Drip Irrigation--Intermediate

by SCMG Steven Hightower

Drip irrigation systems, when utilized with current-state automatic controls and electric valves, represent the optimum in watering efficiency and water-use minimization. Such irrigation systems are generally not overly complicated, but do have some differences from overhead irrigation that must be grasped.

Drip system design

Drip irrigation systems consist of electronic controllers, which control several zones of electric valves to which main distribution lines are attached, and from which one-quarter inch feeder tubes emanate, each ended in a water emitter. More recently, emitter line, which contains integral emitters at various spacings, often replaces feeder tube and individual emitters in drip design. Each automatic electric valve runs one main line, potentially with sub-mains depending on the size of the garden zone and number of plants. Each valve or zone is run by one station on the automatic controller—which can contain from four to dozens of stations—and can be programmed to run for different amounts of times on different days. There are variations on these basic designs using different fittings and parts.

The design of the system involves laying out the zones or garden beds, determining the amount and frequency of water needed for each zone, devising a drip layout for each zone, and designing a watering program to match those water needs.

Hydrozoning

Each of the emitters in one zone is always on when its station is on, and never otherwise. Therefore if you have both plants that need frequent watering and infrequent watering on the same valve/station, things don't work very well--something is not going to get what it needs. Drip irrigation design criteria number one is to plan your garden with plants grouped by water needs. In other words, low-water plants with other low water plants, high-water plants with their like, and so forth. This practice is known as hydrozoning. This makes sense anyway if you think about it, as each section's plants are also somewhat likely to have similar sun/shade, and drainage requirements. The number of zones you need will depend on the overall size of the garden, the different exposures and microclimates therein and your plant selection.

Maximum zone sizes

Mainline runs of 1/2 in. tubing should generally be limited to about 200 feet in a single zone—shorter if you have abnormally low water pressure (under about 20-25 psi). You want to make sure that one zone and valve are not being asked to drive too many emitters, which would cause inadequate water to get to the plants in the zone.

To check this, you must first find the maximum flow rate your plumbing can provide. To evaluate available flow rate per zone at your house, run water full force from an outside faucet and note the number of seconds it takes to fill a known-size bucket. Calculate the gallons of flow per hour (gph) by determining how long it takes to fill that bucket, then calculate the gallons per hour. If a two-gallon bucket fills in one minute, then the flow rate is 2 gallons per minute, or 120 per hour. If it fills in 30 seconds, then the flow rate is 4 gallons per minute or 240 gph. The maximum useable flow is considered to be 75 percent of the flow rate, or in this last example, drip components aggregating 180 gph (75% x 240 gph) could be spread over the mainline for any given zone.

To determine how much water flow any zone will need, add the flow rate of all emitters, emitter tubing and components used in a zone to make sure you have not exceeded the maximum flow for the zone. For example, fifty 1 gph and twenty-five 2 gph emitters require 100 gallons of flow per hour (50 x 1 + 25 x 2 = 100 gph).

Emitter Placement

Emitters come in specific flow rates--typically 1/2, 1, or 2 gallons per hour (gph), and are either pressure-compensating or non pressure-compensating. So the number of emitters per plant, and the length of the watering cycle will determine the amount of water delivered. Pressure-compensating emitters provide the same flow over a wide pressure range. Diaphragm and turbulent flow and diaphragm emitters are non-plugging, and hence greatly minimize annual maintenance. (Micro-bubblers used in lieu of emitters have the same advantage--they emit higher flows of water in a circular pattern and can be adjusted from 0-10 or more gph.) Emitter tubing is useful for many installations. Non-clogging emitters are built into the emitter line at spacing of 6, 12, 18 or 24 inches. The in-line emitters are self-flushing and clog resistant as long as system water is clean or filtration is used. The tubing comes in ½ inch diameter, and ¼ inch. The half-inch is useful where not too many tight turns and twists are needed in the layout. For example a bed of larger shrubs, or an area that can be laid out in a linear grid. It’s a bit more expensive, but is less likely to be moved by deer or other animals. The quarter-inch is useful in closer quarters, where more plants are more closely spaced. It’s also easier to work with.

