📖 How To Use
How to Use This Crop Water Need Calculator
This calculator implements the FAO-56 single crop coefficient (Kc) method — the global standard for estimating crop water requirements. Here's how to use it in under a minute:
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Select your crop type
Choose from 30+ crops across field crops, vegetables, fruit trees, and forages. The calculator automatically loads the FAO-56 Kc values for that crop.
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Choose the growth stage
Select Initial, Development, Mid-Season, or Late Season. This determines which Kc value applies — mid-season typically gives peak water demand.
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Enter reference evapotranspiration (ETo)
ETo is the water demand of a reference grass crop. Get it from your local meteorological station, FAO CLIMWAT database, or use a local value (5–7 mm/day for hot dry climates, 2–4 mm/day for temperate zones). You can input in mm/day, mm/week, or inches/day.
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Enter field area and duration
Your field size in hectares, acres, m², or dunams. Duration is the number of days in the growth stage you're planning for.
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Read your results
The calculator outputs daily ETc in mm/day, weekly totals, period totals, and total water volume in both m³ and litres for your field size — ready for tank sizing or pump specification.
Tip: ETc is the crop water need, not the irrigation application. You also need to account for rainfall, irrigation efficiency (typically 70–90% for drip, 60–75% for sprinkler), and soil water reserves. Net irrigation requirement = ETc − effective rainfall.
📐 The Formula
Crop Water Need Formula (FAO-56)
The crop water need is calculated using the single crop coefficient approach from FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998):
ETc = Kc × ETo
ETc = Crop evapotranspiration (mm/day)
Kc = Crop coefficient (dimensionless)
ETo = Reference evapotranspiration (mm/day)
Volume = ETc (mm/day) × Area (m²) × Days ÷ 1,000
Result in cubic metres (m³)
The crop coefficient (Kc) accounts for differences between the reference crop (clipped grass) and the actual crop in terms of crop height, aerodynamic resistance, albedo, and canopy resistance. Kc varies through the crop's life cycle — peaking at mid-season and dropping at senescence.
FAO-56 Kc Values Reference Table
| Crop | Kc ini | Kc mid | Kc end | Season (days) |
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| Wheat | 0.30 | 1.15 | 0.30 | 150–180 |
| Maize / Corn | 0.30 | 1.20 | 0.35 | 125–180 |
| Rice (Paddy) | 1.05 | 1.20 | 0.75 | 150–180 |
| Soybean | 0.40 | 1.15 | 0.50 | 135–150 |
| Cotton | 0.35 | 1.20 | 0.50 | 180–195 |
| Tomato | 0.60 | 1.15 | 0.70 | 135–180 |
| Potato | 0.50 | 1.15 | 0.75 | 105–145 |
| Onion | 0.70 | 1.05 | 0.75 | 150–210 |
| Alfalfa | 0.40 | 1.20 | 1.15 | Perennial |
| Citrus | 0.65 | 0.70 | 0.65 | Perennial |
| Date Palm | 0.90 | 0.95 | 0.95 | Perennial |
| Banana | 0.50 | 1.10 | 1.00 | 300–365 |
Source: FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998), Table 12. Values are for standard agronomic conditions.
Unit Conversions
1 mm/day over 1 ha = 10 m³/day
1 mm/day over 1 acre = 4.047 m³/day
1 m³ = 1,000 litres
1 inch/day = 25.4 mm/day
🌾 When to Use This
Use Cases & Applications
This calculator is used by farmers, agronomists, and irrigation engineers across a wide range of planning scenarios:
Irrigation System Design
When sizing drip lines, sprinkler heads, or furrow irrigation systems, you need peak ETc at mid-season to determine the required flow rate. A wheat field at peak demand in a 6 mm/day ETo climate needs 1.15 × 6 = 6.9 mm/day — that's 69 m³/day per hectare that your system must deliver.
Tank & Reservoir Sizing
Combine this calculator with the Farm Water Storage Calculator to size your on-farm storage tank. If you have 3 days of expected supply interruption, your tank must hold 3 × daily ETc × field area.
