Water measurement helps you manage the amount of water to apply for the best crop yield. Measurement often results in a decision to use less water. This information can help make the best use of water supplies. By knowing the total amount of water flowing onto a measured border or field area, the irrigator can easily calculate the depth of water applied to the field.

Irrigation water can be measured in three ways: in-ditch devices, flow meters, or on-field methods. The best suited device depends on accuracy desired, ease of use, durability, availability, maintenance required, hydraulic characteristics, ease of construction, and installation cost.

In-Ditch Devices

In-ditch devices provide a method of measuring delivery rates and quantities. The devices measure the rate of flow in an open channel. The rate of water flowing over or through these structures has an established relation between the beginning head (depth of flow) and the discharge. These calibrated devices can measure the discharge rate in cubic feet per second (cfs) or gallons per minute (gpm). Knowing the rate of flow and the time over which this rate is maintained, the total water volume used can be calculated. Using metering devices that measure in cubic feet per second is convenient because 1 cfs equal 1 acre-inch per hour or 1 acre-foot in 12 hours.

Weirs - a weir is an over-pour notch of fixed dimension in a vertical bulkhead or head wall through which water may flow. When properly constructed, installed, and maintained, it provides a simple and accurate means of measuring water. Weirs are easy to construct and accurate if dimensions are followed carefully. They will handle floating trash and not clog easily.

Broad-crested weirs (replogle flumes) are simple structures designed to measure the flow of water in irrigation canals and laterals. By installing a simple concrete weir or bump in the bottom of an existing concrete ditch, the height of the water flowing over the bump or weir is measured. An easy-to-read discharge table is provided with the installed weir that displays the water flow rate for various heights of water over the weir.

Automated, solid-state data recording devices can be added to any measuring weir. These devices automatically measure and constantly record the amount of water flow passing through the measuring weir. The digital data can be either read manually or downloaded to a portable computer which will produce a permanent graph or record of the measured flow.

Flumes - flow measuring flumes are open-channel devices containing a specially-shaped constricted-throat section. They can be constructed from metal, concrete or fiberglass. Standard designs are available to measure water over a wide flow range. Flumes can operate in a flat ditch and require a pool upstream to reduce the approach velocity.

Flumes can operate accurately over a wide range of flows.

The velocity of water as it approaches the flume has little effect upon its operation.

Submerged orifice - This is a sharp-edged opening in a vertical bulkhead placed in a stream perpendicular to the direction of flow, having the upstream and downstream water surface above the orifice. The cross-sectional area of the orifice is small in relation to the stream cross-section. These conditions provide complete contraction of stream flow and the approach velocity of the water becomes negligible. This device is used on relatively flat ditches where fall in not adequate for a weir. It is simple, easy to construct and accurate. Water should be free from floating trash as it will clog very easily.

Flow Meters

Flow meters are attached to a closed pipeline or discharge pipe and measure the flow of water.

Propeller meters, the most commonly used, have a rubber, fiberglass, or metal propeller attached to an indicator head that records accumulated volume and flow, such as acre-feet or gallons per minute (gpm).

Sonic or doppler velocity meters are portable, sensing devices clamped to the outside of the pipeline. Sensor determine the difference between the velocity of the approaching water and the departing water by reflected sound waves. Meters are adjusted to the diameter of pipe and give a readout in gpm flow in the pipe.

Magnetic field velocity meter, another portable system, uses a coil of wire that is lowered into the flow stream. The coil is electrified to create a magnetic field. The sensor measures the reduction in the magnetic field strength caused by the flowing water. Meters can be adjusted for pipe size and give a readout in gpm flow. Accumulated volume recording devices are available.

Velocity tubes measures water velocity in the pipe. These devices are often used to test pumps or monitor flow in pipelines where occasional checks are needed. When the velocity and inside pipe diameter are known, gpm flow can be determined.

On-Field Methods

When siphons are used, the delivery rate of individual siphons can be measured. Water flow in a furrow can be measured in various ways such as with an orifice plate.

For assistance in selecting and/or installing a measurement device, contact your local office of the USDA Natural Resource Conservation Service.

Technical Questions about HIT may possibly be quickly answered by contacting:
Rudy Garcia
Natural Resources Conservation Service
Soil Conservationist & Water Quality Specialist
e-mail: rgarcia@nm.nrcs.usda.gov
or call: 1-505-522-8775, extension 116

Note: Technical Questions may possibly be quickly answered by contacting USDA, Natural Resources Conservation Service Soil Conservationists:

Holistic Irrigation Technology (HIT) Rudy Garcia, Soil Conservationist (Water Quality),  rgarcia@nm.nrcs.usda.gov or call: (505) 522-8775, ext. 116

Remote Sensing & GIS Technology Dave Christenson, Soil Conservationist (Remote Sensing), dchriste@nm.nrcs.usda.gov or call: (505) 522-8775, ext. 115

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Copyright © 1999 Regional Precision Farming Pilot Project
Last modified: April 28, 2000