Irrigation Water Requirement Calculator
Estimate how much water a field needs from crop ET, rainfall, area and system efficiency. Get net and gross depth plus daily water volume with the formula shown.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Estimate the irrigation water a field needs from crop water use (ETc), rainfall, and system efficiency.
Show the formula & steps
How the Irrigation Water Requirement Calculator Works
This calculator estimates how much water a field needs each day. It starts from the crop’s water use (ETc), removes the rain the crop can actually use (effective rainfall), then accounts for losses in your delivery system through an irrigation efficiency factor. Finally it scales the depth over your field area to give a daily water volume.
Enter ETc and rainfall in millimetres per day, the area in hectares, and your system efficiency.
The Formula
Net irrigation depth (mm) = ETc − effective rainfall Gross irrigation depth (mm) = net depth ÷ irrigation efficiency Water volume (m³) = gross depth ÷ 1,000 × area (m²)
The net depth is what the plant needs; the gross depth is what you must pump or release to cover system losses. Because 1 mm of depth over 1 m² is exactly 1 litre, converting depth and area into volume is straightforward.
Worked Example
A 2-hectare field has a crop water use of 6 mm/day, gets 1 mm/day of effective rainfall, and is watered by a 75 percent efficient sprinkler system:
- Net depth = 6 − 1 = 5 mm/day
- Gross depth = 5 ÷ 0.75 = 6.67 mm/day
- Area = 2 ha = 20,000 m²
- Volume = 6.67 ÷ 1,000 × 20,000 = 133.3 m³/day (about 133,300 litres per day)
How Efficiency Changes the Water Bill
For the same 5 mm net depth on 2 hectares, lower efficiency forces more gross water:
| System | Typical efficiency | Gross depth | Volume per day |
|---|---|---|---|
| Drip | 90% | 5.56 mm | 111.1 m³ |
| Sprinkler | 75% | 6.67 mm | 133.3 m³ |
| Furrow | 60% | 8.33 mm | 166.7 m³ |
| Flood | 50% | 10.00 mm | 200.0 m³ |
Getting Accurate Inputs
- Use local ET data. ETc varies daily with weather, crop, and growth stage; pull ET0 and the crop coefficient from a regional source.
- Count only effective rainfall. Heavy storms run off and do not all reach the root zone, so use the portion the crop can store.
- Measure your real efficiency. Worn nozzles, leaks, and wind drift lower efficiency below the rated value.
- Add a leaching fraction in saline soils, where extra water is needed to flush salts below the root zone.
Frequently asked questions
How do I calculate a field's irrigation water requirement?+
Subtract effective rainfall from crop water use (ETc) to get the net depth, divide by the irrigation efficiency to get the gross depth, then multiply by the area. One millimetre of depth over one square metre equals one litre, so volume in cubic metres equals gross depth in mm ÷ 1000 × area in m².
What is the difference between net and gross irrigation requirement?+
Net irrigation requirement is the water the crop actually needs after rainfall. Gross irrigation requirement is the larger amount you must apply at the source because some water is lost to evaporation, runoff, and deep percolation. Dividing net by efficiency gives gross.
What is ETc and where do I get it?+
ETc is crop evapotranspiration, the daily water a specific crop loses through soil evaporation and plant transpiration. It equals a reference ET (ET0) multiplied by a crop coefficient (Kc). Local weather services, the FAO Penman-Monteith method, or extension agencies publish ET0 and Kc values for your region and growth stage.
What irrigation efficiency should I use?+
Typical field efficiencies are about 50 to 60 percent for surface or furrow irrigation, 70 to 80 percent for sprinklers, and 85 to 95 percent for well-managed drip systems. Higher efficiency means less gross water is needed to deliver the same net depth to the crop.
How do I convert depth in millimetres to water volume?+
A depth of 1 mm spread over 1 square metre is exactly 1 litre, or 0.001 cubic metres. So volume in cubic metres equals depth in mm ÷ 1000 × area in square metres. One hectare is 10,000 square metres, so 1 mm over a hectare is 10 cubic metres.