ET-Based Watering Calculator

How to use the ET-based watering calculator

Enter reference evapotranspiration ET₀ in inches per day (from local weather or extension data), crop coefficient Kc for your plants, irrigated area in square feet, and irrigation efficiency as a percentage.

The result is estimated daily water in gallons, adjusted upward when your system loses water to evaporation, runoff, or uneven coverage.

ET-based watering formulas

Crop evapotranspiration ETc = ET₀ × Kc (inches per day). Daily gallons = ETc × Area (sq ft) × 0.623. Adjusted daily gallons = Daily gallons ÷ (Efficiency ÷ 100).

ET₀ reflects climate demand; Kc scales it for your crop stage and canopy. Efficiency accounts for sprinkler overlap, wind drift, and drip losses.

• ETc = ET₀ × Kc
• Daily gallons = ETc × Area × 0.623
• Adjusted gallons = Daily gallons ÷ (Efficiency / 100)
• Weekly gallons ≈ Daily adjusted gallons × 7

Worked example

ET₀ = 0.20 in/day, Kc = 0.70 for mid-season vegetables, 500 sq ft irrigated area, 80% efficiency.

ETc = 0.20 × 0.70 = 0.14 in/day. Daily gallons = 0.14 × 500 × 0.623 = 43.6 gallons. Adjusted = 43.6 ÷ 0.80 ≈ 54.5 gallons per day.

Finding ET₀ and Kc values

Many counties publish daily or weekly ET₀ from weather stations — search for "reference ET" or CIMIS/CoAgMet-style data in your region. Agricultural extension tables list Kc by crop and growth stage.

Lawns often use Kc around 0.6–0.8 in peak summer; young seedlings may be lower; mature fruiting vegetables may approach 0.8–1.0.

Applying ET results in the garden

Convert daily gallons into controller runtimes using your system GPM or GPH. Skip or reduce irrigation after measurable rainfall. ET is a starting point — confirm with soil moisture and plant appearance.

• Drip systems: 70–90% efficiency is common with mulch.
• Sprinklers: 50–75% efficiency depending on wind and overlap.
• Pair with drip runtime or drip line runtime for session length.
• Recalculate weekly as ET₀ changes through the season.