Irrigation Controller Programming and Scheduling

Most irrigation systems waste water at the controller, not in the pipe. A good installation can run a bad schedule for years, and nobody notices until the bill comes or the lawn turns. Programming is a calculation with a few steps, not a guess at 15 minutes for everything. Once you can work from plant demand back to run times, you can set up a new system, fix a complaint, and explain your numbers to a property manager.

What You Will Be Able to Do

• Work out how much water a zone needs from ETo and a plant coefficient
• Turn that amount into a run time using precipitation rate and uniformity
• Split run times into cycles to prevent runoff
• Set up programs, start times, and seasonal adjust without double watering
• Configure rain, soil moisture, and flow sensors
• Explain what smart control does and what it cannot fix

Start From What the Plant Uses

Plants and soil lose water to the air. That loss is called evapotranspiration (ET). Weather services publish a reference number called ETo, in inches per day. It stands for the water used by a reference crop in your area on that day. State climate networks and university extension sites publish it.

Your plants use a share of that number. The share is the plant coefficient.

These are starting values used by university extension programs. They change by month and climate, and shrubs and trees have their own values. Use local figures when you can.

Worked example. Peak summer ETo is 0.22 inches per day. The lawn is cool-season turf. 0.22 x 0.8 is about 0.18 inches a day, or about 1.23 inches a week.

Correct for Uneven Coverage

No system waters perfectly evenly. If you water to the average, the dry spots stay dry. So you run longer to bring the driest part up to the target. Distribution uniformity (DU) measures how even a zone is, from 0 to 1. Published examples use about 0.6 to 0.75 for working systems. A catch-can test on the actual zone gives you a real number.

Divide the water need by the DU.

Worked example. The lawn needs 1.23 inches a week, and the DU is 0.7. 1.23 divided by 0.7 is about 1.76 inches a week.

A better DU saves water directly. Raising the DU from 0.6 to 0.75 drops the same lawn from 2.05 to 1.64 inches a week. Fixing tilted heads and mismatched nozzles often saves more than any controller feature.

From Inches to Minutes

Now use the precipitation rate of the zone.

Minutes a week = inches needed divided by precipitation rate, x 60.

Worked example. The zone applies 1.5 inches an hour. 1.76 divided by 1.5 is about 1.17 hours, or about 70 minutes a week. Watering 3 days a week means about 23 minutes each watering day.

The same lawn on rotors at 0.5 inches an hour would need about 211 minutes a week, or about 70 minutes on each of 3 days. That is why one number for every zone is always wrong.

Soil and Slope Set the Limit

The soil can only take water so fast. The steady intake rate published by the USDA is above about 0.8 inches an hour for sand, about 0.2 to 0.4 for loam, and as low as 0.04 to 0.2 for clay. Compaction and slope lower it further. Spray heads apply 1.5 inches an hour or more, so on clay, water starts to run off within minutes.

The fix is to divide the run time into cycles, with soak time between. Watch the zone once to find how many minutes it runs before water starts to pool or run. That is your cycle limit.

Worked example. The 23-minute zone sits on clay. You see runoff at 9 minutes. Set three cycles of 8 minutes, which is 24 minutes in total. Allow 30 to 60 minutes of soak time between cycles.

Setting Up Programs

Controllers organize watering into programs, often named A, B, and C. Each program has its own water days, start times, and station run times. The most common setup uses one program per water need.

Start Times Are the Classic Trap

A start time starts the whole program. It does not start one station. Each added start time runs every station in that program again.

Worked example. Program A has 8 stations set to 24 minutes. You meant to add a second start time for one zone, but you added two extra start times to the program. Now it runs 8 x 24 x 3 minutes, which is 576 minutes, or 9.6 hours. You have tripled the water use, and the program may still be running when people arrive at work.

Using Station Order as Soak Time

With several zones in one program, the other zones give you free soak time. Set each station to the short cycle length and give the program three start times.

Worked example. 8 stations at 8 minutes take 64 minutes for one pass. Start times at 3:00, 4:05, and 5:10 AM give each zone 3 cycles of 8 minutes. Zone 1 soaks for about 57 minutes between its cycles. The last pass ends about 6:14 AM, before most people are up. The total is the same 24 minutes per zone, with no flooding.

Many newer controllers have a built-in cycle and soak feature for each station, which does the same job.

