Dealing With Slope Irrigation

Here are 10 tips for setting up an effective irrigation system on a slope.

Meeting plant requirements and managing irrigation systems can be a difficult task under normal circumstances. When slope plantings are involved, applying water efficiently and effectively can be extremely challenging. Too little water and plants won’t grow – too much water and you’ve got other problems.

Several tactics can be utilized to help you deal with these ups and downs. The following are 10 design and product application tips for effective slope irrigation.

1. To reduce runoff, select an irrigation timer that controls water applications.
Choose an irrigation timer with at least four start times per program. Determine how long a station can be on before runoff occurs. Then divide the run time necessary to meet the plant water requirements by this factor. If necessary, use each start time and reduce runoff that results from running valve stations too long. Even better, use an advanced controller with a feature that allows for intermittent applications so applied water has time to filter into the soil. This will allow the total irrigation run time to be split into usable cycles, and the application of water divided into intervals that the soil will easily accept.

2. Use master valves and flow sensing equipment as an insurance policy.
Using a master valve is important when irrigating slopes. Strategically locate the master valve to reduce the length of constantly pressurized mainline around the slope and before the zone valves. A normally closed master valve will supply a mainline with water only when a cycle is initiated from the controller. This equipment will reduce the time that a damaged sprinkler, broken pipe or defective valve will have to wash away the landscape.

Flow sensing equipment can detect excessively high flows when a problem occurs. If calibrated properly, the flow sensing equipment will work in conjunction with the master valve to shut the system down and eliminate significant damage during an excess flow condition.

3. Use reverse-flow valves to reduce problems associated with a worn diaphragm.
The inner workings of a reverse flow valve will prevent water from continuously flowing if the diaphragm is torn. In other words, a reverse flow valve is effective because it will fail in the "off" position. A valve without a reverse flow feature can fail in the open position, run continuously and erode the slope until it is repaired.

4. Use pressure compensating/regulating devices to get the best sprinkler coverage.
When used in accordance with the manufacturer’s specifications, pressure-regulating modules can be installed on valves to adjust the operating pressure of sprinkler heads downstream.

Sprinkler nozzles that provide optimum pressure distribute water as efficiently and uniformly as possible. Sprinkler nozzles operating at pressures significantly above or below the optimum pressure usually perform poorly and, as a result, plant material suffers.

Built-in pressure compensating or regulating devices provide the best option since the optimum operating pressure is delivered directly to each sprinkler head. This will eliminate misting caused by high pressures, reduce water drift due to wind, and give the water droplets the best chance of getting to where they are designed to go.

5. Adjust the distance between lateral lines to compensate for the slope.
On a 2:1 slope, for example, a properly adjusted sprinkler will throw about 80 percent of its radius above the head and 120 percent of its radius below the head. This concept is difficult for many people to understand because on the site plan of an irrigation project, slopes appear to cover less ground than they actually do, and the effect of the slope cannot be accurately depicted.

Therefore, sprinklers can be spaced consistently along the lateral, but the distance between bottom and middle laterals should be reduced and moved up toward the top of the slope to obtain head-to-head coverage and compensate for the true effects of the slope.

6. Space lateral lines across the slope rather than with the slope.
When installing the lateral lines, make sure they follow the contours of the slope. If lateral lines are incorrectly installed from the top to the bottom of a slope, the pressure differential resulting from the elevation change could create severely uneven pressure at each of the various sprinkler nozzles, causing irregular water distribution.

Additionally, the higher pressures at the bottom of the slope could shorten the longevity of the pipes and sprinklers.

7. Limit sprinkler heads on a valve zone to decrease potential damage.
As you add more sprinkler heads to a zone, the size of the delivery system components and infrastructure will increase. In this case, a more elaborate system also means that more water will be available to cause damage in the event that a component in a system breaks. Consequently, large zones are not recommended unless other safeguards are included to circumvent potential problems.

8. Limit sprinkler heads on a valve zone to increase performance.
Tailor the irrigation system to meet the specific water requirements of the slope areas. Separate zones to apply water to slope sections with considerably different plant material, as well as different exposures such as sun, wind, rain and other climatic influences.

If you mix zones together, it will be difficult to keep plant material healthy because one side of the slope will get too much or too little water.

9. Place part-circle sprinklers on separate zones or use matched precipitation rate nozzle packages.
Uniform water distribution is critical for effective slope irrigation. To achieve this, separate part-circle sprinklers from full-circle sprinklers, and then adjust the station run times. An easier method is to use nozzle sets that are specifically designed to create consistent precipitation rates, despite the various arcs and radii.

Using matched precipitation rate nozzles on the same valve system reduces the chance that your system will put down too much water with some heads and not enough from others. This will help diminish the potential for severe soil erosion.

10. Install sprinklers with check valves to contain erosion.
When used properly, a check valve will contain unpressurized water in the lateral lines after the zone valve has completed its watering cycle (if the valve is located at a higher elevation). Without a check valve, the elevation difference from a valve above to the sprinklers below could generate enough internal pressure to force water out of the sprinkler.

A check valve will prevent water that remains in the pipe from draining out of the sprinklers after the valve has shut off, thus reducing the chance of erosion of the landscape areas on or around the slope. Look for sprinklers that have built-in check valves. This eliminates the need to install separate check valve under each sprinkler.

An added benefit... Because many slopes are planted with ground cover instead of turf, consider installing 6- or 12-inch, pop-up sprinklers rather than riser-mounted sprinklers. Not only will the pop-ups improve the visual impact of the site, they aren’t as visible, and may cut down on vandalism.

Effective slope irrigation is a mixture of thorough up-front planning, proper installation, and a solid maintenance technique. If all three are done properly, many emotional,

financial and physical problems can be avoided, and healthy, aesthetically pleasing slope planting will result.

The author is area specifications manager, Rain Bird, Glendora, Calif.

October 2000
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