Depth per pass, not depth per hour
Fixed overhead irrigation is quoted as a precipitation rate in inches per hour, and run time is what you vary. A travelling boom works the other way. The depth it lays down in one pass is fixed by three things — the total nozzle flow, the width the boom wets, and how fast it moves — and the run time is simply the length divided by the speed. Running the boom for longer does not put more water on; it only means it is somewhere else on the bench.
The arithmetic is one line. Gallons per square foot in a pass is flow divided by width divided by speed. On the defaults that is 2.5 divided by 5 divided by 4, which is 0.125 gallons per square foot. One gallon spread over one square foot is 231 cubic inches over 144 square inches, which is 1.604 inches deep, so 0.125 gallons per square foot is 0.201 inches. The reciprocal is the number worth memorising: one inch of water is 0.6234 gallons per square foot.
Notice what is missing from that. The bench length appears nowhere. A 20 ft bench and a 200 ft bench get the same depth per pass from the same boom at the same speed; only the clock time differs, at 5 minutes and 50 minutes respectively.
Most of the water misses
This is the part that surprises people who move from ground beds to containers. Water falling on a bench of containers only counts if it lands inside a container. Everything else hits the bench top and goes to the floor.
On the defaults, containers 6.5 inches across sitting on 7 inch offset centres have an open top of 33.2 square inches each and occupy 42.4 square inches of bench. That is 339 containers on a 100 square foot bench, with 11,249 square inches of open container top out of 14,400 square inches of bench — 78.1 percent. And that is close to the best case, because those containers are nearly touching. Space the same containers out to 10 inch centres and the interception drops to 38.3 percent: three fifths of the water is now going on the floor.
| Centre spacing | Containers on 100 sq ft | Water landing in them |
|---|---|---|
| 7 in, pot to pot | 339 | 78.1% |
| 8 in | 259 | 59.7% |
| 10 in | 166 | 38.3% |
| 12 in | 115 | 26.5% |
None of that is an argument for one spacing over another — spacing is decided by the crop, not by the irrigation. It is an argument for knowing the number, because it is the difference between a water bill you understand and one you do not, and because in a recirculating yard it is the volume that has to be collected and dealt with.
Leaching and the rounding to whole passes
If you intend a share of what enters a container to drain out of the bottom, the volume that has to go in is the retention target divided by one minus that fraction. Eight fluid ounces retained at a 20 percent leaching fraction means 10 fluid ounces have to enter.
Each pass on the defaults puts 0.201 inches over 33.2 square inches of container top, which is 6.65 cubic inches, or 3.69 fluid ounces. Ten fluid ounces takes 2.71 passes, and you cannot run 0.71 of a pass. Three passes deliver 11.06 fluid ounces, which retains 8.85 — 10.6 percent more than asked for. The 1.06 fluid ounce overshoot across 339 containers is 2.8 gallons.
Whether that matters is a question about your crop and your water, not one this page can answer. What it can tell you is the lever: a faster boom carries less water per pass, so the rounding error is smaller. At 8 ft per minute each pass delivers 1.84 fluid ounces and 10 ounces takes 6 passes for 11.06 — the same overshoot, as it happens, because 10 divided by 1.84 is 5.42. At 6 ft per minute it takes 5 passes for 12.29. The relationship is not smooth, which is exactly why it is worth calculating rather than estimating.
Getting the flow figure honestly
The single input most likely to be wrong is the total nozzle flow, because it is usually taken from a nozzle chart at a pressure the system does not actually run at. Put a jug under one nozzle, run it for a timed minute, measure what is in it, and multiply by the nozzle count. Then do a catch test with a grid of identical containers along and across the bench and compare the average to what this page predicted. If they disagree, the flow figure or the speed is wrong, and it is usually the flow.
For fixed heads rather than a moving boom, the precipitation rate calculator is the right page, and for emitters the drip emitter calculator. For the water one individual container takes to hit a leach fraction, pot watering volume works from the container rather than from the bench.
This page states no watering volume, no frequency, no leaching target and nothing about water quality or treatment. All of those are yours. It also has nothing to say about fertiliser, and it issues no verdict about whether any schedule is adequate.
Questions people ask
Why does the bench length not affect the depth per pass?
Because the boom is over any given square foot for the same length of time regardless of how long the bench is. A nozzle passing at 4 feet per minute takes the same fraction of a minute to cross a one foot strip whether that strip is at the start of a 20 foot bench or in the middle of a 200 foot one. Length changes the clock time and the total gallons used, and it changes neither the depth nor the amount any one container catches.
Is the interception figure really that pessimistic?
It is geometry, and it is if anything generous. It assumes every container is a perfect circle of the stated diameter with a fully open top, that nothing is blocked by foliage, and that a drop landing exactly on the rim goes in. Real crops intercept water on leaves and shed some of it outside the container, and containers that have been spaced out lose far more than the numbers here. It is the upper bound on what goes in, not the expected value.
Should I run one slow pass or several fast ones?
The gallons are identical for the same total depth, so it is not a consumption question. Multiple faster passes give finer control over the total, because the rounding to a whole pass costs you less each time, and they let water soak in between passes rather than arriving all at once. A single slow pass is quicker on the clock because there are no turnarounds. Which suits a given crop is a horticultural judgement and this page does not make it.
What should the leaching fraction be?
Not something this page will tell you. It is a decision about your water, your media, what you are feeding and what happens to what drains out, and in some places it is also a regulatory question about runoff. The field takes your figure and grosses the delivery up so that share can drain. Set it to zero and the calculator simply delivers the retention target with nothing over.
How do I account for a boom that is wider than the bench?
Enter the bench width rather than the boom width, and put the overhang into the loss field as a percentage. If a 6 foot boom runs over a 5 foot bench, roughly a sixth of the output goes past the edge, so 17 percent in the loss field is a reasonable starting point. Better still, catch test it: put containers right at the bench edge and one just off it, and you will see the real edge profile, which is never a clean cutoff.