What is application rate and where do I find it?

Application rate, also called precipitation rate, is how many inches of water your system delivers per hour. You can get it from the nozzle manufacturer chart or measure it directly with a catch-can test by running the system for a set time and averaging the depth collected.

Why divide by distribution efficiency?

No system applies water perfectly evenly. Distribution efficiency (often 0.7 to 0.9) accounts for losses from wind, evaporation, and uneven coverage. Dividing the gross run time by efficiency means the driest spot still receives the target depth.

How do I know the target depth to apply?

The target depth is usually the soil water deficit since the last irrigation or rain. It equals the crop water use over that period, or the available water capacity of the root zone you want to refill. One inch of water wets a sandy soil to roughly 12 inches deep.

Does this work for drip and sprinkler systems?

Yes. The formula is identical for any system as long as you know its application rate in inches per hour. Drip systems report this differently (gallons per emitter), so convert to an equivalent depth over the wetted area first.

Should I split one long run into several cycles?

Often yes. On slopes or tight soils, water runs off before it can soak in. Use the cycle-and-soak approach: split the total run time into shorter bursts with soak periods between them. The total run time from this tool stays the same.

Irrigation Run-Time Calculator

Watering by the clock guesses; watering by depth is precise. This calculator converts a target water depth into an exact irrigation run time using your system’s application rate, then adjusts for distribution efficiency so even the driest part of the field receives what the crop needs.

How it works

The core relationship is depth equals rate times time, rearranged to solve for time:

gross run time (hr) = target depth (in) / application rate (in/hr)
net run time  (hr) = gross run time / distribution efficiency

Efficiency is a fraction between 0 and 1. Dividing by it lengthens the run so the under-watered low spots still reach the target depth. The result is then converted to hours and minutes for the controller.

Example and tips

To apply 0.6 inch with a system rated at 0.4 in/hr and 80 percent efficiency: gross time is 0.6 / 0.4 = 1.5 hours, and net time is 1.5 / 0.8 = 1.875 hours, or about 1 hour 53 minutes. Run a catch-can test once per season to confirm your real application rate, because clogged nozzles and pressure changes drift it over time. On heavy clay or slopes, split this run time into cycle-and-soak bursts to prevent runoff.