How Irrigation Scheduling Works: Evapotranspiration, Soil Moisture, and When to Water

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How Irrigation Scheduling Works: Evapotranspiration, Soil Moisture, and When to Water

Irrigation scheduling looks complicated from the outside — pivots, soil probes, ET tables — but underneath it's a water balance problem with only three moving parts: how much water your soil can actually hold in the root zone, how fast the crop is pulling that water out, and how far you're willing to let the tank drain before you refill it. Get those three numbers right and the "when" and "how much" fall out as arithmetic.

What Evapotranspiration Actually Measures

ET rate is the combined rate of water loss from a field — transpiration through the crop's leaves plus evaporation off the soil surface — expressed in inches per day. It isn't a fixed crop constant; it moves with the weather (heat, humidity, wind, solar radiation) and with the crop's growth stage, which is why it's read from local weather-station data or a crop ET table rather than looked up once and reused all season. A young crop with little leaf area evaporates mostly from bare soil and has a low ET rate; a full canopy in peak summer heat can push well past that. The calculator's accepted range, 0.05–0.5 inches per day, reflects that real-world spread from a barely-transpiring field to a crop at peak water demand under hot, dry, windy conditions.

Field Capacity, Water Holding Capacity, and the Root Zone

Field capacity is the amount of water a soil holds after gravity has drained away the excess — the "full" point of the bucket. How big that bucket is depends on soil texture: sandy soils hold roughly 0.8–1.2 inches of available water per foot of depth, loams hold about 1.5–2.0, and clay soils hold about 1.8–2.5. Multiply that per-foot capacity by how deep the crop's roots actually reach — corn and wheat typically root 36–48 inches, soybean 24–36 — and you get the total available water (TAW) stored in the zone the crop can actually draw from. A shallow-rooted crop on sandy soil and a deep-rooted crop on loam are working with very different bucket sizes even under identical rainfall and ET.

Management Allowable Depletion: the Trigger Threshold

You don't wait for the bucket to run dry before refilling it, and you don't refill after the crop has barely used any water either — both waste either yield or water. Management allowable depletion (MAD) is the percentage of total available water you let deplete before triggering irrigation, and it's expressed as a fraction of the bucket, not a fixed number of inches. A 50% MAD — the calculator's default, and the standard starting point for most row crops — means you irrigate once the root zone has used up half of what it can hold, leaving the other half as a buffer against a few more dry days before real stress sets in.

Turning the Numbers Into a Schedule

The calculator runs the same water-balance logic every time:

• Total available water = soil water-holding capacity × root depth (converted to feet)
• Allowable depletion = total available water × MAD%
• Current available water = total available water × current soil moisture (% of field capacity)
• Trigger point = total available water × (1 − MAD%) — the moisture level at which you irrigate
• Days until irrigation = (current available water − trigger point) ÷ ET rate
• Water to apply = total available water − current available water — enough to refill the root zone back to field capacity, not beyond it
• Steady-state interval = allowable depletion ÷ ET rate — the recurring cycle length once the field has settled into a rhythm of irrigate-deplete-irrigate

A Worked Example

Run the calculator's own defaults through that logic: ET rate 0.25 in/day, current soil moisture 80% of field capacity, root depth 36 inches (3 feet), soil water-holding capacity 1.8 in/ft, MAD 50%, on a 130-acre field.

1. Total available water = 1.8 × 3 = 5.4 inches
2. Allowable depletion = 5.4 × 0.50 = 2.7 inches
3. Current available water = 5.4 × 0.80 = 4.32 inches
4. Trigger point = 5.4 × (1 − 0.50) = 2.7 inches (equal to the allowable depletion here only because MAD happens to be exactly 50%)
5. Water remaining before the trigger = 4.32 − 2.7 = 1.62 inches
6. Days until irrigation = 1.62 ÷ 0.25 = 6.5 days
7. Water to apply = 5.4 − 4.32 = 1.08 inches, converted to volume at 27,154 gallons per acre-inch across 130 acres = 3,812,422 gallons
8. Steady-state interval, once the field is cycling regularly = 2.7 ÷ 0.25 = 10.8 days

Notice the two interval numbers answer different questions: "days until irrigation" is a one-time countdown from today's specific soil moisture reading, while the "steady-state interval" is the longer-run rhythm the field will settle into once each irrigation event refills the bucket completely before the next depletion cycle starts.

Reading the Depletion Curve

The chart projects soil moisture forward day by day at a constant daily depletion rate — ET rate divided by total available water, expressed as a percentage of field capacity per day. In the example above that's roughly 4.6 percentage points a day, so an 80%-FC start line crosses the 50% trigger threshold around day 6–7, matching the days-until-irrigation figure. That projection assumes tomorrow's ET rate equals today's, which real weather rarely guarantees — a hot, windy stretch will steepen the line and pull the trigger date closer, while a cool or rainy spell will flatten it. Treat the chart as a planning baseline to update with fresh ET readings, not a fixed date to circle on the calendar.

The Bottom Line

Every irrigation schedule, regardless of crop or system, reduces to the same three questions: how much water can this root zone hold, how fast is the crop using it, and how much of that reserve are you willing to spend before refilling. Plug in your own soil test's water-holding capacity, your crop's actual rooting depth, and a current ET reading from a nearby weather station, and the countdown to your next irrigation event — along with exactly how many gallons it will take — falls straight out of the arithmetic above.

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