Evapotranspiration Calculator
Calculate actual crop evapotranspiration (ETc) in inches/day, mm/day, m³/ha, and gallons/acre using FAO-56 Penman-Monteith equations.
Calculate Crop Evapotranspiration (ETc)
Enter reference ETo, crop coefficient (Kc), water stress coefficient (Ks), unit, and field acreage.
Calculation Results
Calculated using FAO Irrigation Paper 56 Penman-Monteith standard: \text{ETc} = \text{ETo} \times \text{Kc} \times \text{Ks}
Quick Summary
The Evapotranspiration Calculator evaluates daily crop water loss ($\text{ETc} = \text{ETo} \times \text{Kc} \times \text{Ks}$) in inches/day, mm/day, m³/ha, and gallons/acre for precision irrigation management.
Formula Explanation
\text{Crop Evapotranspiration (ETc in/day)} = \text{Reference ETo} \times \text{Kc} \times \text{Ks}
\text{Metric ETc (mm/day)} = \text{ETc (in/day)} \times 25.4
\text{Daily Water Loss (m³/ha)} = \text{ETc (mm/day)} \times 10How It Works
The Evapotranspiration Calculator applies the FAO-56 Penman-Monteith method. By multiplying reference grass evapotranspiration (ETo) by crop-specific stage coefficients (Kc) and soil moisture stress factors (Ks), it quantifies daily water loss from plant stomatal transpiration and soil evaporation.
Step-by-Step Worked Example
Practical Problem: Weather station records ETo of 0.25 inches/day. A well-watered corn field (Ks = 1.0) is at peak silking stage (Kc = 1.15) on 100 acres. Calculate daily ETc in inches, mm, and total daily gallons.
- Step 1: Calculate daily ETc: $0.25 \times 1.15 \times 1.0 = \mathbf{0.2875\text{ in/day}}$.
- Step 2: Convert to mm/day: $0.2875 \times 25.4 = \mathbf{7.30\text{ mm/day}}$.
- Step 3: Convert to m³/ha: $7.30 \times 10 = \mathbf{73.0\text{ m³/ha/day}}$.
- Step 4: Gallons per acre per day: $0.2875 \times 27,154 = \mathbf{7,807\text{ gallons/acre/day}}$.
- Step 5: Total daily field volume (100 ac): $7,807 \times 100 = \mathbf{780,678\text{ total gallons/day}}$.
Real-World Calculation Examples
Scenario 1: Peak Summer Corn (ETo 0.25")
Parameters: ETo 0.25", Kc 1.15, Ks 1.0
Result: 0.29 in/day ETc (7.3 mm/day, 7,807 gal/acre/day).
Scenario 2: Mid-Season Soybeans (ETo 0.22")
Parameters: ETo 0.22", Kc 1.10, Ks 1.0
Result: 0.24 in/day ETc (6.1 mm/day, 6,571 gal/acre/day).
Scenario 3: Moderate Drought Stressed Corn
Parameters: ETo 0.25", Kc 1.15, Ks 0.70 (30% stress reduction)
Result: 0.20 in/day ETc (Stomatal closure reduces ETc).
Scenario 4: Arid Desert Citrus Orchard
Parameters: ETo 0.35", Kc 0.75, Ks 1.0
Result: 0.26 in/day ETc (6.7 mm/day, 7,128 gal/acre/day).
Key Benefits of Using This Calculator
FAO-56 Dual Unit Precision
Calculates ETc in both US customary (in/day, gal/acre) and metric (mm/day, m³/ha) units.
Soil Stress Factor (Ks)
Integrates soil water stress coefficient (Ks) to account for reduced transpiration during dry spells.
Daily Pumping Requirement
Converts daily ETc depth directly into required daily well pumping gallons.
100% Free & Client-Side
Executes locally in your browser with zero latency or web server transmission.
Frequently Asked Questions (FAQ)
What is Evapotranspiration (ET)?
Evapotranspiration is the total water lost to the atmosphere through soil evaporation plus plant stomatal transpiration.
What is Reference Evapotranspiration (ETo)?
ETo is the water loss rate from a well-watered 4-inch tall reference grass surface, calculated from weather data (temperature, solar radiation, humidity, wind speed).
What is Crop Evapotranspiration (ETc)?
ETc is the actual water loss rate of a specific crop at a specific growth stage ($\text{ETc} = \text{ETo} \times \text{Kc}$).
What is the soil water stress coefficient (Ks)?
Ks reduces ETc when soil water content drops below the Readily Available Water (RAW) threshold, causing plant stomata to close ($\text{Ks} < 1.0$).
How does solar radiation affect daily ETo?
Solar radiation provides the thermal energy required to vaporize liquid water from leaf stomata, driving 60% to 80% of daily ETo.
How do I convert mm/day of ETc to inches/day?
Divide mm/day by 25.4 ($\text{Inches/day} = \text{mm/day} / 25.4$). For example: $7.3\text{ mm/day} / 25.4 = 0.287\text{ in/day}$.
What is 1 mm/day of ETc in m³ per hectare?
1 mm of water depth over 1 hectare (10,000 m²) equals exactly 10 cubic meters ($10\text{ m}^3\text{/ha}$).
What is peak daily ETc for corn in the Corn Belt?
Peak daily ETc for corn during July silking ranges from 0.30 to 0.35 inches per day (7.6 to 8.9 mm/day).
How does wind speed increase evapotranspiration?
Wind sweeps away the humid boundary layer of air surrounding leaves, accelerating moisture evaporation into drier ambient air.
Why is ASCE-EWRI Penman-Monteith preferred over Hargreaves?
Penman-Monteith incorporates full energy balance physics (radiation, humidity, wind, temp), providing higher accuracy than temp-only Hargreaves equations.