Emitters must be properly placed—not too far apart or too few in number—or root development may be restricted by the limited soil area wetted. Emitter placement is also dictated by soil type: sand, loam or clay. To compensate for variations in lateral movement of water in the soil, generally emitters should be spaced closer together in sand, and further apart in clay. A rule of thumb is 6 inches apart in sand, 12 inches apart in loam, and 18 inches apart in clay. If one to two emitters are recommended for a plant in a clay soil, two or three may be required in a sandy soil to wet a sufficiently wide area. The same thinking applies to emitter tubing. Use more widely spaced tubing in clay, and more closely spaced in sand.

Single emitters are fine for small new plants, but planning for growth dictates using multiple emitters spaced evenly around the plant, typically at least two, or three if a shrub that will grow considerably. Alternatively, use a circle of emitter line with 3-4 emitters in it. Trees may require two to three concentric circles, depending on the size of their roots.

Calculating Watering Cycles

The biggest challenge in drip irrigation is accurately determining how much and when to water. Putting just the right amount, and not too much or little on the plants irrigated based on plant type, soil type, exposure and local weather conditions is important to irrigating effectively and efficiently. There has been endless advice in gardening articles over the years to 'check the soil moisture' in order to determine irrigation timing. But that advice ranges from 'poke a rod into the ground—if it goes in easily, it's probably moist' to 'dig a small hole several inches deep and check the soil moisture.' The former probably works sometimes, but certainly doesn't seem reliable. The latter is a lot of work if you have hundreds of plants. In response to this question on drip irrigation timing, an experienced master gardener recently stated "I believe in the get-in-there-and-feel it system—stick your fingers in the soil and see how moist it is. I also try to gauge the plant's water needs by looking at it - when it just starts to show water-stress you know it's time."

Sound enough advice, but some people’s life and gardening styles don't lend themselves to tending to each and every one of hundreds of plants, and their surrounding soil, one-by-one. They need a system that reliably determines how long and how often to have the drip on for each planting area.

Fortunately, there IS a system that accomplishes this. It assumes you have a controller-driven automatic drip system and that you have hydro-zoned your plants, as discussed above. The document Calculating Drip Irrigation Schedules contains the information. There is math, and some of this may seem daunting at first, but a couple of examples will get you comfortable.

Irrigation Controllers

There are several key factors to consider when selecting a controller. As a general rule, better, more durable controllers with more features will be more expensive, but in the long run are good value as they allow the most flexibility in controlling the irrigation of the landscape for efficient watering. In addition to having enough stations for the number of zones you need, the controller should have features that provide for:
· Multiple independent programs
· At least two start times per program
· Station run times in minutes from one to 120 minutes
· Odd/even, weekly and interval program capability up to 30 days
· Seasonal adjustment, or percentage override from 0-200%, to allow for seasonal variations without re-programming
· “Off”, “Auto”, and “ Manual” (both all and single station) operation modes
· Non-volatile memory or battery back-up
· Rain shut-off (preferable with connection for a rain sensor)

More advanced features such as rain sensors, ground water sensor connections and evapotranspiration (ET) modules to factor in site climatic conditions are increasingly available, and are starting to be within the range of a home landscape irrigation system.

A very important but hard-to-grasp issue is ease of programming. Programming methods vary widely by manufacturers, and range from straightforward and logical, to idiotic and impossible to remember. It’s a good idea to spend a bit of time with a sales associate at the store where you buy irrigation equipment to understand the ease of programming of any particular unit; or to spend time online looking at the instructions for a potential purchase.

See Installation and Maintenance of Drip Irrigation Systems for what to do once you’ve decided to go ahead with a drip system installation.


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