Crop Evapotranspiration (ETc) Calculator

About the Crop Evapotranspiration (ETc) Calculator

Crop evapotranspiration (ETc) is the actual water consumption of a specific crop under optimal growing conditions. It is calculated by multiplying the reference evapotranspiration (ET₀) by a crop coefficient (Kc) that accounts for the plant's canopy size, stomatal behavior, and growth stage.

Kc changes as the crop develops: it starts low during seedling establishment, ramps up through vegetative growth, peaks during full canopy and grain fill, and declines as the crop matures and senesces. By tracking ETc day-by-day or week-by-week, irrigators can schedule applications precisely to match crop demand.

This page combines ET₀ and Kc into a daily crop-water-use estimate that can be matched to growth stage and irrigation intervals.

When This Page Helps

ETc matters because it converts weather demand into crop-specific demand. This page makes that step explicit for scheduling and comparison.

How to Use the Inputs

1. Enter the reference evapotranspiration (ET₀) in inches per day.
2. Enter the crop coefficient (Kc) for the current growth stage.
3. Read the daily crop evapotranspiration (ETc) in inches per day.
4. Optionally enter the number of days to see cumulative ETc.
5. Use ETc to update your irrigation water balance.
6. Adjust Kc as the crop advances through growth stages.

Example Calculation

Tips & Best Practices

• Update Kc every 1–2 weeks as the crop progresses.
• Mid-season Kc for corn is about 1.15–1.20; for soybeans 1.10–1.15.
• FAO-56 Table 12 lists Kc values for over 100 crops.
• Mulch or residue cover can reduce Kc by 0.05–0.10 during early growth.
• Stress coefficients (Ks) can be applied to reduce ETc under deficit irrigation.
• Dual crop coefficient methods split Kc into basal (Kcb) and soil evaporation (Ke) components.

Single vs Dual Crop Coefficient

The single Kc method combines plant transpiration and soil evaporation into one coefficient. The dual method separates them into Kcb (basal crop coefficient) and Ke (soil evaporation coefficient). The dual method is more accurate under frequent irrigation or rain but requires daily soil water balance tracking.

Growth Stage Definitions

The initial stage runs from planting to about 10% canopy cover. The development stage spans from 10% to full canopy coverage. Mid-season covers the period of full canopy until senescence begins. Late season extends from senescence onset to harvest.

Using ETc for Deficit Irrigation

Some crops tolerate moderate water stress during specific growth stages without significant yield loss. By tracking ETc and intentionally under-irrigating during tolerant stages, farmers can conserve water while protecting yield during critical stages like flowering and grain fill.

Frequently Asked Questions

• What is Kc and where do I find it?

  Kc is the crop coefficient, a dimensionless multiplier that converts ET₀ to crop-specific ETc. FAO Irrigation and Drainage Paper 56 and many university extension bulletins publish Kc tables by crop and growth stage.

• Does Kc change during the season?

  Yes. Kc typically starts around 0.3–0.5, rises to 1.0–1.2 at mid-season, and drops to 0.5–0.8 at maturity. The four-stage model (initial, development, mid, late) is standard practice.

• Can ETc exceed ET₀?

  Yes. Tall, dense crops like sugarcane or alfalfa can have Kc above 1.0, meaning their actual water use exceeds the grass-reference baseline.

• What if my crop is stressed?

  Under water stress, actual ET is less than ETc. A stress coefficient (Ks, 0–1) is multiplied: Actual ET = ET₀ × Kc × Ks. This calculator assumes no stress (Ks = 1).

• How do I convert ETc to gallons per acre?

  One inch of water over one acre equals 27,154 gallons. Multiply ETc (in) by acreage and by 27,154 for total volume.

• Is ETc the same as irrigation requirement?

  Not exactly. Net irrigation requirement = ETc minus effective rainfall. Gross irrigation also accounts for application efficiency (typically 75–95%).