Know Your Flow Rates
By Charles Wilson
A good supply of irrigation water is critical for optimum rice production. This is true whether you are flood irrigating a field on contour levees or furrow-irrigating as some growers have adapted over the years.
Disease management (rice blast), weed control, nutrient management and insect management all are impacted by the effectiveness of irrigation. Rice can be irrigated by a number of methods and systems, but one of the keys to being effective is to know the capacity of your well and/or pump.
A good method of determining pump discharge is to use an inline flow meter. Many irrigation equipment dealers handle flow meters and can provide cost and proper installation information. Proper installation is very important to assure accurate readings and good service from the flow meter. Portable flow meters are available for use in monitoring the flow of more than one pumping plant.
Most flow meters can be equipped with a totalizing dial that records the total quantity of water pumped, providing useful water management information that can also be used to document irrigation water requirements.
If a flow meter is not available, the discharge rate can be estimated by the plumb bob method. When the water discharges from a vertical riser, the flow rate can be estimated with a vertical measurement. However, the accuracy is usually very poor because of the difficulty in obtaining a good measurement of the vertical height.
A more accurate measurement is usually obtained if a hydrant is installed on the riser creating a horizontal discharge. A joint of pipe inserted in the hydrant stabilizes the flow and makes the plumb bob method possible. A yard stick with an eight-inch plumb bob is extended parallel with the discharge until the plumb bob barely touches the stream of water. The flow is calculated by multiplying the discharge pipe diameter times the discharge pipe diameter times the length from the edge of the discharge pipe to the plumb bob (D*D*L).
Adoption of multiple-inlet rice irrigation using poly-tubing has increased over the past several years. It has proven to conserve water, reduce input costs, increase irrigation efficiency and improve the effectiveness of other inputs. However, to fully utilize this system, knowledge of overall pumping capacity is needed to set the gates as accurately as possible.
As the season progresses, the water use by the crop increases, and the amount of irrigation water needed tends to increase also. Knowledge of the pumping capacity can improve your planning so that adequate water will be available when it is needed. If the pumping capacity is less than you think, you may find it difficult to keep up with the water without supplementing from other sources.
Multiple-inlet rice irrigation with poly-tubing, tying wells together, land leveling, utilizing nearby surface water, tailwater recovery systems and reservoirs are all things that can be done to improve flood management in rice. Each of these practices can be enhanced if you know the flow rates from your wells and relifts. Taking the brief time to measure can mean time savings later as you make the water stretch as far as possible.
Charles “Chuck” Wilson is the University of Arkansas Extension rice specialist. Contact him at firstname.lastname@example.org.