📖 How To Use
How to Use the Irrigation Water Requirement Calculator
Get accurate irrigation requirements for any field in under a minute:
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Enter your field area
Select hectares, acres, m², or square feet — whichever matches your land records. One hectare = 10,000 m² = 2.47 acres.
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Enter reference evapotranspiration (ET₀)
ET₀ is a climate index in mm/day. Obtain it from your local agro-meteorology station, FAO CLIMWAT database, or weather apps that report ETo. Typical values: arid zones 6–10 mm/day, temperate 3–5 mm/day, cool humid 1–3 mm/day.
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Select your crop and growth stage
The calculator automatically loads the FAO-56 crop coefficient (Kc) for your crop and stage. Kc adjusts ET₀ to actual crop water use. Mid-season values are the highest; initial and late-season values are lower.
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Enter effective rainfall and system efficiency
Effective rainfall (mm/day) offsets irrigation need. System efficiency accounts for distribution losses — drip: 85–95%, sprinkler: 70–80%, furrow/surface: 40–60%. Default 75% is a safe general estimate.
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Enter season length and calculate
Set the number of days in the current irrigation season. The calculator outputs daily, weekly, and total seasonal water requirements in multiple units.
Tip: For annual crops like wheat or maize, run the calculator separately for each growth stage (initial, development, mid, late) using the Kc for that stage and the number of days in that stage. Sum the seasonal totals for a full-season figure.
📐 The Formula
Irrigation Water Requirement Formula (FAO-56)
This calculator follows the FAO-56 Penman-Monteith crop water demand framework:
ETc = Kc × ET₀ (net crop water demand, mm/day)
IRn = ETc − P_eff (net irrigation requirement, mm/day)
IRg = IRn ÷ (Ea / 100) (gross irrigation requirement, mm/day)
Volume (m³/day) = IRg × Area (ha) × 10
Where: Kc = crop coefficient, ET₀ = reference ETo (mm/day), P_eff = effective rainfall (mm/day), Ea = irrigation efficiency (%).
Multiply gross depth (mm) by field area and the factor 10 to convert mm × ha → m³ (since 1 mm over 1 ha = 10 m³).
FAO-56 Crop Coefficients (Kc) Reference
| Crop | Kc Initial | Kc Mid-Season | Kc Late |
|---|
| Wheat | 0.30 | 1.15 | 0.40 |
| Rice (paddy) | 1.05 | 1.20 | 0.90 |
| Maize / Corn | 0.30 | 1.20 | 0.60 |
| Cotton | 0.35 | 1.20 | 0.60 |
| Sugarcane | 0.40 | 1.25 | 0.75 |
| Tomato | 0.45 | 1.15 | 0.80 |
| Potato | 0.45 | 1.15 | 0.75 |
| Soybean | 0.40 | 1.15 | 0.50 |
| Alfalfa (full cover) | 0.40 | 1.20 | 1.15 |
| Citrus (70% canopy) | 0.65 | 0.65 | 0.65 |
| Grape / Vineyard | 0.30 | 0.85 | 0.45 |
Unit Conversions Used
1 ha = 10,000 m² | 1 acre = 4,046.86 m²
1 mm depth over 1 ha = 10 m³ = 10,000 litres
1 m³ = 264.17 US gallons = 1,000 litres
1 acre-inch = 102.79 m³
❓ FAQ
Frequently Asked Questions
How do I calculate irrigation water requirements for my crop?
The standard method is the FAO-56 approach: multiply your reference evapotranspiration (ET₀) by your crop's coefficient (Kc) to get crop water demand (ETc). Subtract effective rainfall to get net irrigation need, then divide by system efficiency to get the gross amount to apply. This calculator automates all those steps.
Where do I get the ET₀ value for my area?
The best sources are: (1) your national or regional meteorological service — many publish monthly ETo tables by district; (2) the FAO CLIMWAT 2.0 database, which covers thousands of weather stations worldwide; (3) weather apps or APIs that include ETo (e.g., Agromonitoring, OpenET, or NASA POWER). Typical ETo ranges: cool humid regions 1–3 mm/day; temperate 3–5 mm/day; semi-arid 5–7 mm/day; hot arid deserts 7–12 mm/day.
What is a crop coefficient (Kc) and why does it matter?
Kc scales reference ET₀ (measured for a standardised grass surface) to the actual water demand of your specific crop and growth stage. A young seedling might have Kc 0.3, while the same crop at full canopy cover at mid-season might reach Kc 1.2. Using the wrong Kc — or ignoring growth stage — can cause 30–50% errors in irrigation planning.
How do I estimate effective rainfall for my irrigation calculation?
Effective rainfall is the portion of total rainfall that actually stays in the root zone and benefits the crop — runoff and deep percolation are excluded. A simple estimate: for rainfall under 5 mm/day, treat 80% as effective; for 5–10 mm/day, treat 70% as effective; above 10 mm/day, cap effective rainfall at 6–8 mm/day. USDA's method or FAO CROPWAT can give more precise figures if you have monthly rainfall data.
What irrigation efficiency should I enter?
Efficiency depends on your irrigation method. Drip or micro-irrigation: 85–95%. Sprinkler / overhead: 70–80%. Border or basin flooding: 55–70%. Furrow irrigation: 40–65%. If you're unsure, use 75% — it's a conservative, widely used default for mixed and surface systems. Higher efficiency means less gross water needed for the same crop benefit.
How many litres per hectare per day does an irrigated crop typically need?
For a mid-season crop in a semi-arid region with ET₀ of 6 mm/day and Kc of 1.15, ETc = 6.9 mm/day. At 80% efficiency (drip), gross requirement = 8.6 mm/day = 86,000 litres/ha/day (86 m³/ha/day). In a cool humid region at ET₀ = 3 mm/day and Kc 0.9, it might be as low as 33,750 L/ha/day (after accounting for 30% effective rainfall).
What size water storage tank do I need for drip irrigation on 1 hectare?
Calculate your peak-season daily requirement with this tool, then size the tank for 1–3 days of storage buffer. For a 1 ha tomato crop in a hot semi-arid climate (ETc 7 mm/day, drip at 90% efficiency, zero rainfall), that's ~78 m³/day gross. A 2-day buffer requires a 156,000-litre (156 m³) tank. Use our Drip Irrigation Tank Size Calculator and Farm Water Storage Calculator for the full sizing workflow.
Can I use this calculator for greenhouse or protected cultivation irrigation?
Yes, but with adjustments. Inside a greenhouse, ET₀ is typically 40–60% of outdoor values because radiation and wind are reduced. Use an ET₀ of 2–4 mm/day for standard polycarbonate greenhouses. Effective rainfall is zero (set it to 0). Efficiency for drip in a controlled environment is typically 90–95%. For hydroponics, the FAO Kc approach does not apply — use direct substrate moisture monitoring instead.