By Carl Wilson, Colorado State University Cooperative Extension Horticulturist, Denver
It's such a common occurrence that many people may think that water running from a lawn, over the sidewalk and down the street is an inevitable part of landscape irrigation. It doesn't have to be, particularly in these drought times when water is too precious to waste.
Runoff may come from a level lawn or from a slope and for different reasons. On a level lawn, the most common reason is that a sprinkler is applying more water than the soil can take in. Technically, this is called exceeding the infiltration rate of the soil. The most common soil type in the Front Range, clay, takes in 1/2 inch or less per hour while sandy soil may take in 1 inch or more in an hour.
Because many sprinkler systems deliver 1 to 2 inches per hour, it's easy to see why the water runs off instead of soaking in. The solution? Set irrigation clocks to apply half the needed water (half the run time) in one cycle, then set a second cycle to apply the other half of the water. This will irrigate around the property through all the zones and then repeat for another round. It allows the water from the first application to soak in before additional water is applied.
Slopes are by their very nature difficult to water effectively because water, after all, does run downhill. With slopes it's key to apply less water on the bottom of the slope and increase the application or run time at the top of the slope. This way, the bottom of the slope is partially irrigated with water running off the top of the slope.
If separate irrigation lines or zones water the bottom and top of a slope, this is easily accomplished. If a slope is irrigated all on the same zone, there is a next-best solution. Consider changing irrigation nozzles in the middle or top of the slope to nozzles that deliver semicircular patterns throwing water upslope instead of full circles. As before, this allows runoff from upslope to cover part of the irrigation need of the grass downslope.
Irrigation nozzles are fairly inexpensive and easy to change out even for beginners. They simply screw off the end of the stem that pops up when the irrigation system comes on. While you have the old nozzle off, be sure to remove and clean the plastic screen located just under the nozzle.
Other possible reasons for wetting streets are operating irrigation systems in windy weather (don't), and over-pressurized systems that release a very fine mist that easily blows off target. Adjusting the pressure to avoid mist clouds is simple. Open the irrigation valve box cover and look for the round "faucet type handles" on top of each valve. These are made to be turned for adjustment just like a faucet handle.
Turn down the handle (clockwise) as each zone is operating. Tighten until the reach of the spray pattern starts to shrink, then turn it up again to full reach and add an eighth turn or so. Your goal is two-fold. One is to wet the area that each sprinkler head is meant to cover, generally so the spray reaches the adjacent head. The second is to achieve a large droplet size that falls to the ground instead of producing a mist that blows away. Note that if your sprinkler heads were installed too far apart or you have low water pressure in your area, you may not be able to adjust the spray patterns correctly.
These simple adjustments to a sprinkler system will enable you to keep water off the pavement and on the landscape where it belongs.
Photos: Carl Wilson
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010