Pump Selection
Peak ETc divided by daily irrigation hours gives you the required flow rate. A field needing 69 m³/day running on 6-hour irrigation windows requires a minimum pump rate of 11.5 m³/hour — feed that into the Irrigation Pump Size Calculator.
Seasonal Water Budget Planning
Multiply daily ETc by the full season length to get total water commitment. This helps in pre-season water purchase agreements, borehole yield assessments, and rainwater harvesting system sizing.
Practical note: ETc is theoretical demand under no stress. To get actual irrigation amounts, subtract effective rainfall (rainfall contributing to soil moisture) and adjust for your system's application efficiency. Field-level ETc × (1 / efficiency) = gross irrigation requirement.
❓ FAQ
Frequently Asked Questions
What is crop water need (ETc) and how is it calculated?
Crop water need (ETc) is the amount of water a crop consumes through transpiration and evaporation from the soil surface under optimal growing conditions. It's calculated as ETc = Kc × ETo, where Kc is the crop coefficient (a dimensionless ratio specific to crop type and growth stage) and ETo is the reference evapotranspiration from a standard grass surface.
Where do I get the ETo (reference evapotranspiration) value?
ETo is available from several sources: (1) your nearest meteorological station, which may publish monthly ET data; (2) the FAO CLIMWAT 2.0 database, which covers thousands of locations worldwide; (3) online calculators that apply the Penman-Monteith equation using temperature, humidity, wind, and solar radiation; (4) local agricultural extension departments. Typical values range from 2–3 mm/day in cool humid climates to 7–10 mm/day in hot arid regions like the Middle East or Pakistan.
What is the crop coefficient (Kc) and why does it change with growth stage?
The crop coefficient (Kc) reflects how much a specific crop transpires relative to reference grass, accounting for canopy size, leaf area, and stomatal behavior. In the initial stage, Kc is low because the soil is bare and the canopy is small. It rises through development to peak at mid-season when the canopy is fully covering the ground. At the late season, Kc drops as leaves senesce and water uptake slows. Using the correct Kc for each stage is critical for accurate irrigation scheduling.
Is ETc the same as the irrigation requirement?
No. ETc is the total crop water demand. The net irrigation requirement (NIR) is ETc minus effective rainfall. The gross irrigation requirement (GIR) is NIR divided by the irrigation system efficiency (e.g., divide by 0.85 for 85% efficient drip irrigation). So for ETc = 5 mm/day, rainfall = 1 mm/day, and drip efficiency = 0.90: NIR = 4 mm/day, GIR = 4.44 mm/day applied at the field level.
How much water does one hectare of wheat need per day?
At mid-season (Kc = 1.15) with a typical ETo of 5 mm/day: ETc = 1.15 × 5 = 5.75 mm/day. Over 1 hectare (10,000 m²), this equals 57.5 m³/day or 57,500 litres/day. In a hot arid climate with ETo of 8 mm/day, this rises to 92 m³/day per hectare.
What is the difference between ETc, ETo, and ETa?
ETo (reference ET) is the evapotranspiration from a hypothetical, well-watered reference grass surface — a climate-driven baseline. ETc (crop ET) applies crop-specific coefficients to ETo to get the demand for a specific crop at a specific stage — the potential demand under no stress. ETa (actual ET) is what really happens in the field when water, nutrients, pests, or other stresses reduce growth — ETa ≤ ETc. This calculator computes ETc (the target to meet with irrigation).
How do I use this calculator to size a farm storage tank?
Calculate daily ETc in m³ for your total field area using this tool. Then multiply by the number of days of storage autonomy you need (e.g., if supply is unreliable 3 days at a time, multiply by 3). Add 10–15% as a buffer. That gives your minimum tank volume. Feed that number into our Farm Water Storage Calculator to select tank dimensions.
Which crops have the highest water need per hectare?
High water demand crops include sugarcane (Kc mid ≈ 1.25), rice paddy (Kc mid ≈ 1.20), banana (Kc mid ≈ 1.10), alfalfa (Kc mid ≈ 1.20), and maize at silking (Kc mid ≈ 1.20). In a 6 mm/day ETo environment, these crops can demand 70–75 m³/ha/day — over 25,000 m³/ha for a full season. Efficient irrigation is critical for these crops to be economically viable.