What the seasonal adjust dial is doing
Almost every irrigation controller has a setting called seasonal adjust, water budget or something similar, expressed as a percentage. It multiplies every station run time by that percentage in one move. Set the controller once for the hottest week of the year, then drop the dial to 60 percent in spring and 40 in autumn, and the whole schedule scales without touching a single station.
The reason it exists is that plant water use varies enormously across a season while the relative demand between zones does not. If your rotor zone needs twice as long as your spray zone in July, it still needs twice as long in April — both are just smaller. One multiplier handles that correctly, which is why the dial is a percentage rather than a set of monthly schedules.
What the dial cannot do is know what percentage to use. That is the number this page produces, and it comes from comparing what the plants need now against what they needed in the month the controller was set for.
Reference ET and the coefficient
Reference evapotranspiration is a weather measurement. It expresses how much water a standardised reference surface would lose, in inches, over a period, and it combines temperature, solar radiation, humidity and wind into a single figure. It is published by state and regional networks, by many agricultural weather stations and by extension services, usually daily and often as a weekly or monthly summary. It is entirely local. There is no general figure and no way to derive one from a national average, which is why this page asks you to supply it rather than offering a table.
Your planting is not the reference surface, so the reference figure is scaled by a crop coefficient — the ratio between what your particular planting uses and what the reference uses under the same weather. The coefficient depends on the species, on how established it is, on the season, and on the local climate, and published values for the same species differ substantially between regions and between sources. Take yours from an extension service or a local horticultural authority. A coefficient borrowed from another climate produces a schedule that is precisely calculated and wrong.
Multiply the two and you have net demand: the depth of water the root zone needs over the period. Subtract useful rainfall. What remains is what irrigation has to supply.
Net, gross and where the difference goes
Net demand is what has to reach the roots. Gross is what has to come out of the sprinklers, and it is larger, because no system distributes water evenly. The zone has to run long enough to satisfy its driest ground, so the average ground receives more than it needed and the wettest more still.
Dividing net demand by an efficiency figure accounts for that. A reasonable efficiency figure for a sprinkler system is close to its low quarter distribution uniformity from a catch cup audit, because that is precisely the ratio between what the driest quarter gets and what the average gets. A system at DU 0.70 needs to apply about 1.43 times its net demand for the dry areas to reach target. Drip systems, having no throw pattern to go uneven, audit far higher and carry a much smaller surcharge, which is most of why they are described as efficient.
| Efficiency used | Net 0.8 in becomes gross | Surcharge |
|---|---|---|
| 85% | 0.94 in | 18% extra |
| 75% | 1.07 in | 33% extra |
| 65% | 1.23 in | 54% extra |
| 55% | 1.45 in | 82% extra |
Read that table as an argument for auditing rather than as a set of options. The surcharge is paid on every watering for the life of the system, and it scales with the seasonal adjustment, so a bad uniformity figure makes every month worse rather than just the peak one.
Rainfall, and why the gauge overstates it
Rain reduces the irrigation requirement, but only the part of it that entered the root zone and stayed there. A short intense storm on compacted or already-wet ground can run off almost entirely. Rain that falls when the soil is at capacity drains past the roots. Very light rain on a hot day can evaporate off the canopy before it reaches the soil at all.
So effective rainfall is always less than the gauge reading, and how much less depends on the intensity, the soil, the slope and how wet things already were. The percentage on this page is a judgement, not a measurement. A gentle overnight soak on dry, well structured soil might be 90 percent effective. A ten minute downpour on saturated clay on a slope might be 10. If you want to stop guessing, a soil moisture probe or a simple rain sensor wired to the controller settles it empirically, and a sensor has the additional merit of skipping cycles while you are not there to notice.
Using the number without over-trusting it
Set the controller for your peak month with real run times, calculated from each zone's precipitation rate and the depth its planting needs. That is the 100 percent baseline. Then recalculate the budget as the season moves — monthly is enough for most gardens, and more often during the shoulder seasons when ET changes fastest. Write the peak run times down somewhere, because the whole scheme falls apart the moment someone adjusts a station time directly and the baseline stops meaning what it meant.
Then check the result against the ground. Dig a plug two days after a run and see how deep the wetting front went. Watch for the specific signals of under-watering — footprints staying visible in the turf, a blue-grey cast, edges browning first — and of over-watering, which is softer ground, more disease and moss appearing where it did not before. The arithmetic is a starting point that gets you close on the first attempt; the soil is the thing that is actually right.
The depth each run applies comes from the precipitation rate calculator, the efficiency figure comes from the catch cup audit, and how much the root zone can usefully hold at once is on the watering depth and interval calculator. For drip zones, run times work from emitter output instead and are on the drip emitter calculator. If the aim of the exercise is the bill, the water bill calculator puts a seasonal saving in money and the water saving guide covers the rest of the household.
Questions people ask
What is a water budget or seasonal adjust percentage?
A single multiplier applied to every station run time on the controller. At 60 percent, a station programmed for 30 minutes runs 18. It exists because plant water use changes by a factor of three or more across a season while the ratio between zones stays roughly constant, so one number can rescale the whole schedule correctly. The controller cannot work out what the number should be — it just applies whatever you set — which is the gap this calculator fills.
Where do I get reference evapotranspiration for my area?
From a local source, and only from a local source. Many states and regions run weather networks that publish daily and weekly reference ET specifically for irrigation scheduling, agricultural weather stations often carry it, and extension services will point you at whichever one covers your area. Some controllers subscribe to a local feed and adjust themselves. What you cannot do is use a figure from elsewhere: ET depends on solar radiation, humidity and wind as well as temperature, so two places with the same daily high can differ substantially.
What crop coefficient should I use for my lawn?
Ask locally, because there is no single answer and this page will not invent one. Coefficients depend on species, on how established the planting is, on the season and on the local climate, and published values for the same grass differ between regions and between sources by enough to matter. Extension services publish coefficients for their own area, which is the figure you want. Treat any number you find that does not say which region it applies to as unusable.
Should the efficiency figure be the same as distribution uniformity?
Close enough to be a good working choice. Low quarter DU is the ratio between what the driest quarter of a zone receives and what the average receives, and since the run time has to satisfy the driest quarter, dividing net demand by DU gives the gross that does it. Strictly, application efficiency also includes wind drift and evaporation losses, which pushes the useful figure slightly lower than DU alone. If you have not audited, entering a guess is fine for getting the seasonal ratio right, since the same efficiency appears in both the peak and current figures and largely cancels out of the percentage.
Should I water more days a week in summer instead of longer?
Usually longer rather than more often, up to what the soil can absorb in one go. Roots follow water down, so fewer deeper applications build a deeper root system that then holds more and buffers hot spells better. Frequent shallow watering does the opposite. The limits are real though: the root zone can only store so much before the surplus drains past it, and the applied rate can only exceed the soil intake rate for so long before water starts leaving. Where the rate is the binding constraint, the answer is cycle and soak rather than extra watering days. And the days you may water are set locally, so check with your water utility before settling on any pattern.