Calculate How Much Water Your Crops Lose Daily — and Irrigate Smarter
Every day, your crops silently lose water through two natural processes — evaporation from the soil surface and transpiration through plant leaves. Together, these two processes are called evapotranspiration (ET), and understanding them is the single most important step toward efficient, science-based irrigation management.
Irrigate too little, and your crops experience water stress — reducing yield, quality, and profit. Irrigate too much, and you waste water, leach valuable nutrients below the root zone, promote root disease, and drive up energy costs. The solution is to irrigate exactly as much as your crops need — no more, no less.
Our Evapotranspiration (ET) Calculator helps farmers, agronomists, and irrigation managers worldwide calculate reference evapotranspiration (ET₀), crop-specific evapotranspiration (ETc), and net irrigation requirement — using globally recognized scientific methods with support for all measurement units.
🌿 Evapotranspiration (ET) Calculator
Calculate reference evapotranspiration (ET₀) and crop water evapotranspiration (ETc) using FAO Penman-Monteith, Hargreaves-Samani, or Blaney-Criddle methods. Estimate how much water your crops lose daily and plan irrigation accordingly. Supports all global units.
Select a calculation method based on the weather data you have available. FAO Penman-Monteith is the international standard (most accurate). Hargreaves-Samani requires only temperature and solar radiation. Blaney-Criddle needs only temperature and sunshine hours.
Crop ET (ETc) = ET₀ × Kc (Crop Coefficient). The crop coefficient varies by crop type and growth stage. Enter ET₀ from the previous tab or from your weather station data.
Calculate the net irrigation water requirement — how much water you need to apply after accounting for rainfall, soil moisture, and system efficiency.
FAO-56 Crop Coefficients (Kc) and growth stage durations for major crops. Use these values in the ETc calculator above.
| Crop | Kc Initial | Kc Mid | Kc Late | Season (days) |
|---|---|---|---|---|
| Tomato | 0.60 | 1.15 | 0.70 | 120–180 |
| Potato | 0.50 | 1.15 | 0.75 | 105–145 |
| Onion (dry) | 0.50 | 1.00 | 0.75 | 150–210 |
| Wheat | 0.30 | 1.15 | 0.25 | 120–150 |
| Maize / Corn | 0.30 | 1.20 | 0.35 | 125–180 |
| Rice (Paddy) | 1.05 | 1.20 | 0.90 | 90–150 |
| Cotton | 0.35 | 1.20 | 0.50 | 180–195 |
| Sugarcane | 0.40 | 1.25 | 0.75 | 270–365 |
| Soybean | 0.40 | 1.15 | 0.50 | 135–150 |
| Sunflower | 0.35 | 1.10 | 0.35 | 125–130 |
| Grape (Vineyard) | 0.30 | 0.85 | 0.45 | Year-round |
| Banana | 0.50 | 1.10 | 1.00 | 300–365 |
| Citrus (no cover) | 0.65 | 0.60 | 0.65 | Year-round |
| Mango | 1.00 | 1.00 | 1.05 | Year-round |
| Pomegranate | 0.35 | 1.05 | 0.65 | Year-round |
| Strawberry | 0.40 | 0.85 | 0.75 | 60–90 |
| Chili / Capsicum | 0.35 | 1.05 | 0.90 | 120–210 |
| Watermelon | 0.40 | 1.00 | 0.75 | 90–100 |
Blaney-Criddle p Values (Mean Daily % of Annual Daylight Hours) by Latitude:
| Latitude | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0° (Equator) | 0.27 | 0.25 | 0.27 | 0.27 | 0.27 | 0.26 | 0.27 | 0.27 | 0.27 | 0.27 | 0.26 | 0.27 |
| 10° N/S | 0.26 | 0.24 | 0.27 | 0.27 | 0.28 | 0.28 | 0.28 | 0.28 | 0.27 | 0.26 | 0.25 | 0.26 |
| 20° N/S | 0.25 | 0.24 | 0.27 | 0.28 | 0.29 | 0.29 | 0.29 | 0.28 | 0.27 | 0.26 | 0.25 | 0.25 |
| 30° N/S | 0.24 | 0.23 | 0.27 | 0.28 | 0.30 | 0.31 | 0.30 | 0.29 | 0.27 | 0.25 | 0.23 | 0.23 |
| 40° N/S | 0.22 | 0.23 | 0.27 | 0.29 | 0.31 | 0.32 | 0.31 | 0.30 | 0.27 | 0.24 | 0.21 | 0.21 |
| 50° N/S | 0.19 | 0.22 | 0.26 | 0.30 | 0.33 | 0.35 | 0.34 | 0.31 | 0.27 | 0.23 | 0.19 | 0.18 |
ET₀ Classification Guide:
| ET₀ (mm/day) | Classification | Typical Conditions |
|---|---|---|
| 0 – 2 | Very Low | Cool, humid, overcast (winter/high altitude) |
| 2 – 4 | Low | Cool/humid climate, spring/autumn |
| 4 – 6 | Moderate | Warm and moderately humid |
| 6 – 8 | High | Hot and dry conditions, summer |
| > 8 | Very High | Extreme heat, arid/semi-arid regions |
What Is Evapotranspiration (ET)?