Check the Watering Window

Add up the run time and compare it with the hours you have. Local watering rules may limit days and hours. Sites with many zones may run two zones at once, but only if the supply can carry both flows. Check that against the budget you designed for. Controllers can stack programs so that one waits for another, or let them overlap. Overlap can drop pressure and cause weak coverage, so learn how your model behaves.

Seasonal Adjust

Seasonal adjust scales all run times by one percentage. Set your program for peak summer as 100 percent, then scale from the ETo for each month.

Worked example. Peak ETo is 0.22 inches a day, and April ETo is 0.12. 0.12 divided by 0.22 is about 55 percent. Set seasonal adjust to 55 percent in April.

Check whether the shortened run still soaks in deep enough. On a very low percentage, fewer watering days with the same run time often work better than very short runs.

Sensors

Some states require a rain sensor. Many controllers have a bypass switch for sensors, and a sensor left in bypass does nothing. Check the switch on every service visit.

Using Flow Sensing

A flow sensor paired with a master valve is the best leak protection a large site can have. Most controllers learn a normal flow for each station by running them once. Then you set limits around that number.

Worked example. Zone 5 learned a normal flow of 18 GPM. You set a high-flow limit of 130 percent, so an alarm trips above about 23 GPM. A broken lateral pushes flow to 31 GPM. The controller shuts that station off, closes the master valve if set to do so, sends an alert, and moves to the next zone. Without the sensor, it would have flooded for the whole run. Set limits too tight and every pressure swing sets off an alarm. Set them too loose, and small breaks go unnoticed.

Relearn flows after any change to nozzles or heads.

Smart Control

A smart controller changes run times by itself. Some use on-site weather sensors or a weather data feed to estimate ET. Others use soil moisture. The EPA's WaterSense program labels controllers that pass its tests.

A weather-based controller needs correct inputs for each zone: plant type, soil, slope, sun, head type, and precipitation rate. Wrong inputs give a wrong schedule that looks automatic. Makers often claim savings of 20 to 50 percent, but real savings depend on how bad the old schedule was. A smart controller cannot fix a broken head, a bad layout, or a stuck valve.

Larger Systems

Large sites may use a two-wire or decoder system. One pair of wires runs out to all the valves, and each valve has a decoder with an address. It saves a lot of wire, but it is sensitive to poor splices and needs surge protection and grounding set up as the maker directs. Central control lets one person manage many sites from a computer. Keep the same rule on any system: each zone gets a reasoned run time.

Warning: Controllers run valves at about 24 volts AC, but the power feeding the controller is line voltage. Leave work on line-voltage wiring to a qualified electrician, and follow local code for outdoor connections.

Commissioning a New Program

1. Set the date, time, and AM or PM. Confirm it after power loss too.
2. Label each station with what it waters.
3. Enter programs from your calculations, and keep a copy.
4. Run each station and walk it for runoff, overspray, and coverage.
5. Test the rain sensor and clear any bypass.
6. Show the owner how to use seasonal adjust, and how to shut off the system.
7. Leave a chart in the controller cabinet with zones, run times, and the program.

Common Mistakes

Check Your Understanding

1. ETo is 0.25 inches a day, and the turf is warm-season. How much water does it need in a week?

0.25 x 0.6 is 0.15 inches a day. Over 7 days, that is 1.05 inches a week.

2. A zone needs 1.4 inches a week after correcting for DU, and it applies 0.7 inches an hour. How many minutes a week is that?

1.4 divided by 0.7 is 2 hours, or 120 minutes a week. On 3 days, that is 40 minutes each day.

3. A property manager says the water bill tripled last month. The controller shows Program A has 3 start times, and all stations run 20 minutes each time. What do you check?

Whether those start times were intended. Each start time runs every station again, so 3 start times mean 3 times the water. If they were meant as cycle and soak, the run times should be shorter.

4. Peak summer ETo is 0.20, and October ETo is 0.09. What seasonal adjust do you use?

0.09 divided by 0.20 is 45 percent.

Key Takeaways

• Run time comes from plant demand, DU, and precipitation rate.
• ETo x plant coefficient gives the water need.
• Soil intake sets the longest cycle before runoff.
• Each start time repeats the whole program.
• Use seasonal adjust from ETo, and check sensors on each visit.
• Smart control only works with correct inputs on a sound system.