Sprinkler irrigation uses some type of sprinkler head to scatter drops of water over a large area, mimicking the effect of rainfall. If the equipment is effective and properly used, it can water large areas quickly and efficiently. As much as 10 percent might be lost to evaporation from drops in the air and from the wet soil surface, but this can be controlled by watering at night or early in the morning, when evaporation is lower.
Sprinklers are the best way to water lawns and ground covers, or any part of the garden where the plants are closely spaced. It spreads the water evenly over the area being watered, promoting even growth. The more widely spaced the plants, the less effective is sprinkling, because water is wasted on areas that don’t contain plant roots.
Overhead watering also washes plant leaves, removing dust and salts that interfere with transpiration and photosynthesis. On the other hand, it can promote leaf diseases. Many fungi and bacteria are more virulent in wet conditions. Most need a period of several hours of wetness — drops of free water sitting on the leaves — to invade the leaf. This condition can be avoided by watering at a time when the leaves will dry quickly, such as early morning.
Sprinkler systems can be either underground and more or less permanent, or a system of hoses and hose-end sprinklers. The latter is inexpensive and flexible, but requires ongoing attention. Underground systems cost more, but can be automated, relieving the gardener of the responsibility.
Sprinkler systems, like any watering system, are prone to misuse. Probably the most common misuse of sprinklers is watering parts of the garden that don’t need water. This not only wastes water, but promotes weed growth. Also, runoff from patios, driveways, and other impermeable surfaces causes problems of its own, with puddles and erosion where the water leaves the surface.
Another common error in operating sprinkler systems is uneven watering. Uneven watering usually leads to wasting water. The system is left on long enough to water the places that are receiving the least water, depositing too much water on the spots that are receiving the most. This extra water percolates down below the root level of the plants and is wasted. The differences in how much water is deposited can be dramatic, but are difficult to see without making some special effort.
The simplest way to measure water distribution is to place an array of straight-sided cans under the sprinkler pattern for a measured period of watering, such as 15 minutes. The cans can be of different sizes as long as they have straight sides and flat bottoms. After 15 minutes, measure the depth of water in each can with a ruler. Multiplied by 4, this is the amount of water the sprinkler is delivering to that spot in an hour. The differences between the cans shows the inefficiency of the sprinkler pattern. See The Container Test for more information.
If you are making this test with a hose-end system, you can either buy a sprinkler head with a more even pattern, or compensate for the differences by overlapping the pattern when you move the sprinkler.
Fixed systems are a little more difficult to adjust. Begin by adjusting the pattern of each sprinkler head. On some, this is done with a screw on the top. With impulse sprinklers, it is done with a screw at the side of the nozzle. If this doesn’t solve the problem, change sprinkler heads to different sizes. If this still doesn’t work, it may be necessary to change the location of some of the heads, a job entailing digging and moving pipes.