The Slope Zone That Flooded the Curb and Browned the Crest
Why this matters
A lawn that is brown at the top of a slope and soggy at the bottom, with water running down the gutter every watering morning, looks like two problems and is one. The usual response, more minutes for the brown crest, puts more water in the gutter and makes the toe wetter, and on a street where a local water-waste rule prohibits runoff, it puts the customer in line for a notice. This case starts with stopping the water in the street that same morning, then finds why a zone that applied water evenly still watered the lawn unevenly, and ends with a program that delivers the same weekly water in pieces the soil can take.
The call
A customer on a maintenance and irrigation account called in late June: the top of the front lawn was going brown in patches, the bottom stayed spongy, and a neighbor had mentioned water in the gutter most mornings. The front lawn falls from the house to the sidewalk, and the whole slope is one rotor zone.
SECTION, front lawn, house down to street
HOUSE
|____ crest: dry, browning
\__ R1 rotor row at the top
\__
\__ water runs downhill
\__ before it soaks in
\__ R2 rotor row at the toe
\______________ walk curb
toe: wet, spongy, moss edge | |
runoff -> gutter
The controller, as found: zone 3 at 42 minutes, one start, three mornings a week. The other zones on the property were flat and had no complaints.
Contain first
Before diagnosing anything, the tech had to decide what to do about the water in the street. Three options, with what each cost:
- Turn zone 3 off. Stops the runoff at once. The crest, already browning, gets nothing at all through the start of July, and the customer is told the fix for a dry lawn is no water.
- Cut the run time in half. Less runoff (a 21 minute run still sheets for its last 11), and half the water. The crest gets less than before, and it was already short.
- Split the same 42 minutes into short cycles now, as a stopgap. Keeps the weekly water, cuts most of the runoff. It needs a cycle length, and the tech did not yet know the right one.
The tech took the third. Standing at the curb in a high-visibility vest with the truck parked to shield the gutter, he ran zone 3 manually from a dry start and timed it. Water began sheeting off the toe onto the walk at about 10 minutes. He set a temporary program of six 7-minute cycles, an hour apart, for the next morning, which kept the weekly minutes unchanged. The sidewalk was hosed off where silt had washed onto it, so it would not stay slick for passersby.
The hypotheses
With the street dry, the question was why the crest was brown. Three candidates:
- The crest heads under-apply. A clogged nozzle, a wrong nozzle, a head throwing short or the top row on lower pressure than the bottom.
- The zone does not run long enough. The crest needs more water than 42 minutes gives.
- Water lands evenly but does not stay where it lands. It runs off the crest and collects at the toe.
The first two predict the crest is getting less water out of the heads. The third predicts the heads are fine. A catch-can test separates them.
The catch-can test
The next morning, in calm air, the tech set 12 catch cans on the zone: four along the crest, four across the middle and four along the toe, under and between heads. He ran zone 3 for 15 minutes, inside the 10 to 20 minute range Irrigation Startup and Audit sets for rotors, and read each can in inches.
- Crest: 0.12, 0.11, 0.10, 0.12, total 0.45
- Middle: 0.11, 0.12, 0.10, 0.11, total 0.44
- Toe: 0.12, 0.11, 0.10, 0.13, total 0.46
All 12 total 1.35 in, so the mean catch is 1.35 / 12 = 0.1125 in. The lowest quarter of the cans, the three at 0.10, averages 0.10, and low-quarter uniformity is 0.10 / 0.1125 = 0.89, well above the 0.70 that card accepts. Precipitation rate is 0.1125 x 60 / 15 = 0.45 in per hour.
The crest cans caught as much as the toe cans. The heads were not short-changing the crest, so hypothesis 1 was out. And the zone was putting down 0.45 x 42 / 60 = 0.315 in a run, 0.945 in a week over three runs, a reasonable midsummer amount for this climate; the crest was not starving for lack of minutes, so hypothesis 2 was out too.
The catch cans measured what fell, and what fell was even. The lawn was uneven because of what happened after the water landed.
Why the water moved
Texture. The tech pulled plugs from the crest and the toe and ribboned them per Soil Texture by Feel for Lawn Techs: a firm ball, a ribbon breaking past 2 in, smooth and sticky with no grit. Clay. The crest plug showed a dense layer about 2 in down with roots running sideways along its top, compaction from years of mowing on a slope that had never been aerated.
