What a seasonal adjustment or water budget setting does
Most irrigation controllers have a single dial or menu percentage that scales every zone's run time up or down at once, without reprogramming each station. Rain Bird calls this feature Water Budget; Hunter calls it Seasonal Adjustment. Both work the same way: 100% is your peak-month run time, the amount of water your zones need in the hottest, driest part of the year, and every other month is set as a fraction of that.
That fraction tracks reference evapotranspiration, or ETo, the standard measure of how much water a well-watered reference surface loses to sun, wind and heat. As ETo drops through fall and rises again in spring, the controller percentage drops and rises with it, so a system that runs at 100% in July might sit at 40% in April and 10% in November.
| Season | Typical range | Notes |
|---|---|---|
| Peak summer | 100% | Your hottest, driest month is always the 100% reference point. |
| Spring and fall | 40–75% | ETo is well below peak but still meaningful. |
| Winter, mild climates | 10–30% | Hunter's Solar Sync charts use a 10% floor for months that still need some water. |
| Winter, freezing climates | 0%, system off | A month showing a low floor value may in reality be shut down for winter. |
In Rain Bird's own published example for a hot desert climate, a controller with a 23-minute peak run time in July was set to just 7% in January, about 2 minutes, and back up to 100% by July. That's the same calculation this tool automates for whatever months you enter.
Check your controller's manual. Most accept a whole-number percentage, commonly from about 10% up to 200%, but the exact range and whether it's set per program or globally varies by brand and model.
How this seasonal adjustment calculator works
The calculator finds whichever month you entered has the highest ETo and treats it as your peak, or 100%, month. Every other entered month is then a simple ratio against that peak.
1. Controller percentage for a month
Seasonal adjustment %
Controller % = Month's ETo ÷ Peak-month ETo × 100
Inches or millimeters cancel out in the ratio, so the result is the same in either unit system. This is the water budget (WB) formula Rain Bird publishes for its controllers.
2. Adjusted run time
Adjusted run time
Adjusted run time = Peak-month run time × Controller % ÷ 100
Rain Bird calls the peak-month run time the Base Schedule Index (BSI) and this result the Adjusted Run Time (ART). Round to a whole percentage and a whole minute before entering it on your controller.
Which month counts as "peak"
This calculator, like Rain Bird's and Hunter's controller features, defines the peak month as whichever entered month has the highest ETo. That's a different definition from EPA's WaterSense Water Budget Tool, which instead looks for the month with the largest gap between ETo and expected rainfall. Where rain is a meaningful part of your water budget, that gap-based peak month can fall in a different month than the ETo-only peak used here.
When this calculator isn't the right tool
It turns ETo you already have into a controller percentage. Use a different tool for:
- Finding your monthly ETo values in the first place. See the reference ET calculator.
- Finding your peak-month run time from a target depth and sprinkler rate. See the sprinkler run time calculator.
- A compliance-style water budget, such as a maximum applied water allowance for an ordinance. See the MAWA calculator or the landscape water budget calculator, which subtract rainfall and factor in plant type.
- Preventing runoff once a month's run time gets long. See the cycle and soak calculator.
Seasonal adjustment calculation example
A homeowner runs 30 minutes per zone in the peak month, June, when ETo is 6.00 inches. They also have ETo figures for July, August, September and October.
- Peak month: June, 6.00 in, is the highest entered value, so June = 100% and 30 minutes.
- July: 5.70 ÷ 6.00 × 100 = 95%. Run time: 30 × 95 ÷ 100 = 28.5, shown as about 29 minutes.
- August: 5.10 ÷ 6.00 × 100 = 85%. Run time: 30 × 85 ÷ 100 = 25.5, shown as about 26 minutes.
- September: 3.60 ÷ 6.00 × 100 = 60%. Run time: 30 × 60 ÷ 100 = 18 minutes.
- October: 2.00 ÷ 6.00 × 100 = 33.3%, shown as about 33%. Run time: 30 × 2.00 ÷ 6.00 = 10 minutes.
The same calculation in metric
A 25-minute peak-month run time, with July as the 150 mm peak month, and ETo of 20 mm in January, 70 mm in April and 55 mm in October:
- Peak month: July, 150 mm = 100% and 25 minutes.
- January: 20 ÷ 150 × 100 = 13.3%, shown as about 13%. Run time: 25 × 20 ÷ 150 = 3.33, shown as about 3 minutes.
- April: 70 ÷ 150 × 100 = 46.7%, shown as about 47%. Run time: 25 × 70 ÷ 150 = 11.67, shown as about 12 minutes.
- October: 55 ÷ 150 × 100 = 36.7%, shown as about 37%. Run time: 25 × 55 ÷ 150 = 9.17, shown as about 9 minutes.
Notice that only the months you enter show up in the schedule. Leave a month blank, such as during a winter shutdown, and it's simply skipped rather than shown as a false 0%.
