AgriAuditor water planning tool
Irrigation water calculator
How much water does the crop need each day, and how much must you apply once system losses are counted? Enter five inputs and the calculator returns daily and seasonal crop water requirement in millimetres, cubic metres and litres, with the method and its limits stated on the page.
What to enter, and in which units
Every input is a stated assumption. None of them is measured from your field, so change them to match your own records before you use the result.
- Target crop from the crop list
- Sets the crop factor (Kc) proxy and the season length in days. Values come from the AgriAuditor crop catalogue, with built-in presets as the fallback.
- Irrigated area hectares (1–10,000)
- Scales the per-hectare demand into a field total. 1 ha = 2.471 acres.
- Average daily temperature °C (5–45)
- The mean daily air temperature over the period you are planning for, not the peak.
- Vapour pressure deficit kPa (0.2–4.0)
- How dry the air is. Roughly 0.2–0.8 humid and cool, 0.8–1.8 mild, above 2.0 hot and arid.
- Irrigation system method application efficiency %
- Drip System (90% Efficiency) · Micro-Sprinkler (75% Efficiency) · Flood / Surface Irrigation (50% Efficiency)
Irrigation water estimator
Estimate daily and seasonal water demand with a simplified temperature and VPD screening model plus irrigation-system efficiency.
| Parameter | Value |
|---|---|
| Target Crop | Corn |
| Irrigated Area | 10 Hectares |
| Daily Temperature | 25 °C |
| Vapor Pressure Deficit (VPD) | 1.5 kPa |
| Irrigation Method | Drip System (90% Efficiency) |
| Daily Water Demand | 595 m³ (595,000 Liters) |
| Atmospheric Demand Proxy (ET0) | 5.1 mm/day |
| Crop Factor Proxy (Kc) | 1.05 |
| Seasonal Duration | 120 Days |
| Estimated Pumping Energy | 14,280 kWh / Season |
| Method Limitation | Screening estimate from temperature and VPD. Not the FAO-56 Penman-Monteith calculation. |
| Total Seasonal Water Demand | 71,400 m³ |
A worked example, step by step
10 hectares of a crop with a 1.05 crop factor and a 120-day season, at 25°C average daily temperature and 1.5 kPa VPD, irrigated by drip at 90% efficiency.
| Step | Arithmetic | Result |
|---|---|---|
| Atmospheric demand (ET₀ proxy) | (25 × 0.12) + (1.5 × 1.4) | 5.1 mm/day |
| Crop water need | 5.1 × 1.05 (Kc) | 5.35 mm/day |
| Water to apply, after system losses | 5.35 ÷ 0.9 | 5.95 mm/day |
| Per hectare | 5.95 mm × 10 (1 mm/ha = 10 m³) | 59.5 m³/ha/day |
| Whole field, daily | 59.5 × 10 ha | 595 m³/day |
| Whole field, season | 595 × 120 days | 71,400 m³/season |
Same crop under flood irrigation at 50% efficiency: 10.71 mm/day, or 128,520 m³ across the season. The crop need did not change; the losses did.
Method and limitations
This is a screening estimate built from temperature and vapour pressure deficit. It is not the FAO-56 Penman-Monteith reference calculation, which also requires solar radiation, wind speed, humidity and elevation.
The crop factor is a proxy derived from the crop profile's water requirement and growing period, clamped to 0.4–1.5. It is not a stage-by-stage Kc curve, and it is not cultivar-specific.
The estimate ignores rainfall, soil water storage, runoff, deep percolation, salinity leaching requirements and peak-week demand. Those are what actually size a pump, a licence or a reservoir. Treat the output as an order of magnitude and confirm it against local extension guidance and your own meter readings.
Use the result in a wider decision
Water demand is one screening check. Run a coordinate-level field audit for mapped soil, climate context and crop fit, check the season length with the GDD and frost window calculator, and price the water against the crop ROI estimator.
To track applied water against demand through a real season, see crop monitoring, which logs irrigation events alongside weather risk.
Read the AgriAuditor methodology →Questions about irrigation water requirement
How does this irrigation water calculator estimate water demand?
It estimates atmospheric demand (an ET₀ proxy in mm/day) from average daily temperature and vapour pressure deficit, multiplies by a crop factor (Kc) proxy to get crop water need, divides by the irrigation system efficiency to get the water you must apply, then converts at 1 mm over 1 hectare = 10 cubic metres.
Is this the FAO-56 Penman-Monteith calculation?
No. FAO-56 Penman-Monteith requires solar radiation, wind speed, humidity and elevation. This is a screening estimate from temperature and VPD only. Use it to size an order of magnitude, not to replace a local water-balance plan.
What units does the calculator use?
Area in hectares, temperature in degrees Celsius (Fahrenheit is shown alongside), vapour pressure deficit in kilopascals, and results in millimetres per day, cubic metres and litres. 1 hectare = 2.471 acres and 1 cubic metre = 264.2 US gallons.
Why does irrigation system efficiency change the answer so much?
Efficiency is a divisor, not a subtraction. The same crop water need of 5.4 mm/day requires about 6.0 mm/day applied under 90% drip and about 10.7 mm/day under 50% flood irrigation, because the losses scale with everything you apply.
Where does the crop factor (Kc) come from?
It is derived from the crop profile water requirement and growing period in the AgriAuditor catalogue, clamped to a 0.4–1.5 range. It is a reference proxy for screening, not a stage-by-stage Kc curve for your cultivar.
Can I use the seasonal total to size a borehole or reservoir?
Only as a first screen. The seasonal figure assumes the entered temperature and VPD hold for the whole season and ignores rainfall, soil water storage, runoff, deep percolation and peak-week demand, which is what actually sizes pumps and storage.