Evapotranspiration is the combined loss of water from:
- Evaporation — water that evaporates directly from the soil surface, puddles, and wet plant surfaces into the atmosphere
- Transpiration — water absorbed by plant roots, transported through stems, and released as water vapor through tiny pores (stomata) in the leaves
In practical irrigation management, ET is expressed in millimeters per day (mm/day) — the same unit used for rainfall measurement. When ET is 5 mm/day, it means the crop and soil are losing the equivalent of 5 millimeters of water depth per day from each square meter of land — or 5 liters per square meter per day.
Understanding daily ET values allows farmers to answer the most fundamental irrigation question: How much water should I apply today?
Reference ET (ET₀) vs. Crop ET (ETc) — What Is the Difference?
Reference Evapotranspiration (ET₀)
ET₀ is the evapotranspiration from a standardized reference surface — a hypothetical well-watered grass crop of uniform height under specific conditions. It represents the atmospheric demand for water on a given day and is calculated purely from weather data: temperature, humidity, wind speed, and solar radiation.
ET₀ is the starting point for all crop water requirement calculations. It tells you how “thirsty” the atmosphere is on a given day, regardless of what crop you are growing.
Typical ET₀ values by climate:
- Cool, humid conditions (winter/overcast): 1–3 mm/day
- Temperate, spring/autumn: 3–5 mm/day
- Warm and sunny summer: 5–7 mm/day
- Hot, arid, windy conditions: 7–10+ mm/day
Crop Evapotranspiration (ETc)
ETc is the actual water requirement of a specific crop at a specific growth stage. It is calculated by multiplying ET₀ by a Crop Coefficient (Kc):
ETc = ET₀ × Kc
The Kc value captures the difference between the reference grass surface and your actual crop — accounting for crop height, leaf area, root depth, and stomatal behavior. Kc values change throughout the growing season, typically starting low in the initial stage, peaking during mid-season, and declining at harvest.
The Three ET Calculation Methods — Which One Should You Use?
Our calculator supports three internationally recognized methods. The right choice depends on what weather data you have available.
Method 1 — FAO Penman-Monteith (Recommended)
The FAO Penman-Monteith method is the global standard for ET₀ calculation, adopted by the Food and Agriculture Organization of the United Nations in their landmark FAO Irrigation and Drainage Paper No. 56. It is the most accurate method available and is used by research institutions, irrigation authorities, and weather networks worldwide.
Data required:
- Maximum and minimum daily temperature
- Maximum and minimum relative humidity
- Wind speed (measured at 2 meters height)
- Solar radiation (or sunshine hours with latitude and day of year)
- Site elevation and latitude
Best for: Farmers near weather stations, agrometeorological networks, or those using smart irrigation controllers with sensor data.
Method 2 — Hargreaves-Samani
The Hargreaves-Samani method was developed for situations where only limited weather data is available. It requires only temperature data (maximum and minimum) plus your geographic location (latitude and day of year) to estimate extraterrestrial radiation.
Data required:
- Maximum and minimum daily temperature
- Latitude and day of year
Best for: Farmers in areas without weather stations, or those using simple thermometers. Results are slightly less accurate than Penman-Monteith but reliable enough for practical irrigation planning.
Method 3 — Blaney-Criddle
The Blaney-Criddle method is the simplest of the three. Originally developed in the 1950s for irrigation planning in the American West, it requires only mean temperature and the percentage of annual daylight hours for the month.
Data required:
- Maximum and minimum daily temperature
- Mean daily sunshine hours
- Monthly daylight fraction (p value — found in the reference table)
Best for: Quick estimates when minimal data is available. Less accurate than the other two methods but useful for rough planning or verification.
