Irrigation Water Requirement Calculator
Estimate crop water need with ET₀ and crop coefficient, then convert it to litres and pumping hours for your field.
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Please note: This calculator provides a mathematical estimate based on the values entered. Recommended agricultural application rates can vary by crop, soil, product formulation, location and official agricultural guidance. Check the product label and local agricultural recommendations before application.
How much water does your crop actually use? This calculator follows the FAO method – reference evapotranspiration (ET0) × crop coefficient (Kc) – subtracts useful rainfall, allows for losses in your irrigation method, and tells you the water in mm, litres and pumping time.
About the Irrigation Water Requirement Calculator
Crops lose water through their leaves and from the soil surface – together called evapotranspiration. How much they lose depends on the weather (sun, temperature, wind and humidity) and on the crop itself (its size and growth stage). Irrigation is about replacing that water, not more and not less.
The internationally used FAO-56 method splits the problem into two parts:
- ET0 (reference evapotranspiration) captures the weather. It's the water a standard grass surface would use, in mm per day. Many agro-met advisories and state agricultural universities publish it.
- Kc (crop coefficient) captures the crop. A young crop with little leaf cover might be 0.3–0.5; a full canopy at flowering is often 1.0–1.2.
Multiply them and you get ETc, the crop's actual daily water use. Over a number of days, subtract any rain that soaked into the root zone, and you have the net irrigation the crop needs. Because no irrigation method is perfect, you then divide by the efficiency of your system to get the gross amount you must pump.
The calculator converts that depth into litres for your field – remember, 1 mm over 1 m² is exactly 1 litre – and, if you enter your pump's discharge, into pumping hours.
Typical crop coefficients (FAO-56)
| Crop | Initial | Mid-season | Late season |
|---|---|---|---|
| Wheat | 0.3–0.7 | 1.15 | 0.25–0.4 |
| Paddy (rice) | 1.05 | 1.20 | 0.90–0.60 |
| Maize | 0.3 | 1.20 | 0.35–0.6 |
| Cotton | 0.35 | 1.15–1.20 | 0.50–0.70 |
| Sugarcane | 0.40 | 1.25 | 0.75 |
| Groundnut | 0.40 | 1.15 | 0.60 |
| Tomato | 0.60 | 1.15 | 0.70–0.90 |
| Potato | 0.50 | 1.15 | 0.75 |
These are general values for sub-humid climates with moderate wind. Local research may give adjusted figures for your region.
How to use the Irrigation Water Requirement Calculator
- 1 Enter your field area.
- 2 Enter the average daily ET0 for the period, from a local weather advisory.
- 3 Enter the crop coefficient for your crop's current stage (see the table).
- 4 Enter the number of days – usually the days since the last irrigation – and any effective rainfall.
- 5 Enter your irrigation efficiency and, optionally, the pump discharge. Press Calculate.
The formula we use
ETc (mm/day) = Kc × ET₀
Net irrigation (mm) = ETc × Days − Effective rainfall
Gross irrigation (mm) = Net irrigation ÷ Efficiency
Water (litres) = Gross irrigation (mm) × Area (m²)
Pumping time (h) = Water (L) ÷ Pump discharge (L/h)Worked example
Wheat at mid-season on 1 acre, ET0 5 mm/day, Kc 1.15, 10 days since the last irrigation, no rain, flood irrigation at 60% efficiency, pump giving 10 litres per second.
- ETc = 1.15 × 5 = 5.75 mm/day.
- Crop water use over 10 days = 57.5 mm (this is the net requirement, as there was no rain).
- Gross irrigation = 57.5 ÷ 0.60 = 95.8 mm.
- Water = 95.8 mm × 4,046.86 m² = 3,87,824 litres (about 388 m³).
- Pumping time = 3,87,824 ÷ 36,000 L/h = 10.8 hours (about 10 h 46 min).
Things to keep in mind
- Soil holds a limited amount. Sandy soils may not hold a large irrigation – apply smaller amounts more often. Clay soils hold more but take water in slowly.
- Critical stages. Every crop has stages where water stress hurts yield most (for example crown root initiation and flowering in wheat). Prioritise water at those stages.
- Paddy is different. Puddled paddy also loses water through percolation and needs standing water management, so its total requirement is higher than ETc alone.
- Measure your pump. Time how long it takes to fill a drum of known size to get a real discharge figure.
Frequently asked questions
What is ET0?
ET0, or reference evapotranspiration, is the water a well-watered reference grass would lose to the air in a day, in millimetres. It summarises the effect of weather on water use.
What is the crop coefficient (Kc)?
Kc adjusts ET0 for a specific crop and growth stage. Small young crops have a low Kc; crops with full leaf cover have a Kc around 1.0–1.25.
How many litres is 1 mm of irrigation on one acre?
1 mm over one acre (4,046.86 m²) is 4,047 litres. So a 50 mm irrigation is about 2,02,000 litres per acre.
What is effective rainfall?
It is the part of rainfall that soaks into the root zone and is available to the crop. Light showers that evaporate, and heavy rain that runs off or drains below the roots, don't count.
How do I measure pump discharge?
Fill a container of known volume, like a 200 L drum, and time it. Discharge (L/s) = volume ÷ seconds. A 200 L drum filling in 20 seconds means 10 L/s.
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