Why the run time jumps by an order of magnitude
Sprinklers are rated in gallons per minute and emitters in gallons per hour, and that factor of sixty is most of the answer. A spray zone drawing 6 GPM delivers 360 gallons an hour. The same bed on 108 one-gallon-per-hour emitters delivers 108 gallons an hour. To put the same volume down takes three and a third times as long, and that is before the target depth is adjusted for the smaller area being wetted.
The consequence is scheduling rather than plumbing. Twelve-minute stations become two-hour stations, several of them will not fit in a pre-dawn window, and any controller limit on station run time starts to bite. Splitting the target into two runs on the same day is the usual fix and it has a second benefit, which is that the soil gets a chance to move the water sideways between them.
The wetted area is the input everybody guesses
Drip does not water a bed, it waters patches. How wide each patch is depends on the soil far more than on the emitter: water spreads sideways in a fine-textured soil and drives downward in a coarse one, so the same emitter for the same time makes a broad shallow saucer in one bed and a narrow deep column in another.
This matters because the target depth is over the wetted area, and the run time is directly proportional to it. Halve the wetted share and you halve the water and halve the time. Nothing else in the calculation has that much leverage on the answer, and it is the one number in the form that can be checked directly: run the zone for its normal time, then dig a small hole beside an emitter the next morning and look at how wide and how deep the wet zone actually is.
Two precipitation rates, and the one that causes trouble
Spread the zone flow over the whole bed and drip looks extraordinarily gentle — a tenth of an inch an hour or less. Spread the same flow over only the soil the emitters actually wet and the number can land above what the spray zone was applying.
| Zone flow | Bed area | Wetted share | Over the bed | Over wetted soil |
|---|---|---|---|---|
| 1.8 GPM | 1,200 sq ft | 60% | 0.144 in/hr | 0.241 in/hr |
| 1.8 GPM | 1,200 sq ft | 40% | 0.144 in/hr | 0.361 in/hr |
| 1.8 GPM | 1,200 sq ft | 20% | 0.144 in/hr | 0.722 in/hr |
| 1.8 GPM | 600 sq ft | 20% | 0.289 in/hr | 1.444 in/hr |
The bottom row is a densely emittered small bed, and at that point the soil under each emitter is taking water faster than most spray zones apply it. On level ground with time to soak, nothing happens. On a slope you get a wet track running downhill from each emitter, and the water arrives somewhere other than the root ball it was aimed at.
The saving is smaller than the flow drop suggests
Flow drops by a factor of ten and people expect the bill to follow. It does not, because the run time goes up to compensate and because the two systems are being asked to do different jobs: the spray zone was watering every square foot of a lawn, the drip zone is watering the root zones of a smaller number of plants that want less. The saving comes from the plant list and from watering less ground, not from the emitters themselves.
It also arrives late. A new planting is watered harder and more often than a settled one, sometimes for two full seasons, and during that period a converted zone can genuinely use more water than the lawn did. That is the single most common surprise in a conversion budget and it is the reason the payback calculation on this site takes an establishment period as an explicit input rather than assuming the saving starts on day one.
Questions people ask
How do I get the precipitation rate of the spray zone I am replacing?
Measure it if the system is still running, because the nameplate arithmetic and the reality diverge as heads settle and nozzles wear. Set out identical straight-sided containers across the zone, run it for a known time, and average the depths — the catch cup audit calculator turns those readings into a rate and a uniformity figure. If the system is already out, the calculated version from head flow and spacing is on the precipitation rate calculator, and it is close enough for a before-and-after comparison.
What wetted percentage should I use?
One you have looked at, ideally. It depends on the soil texture, the emitter flow, the run length and the spacing between emitters, and published ranges are wide enough to be nearly useless as a single figure. The practical method is to water normally, wait until the next morning and dig a narrow hole beside an emitter to see how far the wet zone spread sideways. Until then, run the calculation across a range rather than at one value, because the run time scales with it one for one.
Can I put drip on the same valve the sprinklers were on?
That is a hydraulic and hardware question rather than a water-balance one, and this page does not answer it. What it will tell you is the flow the new zone draws, which is usually so much lower that pressure regulation and filtration become the concern rather than capacity. The drip emitter calculator covers zone flow against a supply limit and line velocity. Anything involving the backflow assembly, the point of connection or the supply itself belongs with someone licensed to work on it.
Should the drip run more often or less often than the spray zone did?
Usually less often and much longer, because the reservoir being filled is a smaller volume of soil that is refilled to a greater depth each time. But the interval is set by how much water the wetted soil can hold and how fast the plants use it, not by a rule about drip, and that is a different calculation — the watering depth and interval calculator is the one that does it from soil type, root depth and allowable depletion.
Why did my water bill not drop after the conversion?
Several reasons compound, and the calculator will show most of them if you enter what actually happened rather than what was planned. The new planting is being established, so it is watered more than it will be later. The run time went up and the number of days may not have come down. Fixed charges do not move with the volume at all. And if the old zone was being run for twelve minutes when the lawn wanted twenty, the baseline you are comparing against was already low. Run the before column with the schedule you were really using.