How to Use the ET Calculator — Step by Step
Step 1 — Calculate Reference ET₀ (Tab 1)
Select your calculation method based on your available data. Enter temperature in Celsius, Fahrenheit, or Kelvin — whichever your thermometer or weather app displays. The calculator converts automatically.
For the FAO Penman-Monteith method, also enter relative humidity, wind speed (in m/s, km/h, mph, or knots), solar radiation (in MJ/m²/day, W/m², kWh/m²/day, or cal/cm²/day), elevation (meters or feet), latitude, and the day of the year.
Click Calculate ET₀ to see your reference evapotranspiration in mm/day, inch/day, mm/week, mm/month, m³/ha/day, and L/m²/day — all displayed simultaneously.
Step 2 — Calculate Crop ET (Tab 2)
Enter the ET₀ value from Step 1 (or from your local weather station). Select your crop type — the calculator automatically fills in the FAO-56 Kc value for the selected growth stage. You can override this with a locally calibrated Kc value if available.
Enter your field area and effective rainfall. The calculator shows your crop water demand (ETc), net irrigation requirement after subtracting rainfall, and daily and weekly water volumes for your field.
Step 3 — Calculate Irrigation Deficit (Tab 3)
This tab performs a complete water balance calculation. Enter your ETc, field area, effective rainfall, soil moisture contribution, irrigation system efficiency, and leaching requirement for saline soils.
The calculator outputs the net and gross irrigation depth required, total volume per irrigation event, weekly volume, and monthly volume — in liters, cubic meters, US gallons, or acre-feet.
Step 4 — Reference Tables (Tab 4)
The reference tab contains FAO-56 Kc values for 18 major crops across all growth stages, Blaney-Criddle p-values by latitude for all 12 months, and an ET₀ classification guide showing what each ET range means agronomically.
Understanding Crop Coefficients (Kc) by Growth Stage
The Kc value is not constant — it changes as your crop grows and its canopy develops. FAO-56 defines four growth stages:
Initial Stage (Kc ini): From planting to approximately 10% ground cover. The crop is small, and most ET comes from soil evaporation rather than plant transpiration. Kc values are low (0.3–0.6 for most crops).
Crop Development Stage: Ground cover expands from 10% to approximately 70–80%. Kc increases progressively as the canopy develops and transpiration becomes the dominant component of ET.
Mid-Season Stage (Kc mid): Full canopy cover through the flowering and fruiting period. This is when Kc is at its maximum and crop water demand is highest. Values range from 0.85 (grape) to 1.25 (sugarcane). This is the critical period when water stress has the greatest impact on yield.
Late Season Stage (Kc late): From the beginning of maturity (yellowing, leaf drop) to harvest. Kc declines as the crop senesces and water demand decreases. Deliberately reducing irrigation in this stage can improve fruit quality, sugar content, and shelf life for certain crops.
Evapotranspiration by Crop — What the Numbers Mean
Understanding typical ETc ranges helps you cross-check your calculations and spot errors:
| Crop | Typical Peak ETc (mm/day) | Season Length |
|---|---|---|
| Tomato | 5–8 | 120–180 days |
| Wheat | 4–7 | 120–150 days |
| Maize / Corn | 5–8 | 125–180 days |
| Rice (Paddy) | 6–10 | 90–150 days |
| Cotton | 5–8 | 180–195 days |
| Sugarcane | 5–9 | 270–365 days |
| Potato | 5–7 | 105–145 days |
| Grape (Vineyard) | 3–6 | Year-round |
| Banana | 5–8 | 300–365 days |
| Citrus | 3–5 | Year-round |
| Soybean | 4–7 | 135–150 days |
| Strawberry | 3–5 | 60–90 days |
If your calculated ETc falls significantly outside these ranges, check your ET₀ input and Kc value.
Effective Rainfall — Not All Rain Counts
In the ET deficit calculation, we subtract effective rainfall from ETc — but not all rainfall is effective. Effective rainfall is the portion of total rainfall that is actually stored in the soil root zone and available for crop use.
Rainfall is not effective when:
- It falls faster than the soil can absorb it, causing runoff
- It exceeds the soil’s field capacity, draining below the root zone
- The rain event is too small to penetrate dry surface soil and most evaporates before being absorbed
As a general rule, for irrigation planning in drip systems, effective rainfall is approximately 70–80% of total rainfall for moderate events (5–30 mm). Very light showers (< 5 mm) are often 0% effective in arid conditions, while large storms (> 50 mm) may be only 50–60% effective due to runoff and deep percolation.
