How to use this calculator
- Select Penman-Monteith for complete daily weather observations.
- Choose Hargreaves when only temperature and location data exist.
- Enter the date, latitude, elevation, temperatures, humidity, wind, and radiation.
- Select a crop coefficient method and provide Kc values.
- Enter field area, irrigation efficiency, flow rate, and system details.
- Enable multi-day mode to paste or upload daily CSV observations.
- Review warnings, assumptions, data quality, charts, and irrigation runtime.
Example data
| Input | Example | Purpose |
|---|---|---|
| Method | FAO-56 Penman-Monteith | Complete weather calculation |
| Temperature | 34°C maximum, 22°C minimum | Thermal and vapour pressure terms |
| Humidity | 78% maximum, 36% minimum | Actual vapour pressure |
| Wind | 2.4 m/s at 2 metres | Aerodynamic term |
| Solar radiation | 21.5 MJ/m²/day | Net radiation estimate |
| Crop coefficient | 1.20 | Converts ET₀ into crop ET |
| Area | 1 hectare | Converts depth into water volume |
| Efficiency | 85% | Calculates gross irrigation requirement |
Understanding evapotranspiration estimates
Evapotranspiration combines water lost through evaporation and plant transpiration. Reference ET describes atmospheric demand over a standard grass surface. Crop coefficients then adapt that demand for particular plants.
Penman-Monteith balances radiation, temperature, humidity, and wind effects. It usually performs best with complete, quality-controlled daily observations. Missing variables can create large uncertainty during extreme weather.
Hargreaves uses temperature range and extraterrestrial solar radiation. It helps when weather stations lack humidity or wind records. Local calibration improves results across unusual climates and elevations.
Crop evapotranspiration equals reference ET multiplied by crop coefficients. Coefficients change as canopy cover and growth stages develop. Stress and soil evaporation can be separated using dual coefficients.
Irrigation planning must also consider rainfall and application efficiency. Net irrigation replaces the crop water deficit after effective rainfall. Gross irrigation includes field losses and optional leaching demand.
Water depth becomes volume after multiplying by irrigated area. One millimetre over one square metre equals one litre. Flow rate then converts required volume into operating time.
Soil water capacity helps schedule irrigation before harmful depletion. Root depth determines how much stored water remains accessible. Allowable depletion reflects crop sensitivity and management objectives.
Daily estimates are more useful than isolated monthly averages. Multi-day tables reveal changing demand, rainfall, and irrigation requirements. Charts make trends easier to compare across management periods.
Input quality strongly controls the reliability of calculated results. Measured radiation and humidity usually improve confidence substantially. Estimated variables should remain visible beside every reported result.
This tool supports farms, gardens, nurseries, and landscape systems. It provides planning estimates rather than guaranteed crop recommendations. Verify schedules with local observations, soil checks, and experience.
Frequently asked questions
What is reference evapotranspiration?
Reference evapotranspiration, ET₀, represents atmospheric water demand from a standardized reference surface under defined conditions.
How is crop evapotranspiration calculated?
The basic method multiplies ET₀ by a crop coefficient. The dual method separates plant transpiration and soil evaporation.
Which method should I choose?
Use Penman-Monteith with complete data. Use Hargreaves, pan evaporation, or known ET₀ when observations are limited.
Why is latitude required?
Latitude and date determine solar geometry, daylight duration, and extraterrestrial radiation used by several equations.
Can the calculator estimate missing radiation?
Yes. It can estimate radiation using sunshine duration, cloud cover, or the daily temperature range.
What does irrigation efficiency mean?
Efficiency is the fraction of applied water stored in the root zone. Lower efficiency increases gross water demand.
What is effective rainfall?
Effective rainfall is the portion of rainfall available to meet crop demand after runoff, evaporation, and deep drainage.
How does the calculator find runtime?
It divides required irrigation volume by the entered system flow rate and also reports sprinkler-based runtime.
What is the water-stress coefficient?
Ks reduces crop transpiration when root-zone water becomes limited. A value near one indicates little stress.
Can I calculate several days together?
Yes. Enable multi-day mode, paste CSV rows, and calculate a chart plus detailed daily table.
Is this a substitute for local agronomic advice?
No. Use the result with field observations, soil moisture checks, local coefficients, and professional recommendations.
Important limitation
This calculator provides planning estimates. Local crop coefficients, microclimates, sensor errors, salinity, rainfall distribution, and irrigation uniformity can change actual water needs.