Colorado Lawn Irrigation: Audit, Schedule and Winterize

Colorado Master Gardener GardenNotes from Colorado State University (CSU) Extension give practical, numbers-based advice on managing a sprinkler system. This guide summarizes them. Figures below come from that document and apply to cool-season lawns; your own soil and site will differ.

How much water, how often

CSU gives textbook amounts per irrigation for a 6-inch rooting depth: 0.25 inch for sandy soil, 0.38 inch for sandy loam and 0.5 inch for loamy or clayey soil. Deeper roots allow larger amounts, but you should apply no more than the root zone holds, since extra water leaches below the roots.

Weekly cool-season lawn need, counting irrigation plus rain, is given as:

0.75 inch in April and October
1.0 inch in May, June and September
1.5 inches in July and August

For most Colorado soils, CSU says once per week suits spring and fall and twice per week suits summer. Watering between 9 p.m. and 9 a.m. reduces wind and evaporation losses. Use cycle-and-soak on slopes and on compacted or clayey soils.

CSU also says over-watering in spring and fall is a main cause of iron chlorosis, and that a controller left on its summer schedule can over-water those seasons by about 40%.

Run a sprinkler audit

The CSU audit is simple and needs no special equipment:

Place six identical straight-sided cans in each zone.
Run the sprinklers for exactly 10 minutes.
Pour the water into one can and measure the depth.
Multiply by six to get inches per hour.
Run time equals inches to apply divided by the precipitation rate, times 60.

Widely varying can volumes point to poor design or malfunctioning heads. CSU gives typical rates of about 1 to 2.5 inches per hour for pop-up heads and about 0.25 to 0.75 inch per hour for rotors. Pop-up pressure should be 30 to 40 psi, and a mist cloud around a head signals excess pressure.

Cycle-and-soak thresholds from CSU: pop-up heads, 2 cycles above 16 minutes and 3 cycles above 24 minutes; rotors, 2 cycles above 48 minutes and 3 cycles above 72 minutes.

Controllers and sensors

Set the controller to the July and August schedule, then use the percent key (50%, 67% or 100%) for other seasons.
Fine-tune zones in 10% steps. If dry spots appear in hot weather, add time; if they do not, cut back.
Signs of stress: lawn color shifting from blue-green to gray-blue, and footprints that do not rebound within an hour.
ET controllers adjust daily to evapotranspiration. CSU notes that historical-ET models ignore local rain, while models that download live weather for a small annual fee account for actual rainfall.
Soil moisture sensors run the system only when soil dries below a set threshold.

Winterization

Where soil freezes, CSU outlines three steps: shut off water at the valve near the point of connection; open the internal drain line so the supply drains to the outdoor backflow device; and clear lines by blowing them out with an air compressor, opening manual drain valves, or relying on self-draining valves in secondary lines. Compressed air can be dangerous if done wrong, so many owners hire an irrigation contractor.

Where pumps fit

CSU's document concerns sprinkler management, not pumps. If your system uses a well, ditch or booster pump, ask a supplier about pressure, flow and controls for your setup. Not verified: Colorado water rights or permits for private irrigation sources; check with the state and your water provider.

Questions for an irrigation contractor

Can you perform an audit and give me the results per zone?
What are the pressure and head type in each zone?
What controller features do you recommend and why?
Who handles spring start-up and fall blowout, and what is included?
Are there local watering restrictions I must follow? Not verified: ask your water provider.

Common mistakes

Leaving the controller on summer settings all year.
Watering a little every day instead of deeply and less often.
Ignoring misting heads and high pressure.
Skipping fall blowout where soil freezes.

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General information only. Not legal or engineering advice. Check local water restrictions, and have a licensed contractor confirm equipment and work.