Irrigation System Efficiency and Why It Matters
The ET calculation tells you how much water your crop needs. But the amount of water you need to pump is always more than what the crop actually uses — because no irrigation system delivers water with 100% efficiency.
Drip Irrigation: 85–95% efficiency — water is delivered directly to the root zone with minimal evaporation or runoff. This is why drip irrigation is recommended for high ET environments.
Sprinkler Irrigation: 70–85% efficiency — some water is lost to wind drift and evaporation during application, especially in hot and windy conditions.
Furrow / Border Irrigation: 60–75% efficiency — water must flow along the surface to reach distant parts of the field, resulting in uneven distribution and runoff losses.
Flood / Basin Irrigation: 50–65% efficiency — the least efficient method. Deep percolation losses are significant, especially in sandy or cracked clay soils.
The gross irrigation requirement = net irrigation requirement ÷ system efficiency. For example, if your crop needs 50 mm of water and you are using a sprinkler system at 80% efficiency, you need to apply 50 ÷ 0.80 = 62.5 mm.
Leaching Requirement for Saline Soils
In regions where irrigation water contains dissolved salts — common in arid and semi-arid areas — an additional amount of water must be applied beyond the crop’s ET requirement to flush salts below the root zone. This is called the leaching requirement (LR).
Without adequate leaching, salt accumulates in the root zone over successive irrigations, eventually reaching toxic levels that damage crops and reduce yields dramatically. Crops vary significantly in their salt tolerance (measured as electrical conductivity of soil water, ECsw).
The leaching requirement is expressed as a percentage of ETc:
- 0% — normal soil with non-saline water
- 5% — slightly saline conditions (EC irrigation water < 1 dS/m)
- 10% — moderately saline (EC 1–3 dS/m)
- 15–20% — highly saline (EC > 3 dS/m)
Our calculator adds the leaching requirement to the gross irrigation depth automatically when selected.
Practical ET Management Tips for Farmers
Monitor weather daily: ET₀ changes every day with temperature, humidity, wind, and cloud cover. Using a fixed irrigation schedule that does not account for daily weather variation will always result in either over- or under-irrigation on most days.
Use the crop growth stage correctly: Many farmers make the mistake of using the peak Kc value throughout the entire season. Using the correct Kc for each stage — especially the lower values during initial and late stages — can reduce total seasonal water use by 15–25%.
Account for mulching: Plastic or organic mulch on the soil surface can reduce soil evaporation by 20–40%, effectively lowering the Kc and ETc during the initial and late stages when soil evaporation is the dominant component. If you use mulch, reduce your Kc ini value by 0.10–0.15.
Calibrate with soil moisture sensors: ET-based irrigation scheduling gives you a theoretical water requirement. Always validate against actual soil moisture measurements using tensiometers, capacitance sensors, or gypsum blocks. This helps you adjust for local soil variability and microclimate effects.
Adjust for shade nets and greenhouses: If your crop is grown under shade nets or in a greenhouse, actual ET is lower than field ET₀ would suggest. A shade factor of 0.5–0.8 is typically applied to ET₀ depending on shade percentage.
Plan fertigation around ET cycles: Nutrients applied through the drip system (fertigation) are most efficiently absorbed when applied in the early part of an irrigation cycle — after the root zone is wetted but while flow continues. Align your fertigation schedule with your ET-based irrigation timing.
ET Calculator for Different Climates Around the World
One of the key strengths of this calculator is its ability to serve farmers in any climate zone. Here is a general guide to ET₀ ranges by climate type:
Tropical Humid (Amazon Basin, Southeast Asia, West Africa): ET₀ typically 3–5 mm/day year-round. High rainfall often meets crop demand, but irrigation is needed during dry spells and for high-value crops in the dry season.
Semi-Arid and Arid (Sahel, Rajasthan, Middle East, Central Valley California): ET₀ of 7–12 mm/day in summer. Irrigation is essential for all crops. Water management is critical. Drip and subsurface drip irrigation are standard.
Mediterranean (Spain, Italy, Greece, South Africa, Chile, Australia): ET₀ of 5–9 mm/day in summer, 1–3 mm/day in winter. Summer irrigation is essential. Grapes, olives, citrus, and vegetables are major irrigated crops.
Temperate Continental (Central Europe, Northern China, US Midwest): ET₀ of 4–7 mm/day in summer. Supplemental irrigation during dry spells is common. Winter wheat and summer corn are major crops.
