# Irrigation Scheduling Calculator

> Estimate how often to irrigate and how much water to apply from ET₀, crop coefficient, soil water holding and root depth, using the FAO-56 method.

Category: Agriculture · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/irrigation-scheduling-calculator

The FAO-56 approach lets the root zone dry by the readily available water, then refills it. Interval = readily available water ÷ daily crop water use.

Also called: Irrigation interval calculator, Irrigation frequency calculator.

## Inputs
- Reference evapotranspiration (ET₀) (unit mm/day; default 5)
- Crop coefficient (Kc) (default 1.15)
- Field capacity (unit % vol; default 30)
- Wilting point (unit % vol; default 15)
- Effective root depth (unit m; default 0.8)
- Allowable depletion (p) (default 0.5)
- Application efficiency (unit %; default 85)
- Area (default 2)
- Area in (options: Hectares, Acres; default ha)

## Results
- Days between irrigations
- Crop water use (ETc)
- Total available water (TAW)
- Readily available water (net depth)
- Gross depth to apply
- Water per hectare
- Water for the area

## Formula

```
ETc = Kc × ET₀;  TAW = 1000 (θFC − θWP) Zr;  RAW = p × TAW;  Interval = RAW ÷ ETc;  Gross depth = RAW ÷ Efficiency
```

Where:
- θFC, θWP: water content at field capacity and wilting point (m³/m³)
- Zr: root depth (m)
- p: depletion fraction before stress
- ETc: crop evapotranspiration

## Worked example
1. ETc = 1.15 × 5 = 5.75 mm/day.
2. TAW = 1000 × (0.30 − 0.15) × 0.8 = 120 mm; RAW = 0.5 × 120 = 60 mm.
3. Interval = 60 ÷ 5.75 = 10.43 days; gross depth = 60 ÷ 0.85 = 70.59 mm (705.88 m³/ha).

**Result:** Irrigate about every 10 days, applying roughly 71 mm gross (706 m³ per hectare).

## What the result means

Round the interval down to a whole number of days, and recalculate as ET₀ and Kc change through the season. Rain that soaks in extends the interval.

## Assumptions and limits
- The root zone starts at field capacity after each irrigation.
- No rain, capillary rise or deep percolation between irrigations.
- Uses one soil layer and one ET₀ value; real schedules should be updated with weather data.

## Questions

### Where do I get ET₀?

Many national weather services and irrigation advisory services publish daily reference evapotranspiration. It can also be calculated with the FAO Penman–Monteith equation.

### What is the difference between net and gross depth?

Net depth is what the root zone needs; gross depth is what you must pump, allowing for losses in application.

## Sources
- [FAO Irrigation and Drainage Paper 56, Chapter 8: soil water availability](https://www.fao.org/3/x0490e/x0490e0e.htm)

## Related calculators
- [Soil Water Deficit Calculator](https://calculators.geekmidst.com/calculators/soil-water-deficit-calculator)
- [Drip Irrigation Discharge Calculator](https://calculators.geekmidst.com/calculators/drip-irrigation-discharge-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/irrigation-scheduling-calculator