Using your seasonal percentages
Once you have a percentage for a month, round it to a whole number and enter it into your controller's water budget or seasonal adjustment setting for that program. If your controller only has one global setting rather than a per-program one, use the percentage for whichever zone type matters most, usually turf, and check that other zones such as drip beds aren't left over- or under-watered as a result.
Update it as the season turns
A manual dial needs revisiting every month or so to track the changing percentage; a smart, weather-based controller can pull current ET automatically and adjust daily, which is closer to how the underlying ET-based scheduling method is meant to work. Either way, the base peak-month run time you entered here only needs to be set once per zone type, for the year.
Watch for long run times
A month close to 100% can produce a run time that exceeds how fast your soil absorbs water, especially on clay or slopes. If a session starts running off before it finishes, split it into shorter cycles with a soak break in between using the cycle and soak calculator, rather than lowering the percentage below what the ETo ratio calls for.
Different zone types need their own base run time
Drip-irrigated beds and shrub zones typically need a smaller share of reference ET than turf, so their peak-month run time is usually shorter than a spray or rotor zone's. Run this calculator once per zone type, each with its own peak-month run time, rather than applying one turf-based schedule to the whole property.
What changes the pattern from month to month
| Factor | Effect on ETo | What to adjust |
|---|---|---|
| Temperature and sun | Heat and long daylight raise ETo through late spring and summer. | Use your actual monthly data; don't assume last year's numbers without checking. |
| Wind and humidity | Dry, windy conditions raise ETo even without extra heat. | A weather-based controller reacts to this daily; a manual dial only reacts monthly. |
| Rainfall | Not part of ETo itself, but reduces how much of the calculated amount is actually needed. | Check the effective rainfall calculator or add a rain sensor. |
| Freeze risk | Doesn't change ETo, but stops irrigation regardless of the calculated percentage. | Leave winterized months blank rather than entering 0%. |
| Zone type | Turf typically tracks closer to full ETo than trees, shrubs or drip zones. | Give each zone type its own peak-month run time. |
If you're setting up a new schedule rather than adjusting an existing one, the irrigation scheduling calculator and the ETWU calculator build a full month-by-month plan from ETo, plant factor and area rather than from an existing peak-month run time.
Frequently asked questions
What is a seasonal adjustment or water budget setting?
It's a single percentage on your irrigation controller that scales every zone's run time up or down without reprogramming each one. Rain Bird calls this feature Water Budget and Hunter calls it Seasonal Adjustment; both work the same way. You set 100% for your peak watering month, usually mid-summer, and lower the percentage for cooler months when plants use less water.
How do I find my peak-month run time?
It's the time per zone you already run, or would run, during your hottest, driest month, sometimes called the Base Schedule Index. If you don't know it yet, the sprinkler run time calculator can work it out from your target depth and your sprinkler's application rate.
See the sprinkler run time calculator.
How is the monthly percentage calculated?
Divide that month's reference ET (ETo) by your peak month's ETo and multiply by 100. A month with half the peak month's ETo gets a 50% setting; a month that matches the peak gets 100%. This is the same water budget formula Rain Bird publishes for its controllers.
Where do I get monthly ETo values?
Use the reference ET calculator, or ask your local extension service, water utility or CIMIS-style network for historical monthly averages for your area. This calculator only turns ETo you already have into a controller setting; it doesn't estimate ETo itself.
See the reference ET calculator.
What happens to a month I leave blank?
It's simply left out of the schedule and treated as a month you don't water, such as a winterized system in a freezing climate. It doesn't affect the percentages calculated for the months you do enter.
Does this account for rainfall?
No. It scales run time to reference ET alone, the same way Rain Bird's and Hunter's seasonal adjustment features do. EPA's WaterSense Water Budget Tool instead defines the peak month as the month with the largest gap between ETo and expected rainfall, so if rain is significant where you live, check the effective rainfall calculator before finalizing a month's setting.
See the effective rainfall calculator.
Can I use this for drip zones or shrub beds?
Yes, as long as you give each zone type its own peak-month run time, since drip zones and shrub beds often need a smaller fraction of ETo than turf. Run the calculator once per zone type using that zone's own peak run time.
How often should I update the percentage?
Monthly is typical for a manual dial, moving toward the next month's figure as the season turns. Some controllers accept a full 12-month table at once, which is what the schedule in this calculator is built for.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Percentage and run-time formulas verified by hand against Rain Bird's own published water-budget worked example.
- Peak-month definition checked against EPA WaterSense's Water Budget Approach documentation.
- Method follows the water-budget / seasonal-adjustment convention used by Rain Bird and Hunter controllers.
Sources
- Rain Bird Corporation. Irrigation Scheduling – Use "ET" to Save Water.
- Hunter Industries. Seasonal Adjustment: Global.
- University of California Statewide Integrated Pest Management Program (UC IPM). Irrigation Scheduling Using Evapotranspiration (ET).
- U.S. EPA WaterSense. WaterSense Water Budget Approach.