Cool and Humid (Northern Europe, Canada, New Zealand): ET₀ of 2–4 mm/day in summer. Rainfall often meets demand, but irrigation boosts yield reliability for vegetables, berries, and potatoes.
Frequently Asked Questions (FAQ)
Q. What is the difference between ET₀, ETc, and ETa? ET₀ (reference ET) is calculated from weather data for a standard grass surface. ETc (crop ET) is the theoretical water requirement of a specific crop under optimal growing conditions, calculated as ET₀ × Kc. ETa (actual ET) is the real evapotranspiration that actually occurs in the field, which may be less than ETc if the crop is water-stressed or partially irrigated.
Q. Where can I get weather data for the Penman-Monteith calculation? Local weather stations operated by meteorological departments, agricultural universities, or government agencies are the best source. Online platforms such as NASA POWER (power.larc.nasa.gov), the FAO CLIMWAT database, and national agrometeorological networks provide historical and current weather data for locations worldwide. Many modern irrigation controllers also include built-in weather sensors.
Q. My ET₀ result seems too high or too low. What should I check? First, verify that your temperature values are correct and in the right unit. For Penman-Monteith, ensure solar radiation is in MJ/m²/day (not W/m² — divide W/m² by 11.6 to convert, or use the unit selector). Check that wind speed is realistic (typical values are 1–5 m/s). For Hargreaves-Samani, ensure latitude is entered correctly. Check that the day of year matches the calculation date.
Q. What Kc value should I use if my crop is not in the list? Look up the FAO-56 publication (freely available on the FAO website) for your specific crop. If no published Kc is available for your crop, use values from a similar crop with comparable canopy structure, height, and leaf area. You can also conduct a field water balance study to derive local Kc values.
Q. Can I use this calculator for greenhouse or screenhouse crops? Yes, but with an adjustment. ET under protected cultivation is typically 40–70% of open-field ET₀ because the structure blocks wind, reduces solar radiation, and moderates temperature extremes. Apply a correction factor of 0.5–0.8 to ET₀ before multiplying by Kc. For hydroponic or substrate cultivation, ET models are less accurate and direct measurement of drainage and plant water uptake is recommended.
Q. How does ET change during a drought? During an extended drought without irrigation, actual ET (ETa) falls below ETc as the soil moisture depletes and plants begin to close their stomata to conserve water. This is the onset of crop water stress. The degree of stress is measured by the stress coefficient (Ks), which ranges from 0 (complete stress, no transpiration) to 1 (no stress). Our calculator assumes full water availability (Ks = 1), which means results represent the irrigation needed to avoid any stress.
Q. What is a good ET₀ value for scheduling drip irrigation? There is no single “good” value — ET₀ is what it is for your location and weather conditions. What matters is using the correct ET₀ with the correct Kc for your crop and growth stage to calculate ETc. For drip irrigation, the goal is to replace ET daily or every few days, maintaining soil moisture within the optimal range for your crop.
Q. How does wind affect ET? Wind accelerates evapotranspiration by removing the humid air layer that forms just above the soil and leaf surfaces, replacing it with drier ambient air. In the Penman-Monteith equation, wind speed has a significant direct effect on ET₀. A windy day (4 m/s) can have ET₀ that is 30–50% higher than a calm day (1 m/s) with identical temperature and humidity. This is why ET₀ in coastal and open-plain areas tends to be higher than in sheltered valleys.
Conclusion
Efficient irrigation is not about watering on a fixed schedule — it is about replacing exactly what your crops lose to evapotranspiration every day. Irrigating based on ET data rather than intuition or calendar schedules consistently produces higher yields, better fruit quality, lower water bills, and reduced disease pressure from over-irrigation.
Our Evapotranspiration (ET) Calculator brings scientific-grade irrigation planning to every farmer — from smallholders with a basic thermometer to commercial growers with full weather station data. Choose your method, enter your data, and get instant, reliable crop water requirement calculations in every unit used around the world.
Whether you grow wheat in India, grapes in Spain, tomatoes in Mexico, sugarcane in Brazil, or citrus in South Africa — this tool gives you the science behind every irrigation decision.
💧 Know your ET. Irrigate with precision. Grow with confidence.
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- Water Tank / Farm Pond Capacity Calculator — Storage planning made easy
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- Pump Power (HP) Calculator — Find the right pump for your field
References: FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998) | Hargreaves & Samani (1985) | Blaney & Criddle (1950) | ASCE Standardized ET Manual
This calculator is for educational and planning purposes. For large-scale or commercial irrigation design, consult a certified irrigation engineer or agronomist.