How to Set the Controller Seasonal Adjust Through the Year

Why this matters

Most irrigation systems are set once in spring and run the same minutes until somebody complains, so they overwater in April and October and the lawn goes soft, shallow-rooted and diseased while the water bill climbs. The fix is not to re-edit every zone every month. Program each zone once for the hottest, driest stretch of the year, then turn one knob, the controller's seasonal adjust (also called water budget or percent adjust), down in spring and fall and back up in summer. One number moves every zone together, the zone-to-zone balance you worked out at the audit stays intact, and a tech who has never seen the property can read the log and know exactly what was done. Base programming (start times, days, zone run times, cycle and soak) belongs to Irrigation Controller Programming; this card is the monthly adjustment on top of it.

What the seasonal adjust actually does

On most controllers the seasonal adjust is a percentage applied to every zone's programmed run time. A zone programmed for 40 minutes runs 40 at 100 percent, 20 at 50 percent, 48 at 120 percent. It does not change start times, watering days or cycle and soak counts on most models, though some apply it per program rather than to the whole controller and some apply it by changing cycle counts; read the controller's manual and confirm on the first visit which you have.

Two consequences follow. First, the base program has to be right at 100 percent, or every month's percentage is a percentage of the wrong number. Second, because the zones scale together, a shady zone programmed at half a sunny zone's minutes stays at half all year, which is what you want.

The procedure

Step 1: program the peak-demand schedule once

Set run times for the hottest, driest month the lawn needs to survive in good condition, typically mid-summer, using the system's measured precipitation rates from a catch-can audit (see related: Irrigation Startup and Audit) and the turf's weekly water need for your climate. That program is 100 percent. Write it on the controller door or the property record as "peak program".

Reason: a schedule built for July, scaled down, can always be reduced. A schedule built for May and scaled up past 100 percent pushes long run times onto slopes and clay that could not take them at the base length, and on some controllers the adjust tops out before you reach what July needs.

Step 2: derive the monthly percentages

The honest basis for the percentage is the ratio of each month's turf water demand to peak-month demand. The local reference evapotranspiration (ET, the water a reference grass loses to evaporation and transpiration) is published by many state weather networks, extension services and water utilities as monthly averages. Divide each month's average ET by the peak month's and round to the nearest 10 percent. Rainfall is handled by the rain sensor or by skipping, not by this percentage.

Where no local ET table exists, a utility or extension office often publishes a recommended seasonal-adjust or watering-index calendar for the area; use that in preference to a generic chart.

Step 3: change the adjust, not the zones, and log it

On each visit in a transition month, move the seasonal adjust to that month's figure. Do not touch individual zone run times unless the audit found a zone problem, and if you do, note it separately. Record every change.

Reason: the moment one tech edits zone 3 by hand in May and another moves the seasonal adjust in June, the controller carries two adjustments stacked on top of each other and nobody knows which one is responsible for the dry patch in July.

Step 4: check the result against the rules that win

Local watering restrictions override the schedule. A water purveyor or municipality may limit watering days per week, hours of the day, or total volume, and drought stages tighten them. Before you leave, confirm the adjusted program still runs only on permitted days and inside permitted hours. Seasonal adjust usually shortens run times, which helps; raising it in a heat wave can push the last zone past the end of a permitted window, which does not.

The artifact: one property's year

 SEASONAL ADJUST LOG - 114 ELM ST
 Cool-season lawn. Peak program = July = 100 percent
 Zone 3 (rotors, full sun): 40 min x 3 days = 120 min/wk
 Restriction: 3 days/wk, start no earlier than 4 am,
 done by 9 am

 MONTH  ET RATIO  ADJUST  ZONE 3 RUN  ZONE 3 WEEK  BY
 Apr      0.47     50%      20 min      60 min    JR
 May      0.70     70%      28 min      84 min    JR
 Jun      0.88     90%      36 min     108 min    KT
 Jul      1.00    100%      40 min     120 min    KT
 Aug      0.96    100%      40 min     120 min    KT
 Sep      0.68     70%      28 min      84 min    JR
 Oct      0.42     40%      16 min      48 min    JR
 Nov    system winterized, controller off          JR

The ET ratios in that log are illustrative, the shape of a typical cool-season year in a temperate climate; pull your own from the local table. The percentages are the ratios rounded to the nearest 10; August's 0.96 rounds to 100.

