More Drip Irrigation System Basics
From water source to soil type, many factors go into planning the best drip irrigation system for your garden or farm.
By Jan Wiese-Fales | Illustration by Marilyn Cummins and Ray E. Watkins, Jr.
There are many variables to consider when installing drip irrigation systems for large gardens and small market-grower operations. After mapping the area you plan to irrigate, consider contacting an agricultural engineer or knowledgeable dealer to assess your needs. Links to university and commercial irrigation resources appears at the bottom of this article.
Main components: Whatever their size or design, most systems include:
- a water source
- a backflow preventer
- a filter
- a pressure regulator
- irrigation mainline piping
- manifold lines for each plot being irrigated
- lateral lines with emitters and connectors: tees, elbows and valves.
- Stakes are necessary to hold tubing in place.
- A timer to automate the system if desired.
Water source: Municipal water is the most common source, but it can be costly. Those who pump water from a well for daily home use must consider overall water usage. The pressure tank may need to be upsized to reduce pump cycling to prevent pump burnout, or you can irrigate at night when family use is minimal. Pumping from a pond may require an additional, separate filtering system to remove system-plugging suspended material.
Filter: A 100- to 200-mesh screen size will suffice to remove emitter-blocking particles from municipal and well water; pond water may require an additional filter, as mentioned above.
Timer or controller: If irrigating multiple zones, low-voltage, electrically operated solenoid valves can switch irrigation between zones on the same system. A water-pressure-activated valve can alternate between two zones.
Pressure regulator: In-line pressure regulators should keep pressure at 10-30 pounds per square inch (psi) for pipe, and at 6-15 psi for simple drip tape on level ground.
Flow rate: Flow rate and pressure dictate the amount of water delivered to each emitter in gallons per hour (GPH). Using a half-inch mainline pipe at 200 GPH, a system can have 200 emitters applying 1 GPH or 400 emitters applying 0.5 GPH and so on. Systems may be split into multiple zones if irrigation needs exceed GPH limit of the mainline pipe.
To calculate GPH, place a 1- or 5-gallon bucket under the spigot you will use. Turn the spigot on fully and measure the seconds it takes to fill the bucket. Divide 60 by that number and multiply the result by the number of gallons in the container. That’s your gallons per minute (GPM) rate. Calculate GPH by multiplying GPM by 60.
Mainline and manifold tubing: Mainline tubing for larger operations often is 2- to 3-inch PVC pipe connected to PVC manifold lines, both considered “permanent” and buried below the frost line. Lateral lines containing the emitters are attached to risers from the pipe. Doubling the pipe diameter quadruples the water flow potential. Smaller systems may use 3/4-inch main lines with 1/2-inch lateral lines with no need to bury either.
Emitters: According to industry standards, half-inch pipe can supply 200 GPH for 200 feet, 3/4-inch pipe supplies 480 GPH for 480 feet and one-inch pipe supplies 960 GPH for 960 feet. Simple drip tapes with slits are fine for irrigation of level rows. Larger tapes offer more emitter spacing options and longer row lengths. Plots with varying elevation may benefit from pressure-compensating emitters.
- Clay absorbs water slowly; loam absorbs it at a medium rate; and sand absorbs it quickly. Optimum GPH for emitters for these soil types is 0.5, 0.5-1 and 1-2 GPH respectively.
- Put the system in place before mulching.
- Consider supplying animals and small rodents an alternate source of drinking water away from the garden or field to avoid damage to the tubing.
- University of Missouri Agricultural Engineer Bob Schultheis has compiled an extensive list of U.S. irrigation resources, including commercial websites and equipment suppliers at: missouri.edu/webster/irrigation.aspx
- Here are lists of some resources and suppliers in Canada: http://cpcia.ca/resources/links/, http://www.omafra.gov.on.ca/english/crops/resource/irrigation.htm