Arithmetic. Clay, compacted, takes water slowly. The zone applies 0.45 in per hour. At the runoff point the tech timed, 10 minutes, the zone had applied 0.45 x 10 / 60 = 0.075 in. Everything after that, on a 42 minute run, ran downhill: the crest kept what soaked in during the first few minutes, and the toe received its own water plus everything the slope shed. That is the brown crest and the spongy toe from one cause. Adding minutes to fix the crest would only have added to the second part.
The program that fixed it
The goal was the same weekly water the zone had been applying, 0.945 in, delivered in cycles short enough to soak in, per Irrigation Controller Programming.
Cycle length. Runoff began at 10 minutes from a dry start. Once the surface is wet, clay takes water more slowly still, so later cycles hit runoff sooner. The tech set the cycle at 7 minutes, 0.45 x 7 / 60 = about 0.05 in per cycle, leaving margin under the dry-start runoff point.
Number of cycles. 42 minutes per watering day / 7 minutes = 6 cycles. Six cycles of 7 minutes is 42 minutes, so the weekly total stays 3 x 42 = 126 minutes, 0.945 in.
Soak. 60 minutes between cycles, toward the long end of the 30 to 90 minute range the programming card gives, because this is compacted clay on a slope. The controller supports cycle and soak and runs the flat zones during the soak gaps, so the six cycles fit in a window of 6 x 7 + 5 x 60 = 342 minutes, under six hours, before the morning traffic on the walk.
Seasonal adjust. On this controller the seasonal adjust stretches each cycle. The program was written for July at 100 percent, per How to Set the Controller Seasonal Adjust Through the Year, so it only scales down from here; but any future increase above 100 percent would push a cycle past the runoff point, and a note on the controller door says so.
Confirming it
The street. On the next scheduled morning the tech watched cycles 1, 3 and 6 from the curb. No sheeting at the toe, nothing in the gutter. Cycle 6 showed the first glisten on the toe surface at the end of its 7 minutes and no flow.
The soil. The following morning a soil probe on the crest went in to about 4 in through moist soil above the compacted layer, where on the first visit it had been dry below an inch. The toe was moist rather than wet, and the moss edge at the walk had stopped spreading by the next visit.
The lawn. By the third week the browning on the crest had filled in except for two patches where the turf had thinned out entirely; those were noted for overseeding in the fall.
Telling the customer
The customer's first question was whether the crest needed more water, which is the instinct the whole case ran against. The tech kept it to what had been measured.
CUSTOMER: The top is dying. Shouldn't it run longer?
TECH: The cans on top caught as much as the bottom.
The water lands fine. It runs off before it
soaks in, and ends up at the curb.
CUSTOMER: So what changed?
TECH: Same water every week. Six short runs an hour
apart instead of one long one, so it soaks in
where it lands.
CUSTOMER: And the water in the street?
TECH: Watched three of the six this morning. Dry.
That exchange matters because a customer who believes the lawn is short of water will turn the minutes up on the controller the first hot week, and the runoff comes straight back. The controller door note covers the next tech; the conversation covers the customer.
What stays on the ticket
Aeration. The compacted layer is still there, and it is what keeps the runoff point at 10 minutes. Fall core aeration was added to the account; see Fall Core Aeration & Overseed. Once the crest takes water faster, the cycle length can be retested and lengthened.
Nozzles. The zone's rotors put down 0.45 in per hour. Lower-rate nozzles or a conversion to multi-stream rotary nozzles would raise the runoff point at the source; the trade-offs are in Rotary Vs Drip Vs MP Rotator Conversion For Runoff-Prone Slope Decision Matrix and the decision is the customer's to make.
The record. Measured precipitation rate, uniformity, texture, the runoff time from a dry start, and the cycle and soak program, so the next tech who sees a brown crest on this lawn starts from the numbers rather than the minutes.
References
- See related: Irrigation Controller Programming, for cycle and soak.
- See related: Irrigation Startup and Audit, for the catch-can method and uniformity.
- See related: Soil Texture by Feel for Lawn Techs.
- See related: How to Set the Controller Seasonal Adjust Through the Year.
- Your local water utility's or municipality's water-waste rules, where they prohibit irrigation runoff to the street.