Worked example: catching a stacked adjustment

A crew lead takes over 114 Elm in June and finds the controller at 90 percent, which matches the log. But zone 3 runs only 27 minutes, not the 36 the log predicts.

Find the base. At 90 percent, 27 minutes means a base of 27 divided by 0.9, 30 minutes. The peak program on the door says 40. Somebody cut zone 3's base from 40 to 30 by hand, then the seasonal adjust kept scaling the cut figure.

Why it matters. At the stacked setting, zone 3 runs 27 minutes three days a week, 81 minutes a week. At the logged setting it should run 36 minutes, 108 a week. The zone is getting 81 divided by 108, 75 percent of what the log says, and in July at 100 percent it would get 30 times 3, 90 minutes against 120, still 75 percent. That is the dry, brown arc the customer will call about in July, and it would have been blamed on the heat.

What the lead does. Asks the office why zone 3 was cut. The record shows a spring complaint of runoff onto the driveway from zone 3. That is a real problem, but cutting the run time was the wrong fix: the runoff is a cycle and soak problem on that zone (split the 40 into two cycles with a soak between), not a total-water problem. The lead restores the 40 minute base, programs it as two 20 minute cycles with an hour soak, leaves the seasonal adjust at 90, and logs both changes on separate lines with the reason.

Check against the restriction. The five zones' base run times, with zone 3 counted as its two 20 minute cycles, total 260 minutes at 100 percent. The soak between zone 3's cycles is filled by the other four zones running (220 minutes of them), so elapsed time is just the run time. At 90 percent that is 260 times 0.9, 234 minutes: a 4:00 am start ends at 7:54 am. In July at 100 percent, 260 minutes ends at 8:20 am. Both are inside the 9:00 am limit, with the July margin the one to watch if a heat wave tempts someone to push the adjust over 100.

Opening and closing the season

The percentage curve assumes the system is running. The two ends of the year need judgment the curve does not supply.

Spring start. Do not turn the controller on because the calendar says April. Push a screwdriver or soil probe into the root zone at a few spots; if it goes in easily to 4 to 6 in and the soil on it is moist, the lawn does not need water yet, and early irrigation only teaches the roots to stay shallow. Start at the April percentage when the probe meets dry soil in the top few inches and the turf is actively growing.

Fall close. Cool nights and short days cut demand faster than the averages suggest, and wet turf going into cold weather is a disease setup. Drop to the October figure on time, and earlier after a rainy September. Winterization, including blowing out lines in freezing climates, is a separate job (see related: Property Winterization and End-of-Season Shutdown SOP); log the date the controller went off so the spring tech knows the base program is still the July one.

When the percentage alone stops working

Very low adjust on short zones. At 30 or 40 percent, a spray zone programmed for 10 minutes runs 3 or 4, and rotor zones may not get through enough full rotations to water evenly. When the adjust pushes run times that low, take a watering day out instead and hold the percentage higher. That keeps each run long enough to be uniform.

A weather-based (smart) controller. It adjusts run times from local weather on its own. Many still carry a manual percentage on top; leave that at 100 unless the audit says otherwise, or you are adjusting twice.

New seed or sod. Establishment watering is frequent and shallow and has nothing to do with the seasonal curve. Program it as a separate program and remove it once the turf is rooted, rather than bending the seasonal adjust to fit.

A dry spring or a wet summer. The ET ratio is an average. In a dry, windy April the lawn needs more than 50 percent; raise it and log why. Do not wait for the calendar to say May.

How to verify you got it right

• The controller shows the logged percentage for the month, and one zone's actual run time, read off the display or timed, equals base times percentage within a minute.
• Every change has a line in the log: date, percentage, reason, tech.
• The adjusted program runs only on permitted days and finishes inside permitted hours.
• A walk of the property after a cycle shows no runoff at curbs or hardscape and no standing water; a dry arc at a head is a head problem for the audit, not a percentage problem.

References

• See related: Irrigation Controller Programming, for base program, cycle and soak, and controller setup.
• See related: Irrigation Startup and Audit; Matched Precipitation on a Rotor Zone.
• Local water purveyor or municipal watering restrictions and drought-stage rules, which override any schedule.
• State weather network, extension service or utility reference ET tables for the service area.