Depth Per Event: The Single Number That Sets Your Irrigation Timer
Irrigation scheduling answers two questions at once: how many inches (or millimeters) of water to apply each time you irrigate, and how many days to wait between events. Most people guess the interval and hope the depth works out. The smarter approach is to let crop ET (daily water use) and your soil’s available water capacity dictate both numbers, so you replace exactly what the crop consumed without pushing water below the root zone.
A common scheduling mistake is running the pivot on the same fixed cycle all season. Early in the year, when corn Kc is 0.3, a 7-day cycle may over-water. At silking, when Kc hits 1.15 and daily ETc can reach 0.35 in, that same 7-day cycle delivers too little. The result is waterlogging in May and stress in July— both costing yield. This calculator adjusts depth and interval to the crop stage so you avoid both extremes.
Crop Coefficients Through the Growth Stages: Not a Flat Line
The Kc curve published in the FAO-56 guidelines breaks each crop into four stages: initial, development, mid-season, and late. Corn, for instance, starts around Kc = 0.30, climbs to 1.20 by tassel, and drops back to 0.55 at maturity. Plugging mid-season Kc into a schedule that covers the whole year over-applies early and under-applies late.
Enter the Kc that matches your crop’s current stage. If you are running schedules for the full season, the calculator lets you provide a weighted average or you can re-run it at each stage transition. Extension bulletins from land-grant universities publish Kc tables calibrated to local planting dates—use those rather than generic values whenever possible.
Net Depth, Gross Depth, and the Efficiency Factor Between Them
Net depth per event is the water the root zone actually needs to refill. Gross depth is what the system must deliver, accounting for losses. The relationship is identical to the seasonal calculation:
Gross depth = Net depth ÷ System efficiency
If net depth is 1.0 in and your pivot runs at 85 % efficiency, gross depth = 1.0 ÷ 0.85 = 1.18 in per pass.
The interval between events depends on daily ETc and how much available water the soil can hold in the root zone. Sandy loam with 1.2 in/ft of available water and a 3-ft root zone holds 3.6 inches total, but you should only deplete about 50 % of that (1.8 in) to avoid stress. At 0.30 in/day ETc, that buffer lasts 6 days—so you irrigate every 6 days with a 1.8-inch net application.
Mid-July Corn on Sandy Loam: Scheduling a Single Irrigation Cycle
Daily ET₀ = 0.28 in, Kc at silking = 1.15, root depth = 3 ft, available water capacity = 1.2 in/ft, management-allowed depletion = 50 %, pivot efficiency = 85 %.
| Step | Math | Result |
|---|---|---|
| Daily ETc | 0.28 × 1.15 | 0.322 in/day |
| Total available water | 1.2 × 3 | 3.6 in |
| Allowed depletion | 3.6 × 0.50 | 1.8 in |
| Interval | 1.8 ÷ 0.322 | 5.6 days → 5 days |
| Net depth per event | 0.322 × 5 | 1.61 in |
| Gross depth per event | 1.61 ÷ 0.85 | 1.89 in |
Round the interval down to 5 days (not up) to stay on the safe side of depletion. Your pivot needs to lay down about 1.9 inches per pass. If your pivot speed setting only delivers 1.5 in, you either slow it down or accept a shorter interval.
Situations That Break a Fixed Schedule
- Heat wave with ET₀ above 0.35 in/day. Your 5-day interval shrinks to 3–4 days. If the pivot cannot cycle fast enough, you will hit stress. Monitor soil moisture sensors and be ready to shorten the rotation during extreme heat.
- Mid-season rainfall that partially refills the profile. A 1-inch rain resets your depletion clock. If you irrigated yesterday and it rained 1.5 inches overnight, skip the next scheduled event and re-start the countdown from 0 depletion. Over-irrigating on top of rain pushes nitrogen below the roots.
- Shallow root crops on deep sand. Lettuce or strawberries with a 12-inch root zone on sand (0.8 in/ft AWC) hold only 0.8 inches total, half of which is 0.4 in. At 0.20 in/day ETc you irrigate every 2 days with tiny applications—drip is nearly mandatory.
- Late-season Kc drop-off. Once the crop reaches physiological maturity, Kc falls sharply. Continuing to irrigate at mid-season depth wastes water and can delay harvest or promote disease. Taper your schedule as Kc declines.
Scheduling Errors That Cause Runoff or Root-Zone Stress
- Setting the pivot speed by “feel” rather than calculated depth. A pivot chart or travel-speed table converts inches per pass to a percent-timer setting. Guessing the speed commonly over-applies by 0.3–0.5 in per event, adding up to several acre-feet of wasted water per season.
- Using a single depletion percentage for every soil. The 50 % rule-of-thumb works for most field crops on medium-textured soils, but sensitive crops (grapes, peppers) should stay below 35 % depletion to maintain quality. Check crop-specific guidelines before locking in one number.
From Per-Event Depth to Full-Season Volume
Once you know how many inches each event delivers and how often events repeat, multiply through the season to get total gallons—or jump to the Irrigation Water Requirement Calculator which handles seasonal totals, pump hours, and energy cost in one pass. To fold irrigation cost into a complete input budget, combine with the Fertilizer Cost per Nutrient Unit Calculator and the Seed & Fertilizer Rate Calculator. The Crop Yield Estimator closes the loop by projecting whether the yield justifies the pumping expense.
Scheduling outputs depend on accurate ET₀ data, soil water-holding capacity, and realistic Kc values for your crop and growth stage. Field conditions—compaction layers, variable soil textures, wind patterns—can shift actual results. Verify with soil moisture sensors or a tensiometer, and consult your irrigation district or extension specialist for local guidance.