipe and take very accurate measurements of Doppler shift or transit time to calculate flow rate. These devices are clamped onto the outside of the pipe wall (nothing is physically inserted into the pipe) which makes them extremely quick and convenient to use. Several types of ultrasonic meters are currently on the market. They are costly, in the range of $1,500 to $10,000, although prices are dropping, and some training is required for accurate measurements.

Irrigating with Limited Water Supplies

When water supplies are short:

• Don’t over-irrigate. Learn the waterholding capacity of your soils, their allowable depletion, and the effective root depth and critical growth stages
  of your crops.
• Focus irrigation on critical growth stages. Depending on the crop, you’ll usually see one of two types of responses to drought stress:
  • Seed crops, cereals, and oilseeds are most sensitive to drought stress during flowering or seed formation, and relatively insensitive during early vegetative growth. Irrigate enough at the onset of seed formation to carry the crop through seed fill.
  • Perennial crops grown primarily for forage, and some root crops, are relatively insensitive to moderate drought stress for short periods throughout the growing season. They can recover from stress periods with little reduction in yield. Focus on irrigating during periods of maximum growth.
• Irrigate early in the season. Fill the root zone to field capacity before hot weather starts.
• Leave room in the soil for precipitation. Crop residue and cover crops help capture snow and rain and reduce evaporation.
• Aim for optimum rather than maximum yield (i.e., the greatest yield with the least input).
• Plant drought-tolerant crops or quick maturing crops that require most of their water early in the season.
• Reduce the amount of land you irrigate and use the saved water on the remainder, or reduce the amount of water you apply over the whole irrigated area.
• Irrigate every other furrow, switching at each irrigation. You’ll still get water to one side of each row, generally using far less water.

Further Resources

NCAT Resources

Drought Resistant Soil. 2002. By Preston Sullivan.

Explains how organic matter can dramatically increase the soil’s water storage capacity and how ground cover increases the water infiltration rate while lowering soil water evaporation. Both practices reduce the severity of drought and the need for irrigation.

Sustainable Irrigation: A Beginner’s Guide.

Aimed at beginning farmers with little or no experience, this course begins at square one and demystifies the irrigation process: providing a friendly introduction to equipment, terminology, and safety precautions. Unlike other irrigation courses that take a more industrial or engineering approach, this one maintains a strong focus on sustainability throughout.

Other Publications

Irrigation Water Measurement as a Management Tool. 2002. By Danny H. Rogers, Gary Clark, and Mahbub Alam. Kansas State University Agricultural Experiment Station and Cooperative Extension Service, Manhattan, KS.

Discusses tensiometer installation, use, and troubleshooting.

Selection and Use of Water Meters for Irrigation Water Measurement. 2003. By Melissa C. Baum, Michael D. Dukes, and Dorota Z. Haman. University of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL.

Descriptions, use, maintenance, and comparison of various flow-measuring devices.

How Good Is Your Water Measurement? 1999. By Robert Hill. University of Utah Cooperative Extension Service.

Open channel and pipeline devices, typical prices, and sources of equipment.

Agencies

U.S. Bureau of Reclamation. 2001. Water Measurement Manual. U.S. Department of the Interior, Washington, DC. 317 p.

USDA-Natural Resources Conservation Service. 1997. NRCS Irrigation Guide. Natural Resources Conservation Service, Washington, DC. 702 p.

Appendix: Finding Flow Rate with a Bucket Test

For hand move, side roll, or linear move systems:

Measure the flow of one sprinkler per lateral, situated on relatively level ground. The selected sprinkler should be about one-third of the way down the length of the lateral from the mainline. Use a hose to direct the flow into a five-gallon bucket. Using a stopwatch, estimate the time it takes, in seconds, to fill the bucket. For greater accuracy, take more than one reading per sprinkler and average the times. Repeat for the other sprinklers on other laterals.

For pivots:

Step 1. Measure the flow of one sprinkler in each set of nozzle diameters along the pivot using the method described for hand move, side roll, or linear move systems above.