I see many sprinkler contractors advertise “We offer one hundred percent sprinkler coverage for your lawn.” What does that mean? Will a drop of water reach every corner of your garden? And that? A quality sprinkler system will provide relatively even coverage for your lawn.
As an EPA Water Sense partner and sprinkler system troubleshooting professional, I see landscapes every day that provide “100% coverage,” but the irrigation design is terrible. What matters is uniform coverage for areas that have the same water requirements. In a simplistic way, for every ten drops of water that hit this area, you want at least six to seven drops to hit that area. In technical terms, it is known as DuLQ (Distribution Uniformity, Lowest Quarter). This relation of the equality coverage is then expressed as a percentage.
One hundred percent uniformity of coverage, or a better term,uniform precipitation, it is impossible to achieve. Evaporation, weather conditions, soil conditions, pressure fluctuations, and other considerations make it impossible for every drop of water delivered by sprinklers to end up on the target root profile. However, careful planning and design can minimize water waste.
Here are some benchmarks to aim for:
Rotor zones 65-70%
Spray areas 50-65%
Drip zones 85-95%
For example, I recently solved some sprinkler system problems in a home that had rotors and sprinkler heads connected to the same zone. Sprinkler heads (sometimes called “stationary sprinklers”), in general, emit three times more water per minute over their given area (precipitation rate) than rotor heads (those that rotate). This mix of rotors and sprays in the same area is pretty bad. When you add to the situation the fact that the sprays were all in the shade watering mature bushes, and the rotors were all on the lawn in full sun, what you have is a disastrous design. In order to provide the correct amount of water to the lawn, the shady bushes, which did not need water, were flooded. However, this irrigation system provided “100% coverage”.
When looking for an irrigation contractor to install your sprinklers, after the salesperson promises you “100% coverage,” ask if they can give you a rough estimate of application uniformity by zone. If he says with a big smile “100%”, or even worse “Huh?”, Keep looking.