Estimate evaporation fast from temperature, humidity, and wind. Set coefficients to match your site conditions. Export results as CSV or PDF for reports today.
Dalton-style evaporation models relate evaporation to the vapour pressure difference between the water surface and the overlying air. A common empirical form is:
This calculator computes vapour pressures in kPa using the Tetens relation and estimates ea from air temperature and relative humidity. Choose K, a, and b to match your calibration method. In many projects, K is fitted so E is in mm/day.
| Tw (°C) | Ta (°C) | RH (%) | u (m/s) | a | b | K | Area (m²) | Estimated E (mm/day) |
|---|---|---|---|---|---|---|---|---|
| 25 | 25 | 50 | 2 | 1.00 | 0.00 | 1.00 | 10 | Depends on your chosen coefficients |
| 30 | 28 | 40 | 3 | 1.00 | 0.10 | 1.00 | 25 | Higher deficit and wind usually increase E |
| 20 | 22 | 80 | 1 | 1.00 | 0.05 | 1.00 | 5 | High humidity reduces driving deficit |
Dalton-type models are popular because they connect evaporation to a measurable driving force: the moisture gradient between a water surface and the air. With basic weather inputs, teams can estimate loss from ponds, reservoirs, cooling basins, and tanks without complex instrumentation.
The calculator computes saturation vapour pressure at the water temperature and estimates the air’s actual vapour pressure from air temperature and relative humidity. The difference (es − ea) in kPa is the deficit, and larger deficits generally mean stronger evaporation potential.
Wind removes humid boundary-layer air and replaces it with drier air, helping the deficit persist. Dalton formulations capture this with a wind function such as f(u)=a+bu, where u is wind speed. Sites with steady breezes often require a positive b to reflect higher transport.
The intercept a represents baseline exchange in near-calm conditions, while b controls how quickly evaporation responds to wind. The scaling factor K converts the deficit-and-wind product into a depth rate. Because coefficients depend on units, height, and exposure, calibrate them using local pan data or verified observations.
Water temperature commonly spans about 5–35 °C outdoors, while relative humidity often varies from 20–90%. Wind speeds near 2 m height frequently fall between 0–8 m/s, with higher values in exposed terrain. Use nearby stations, onsite loggers, or handheld meters for consistent input sets.
Depth loss becomes actionable when multiplied by surface area. This calculator converts rate into volume loss (liters) and mass loss (kilograms) assuming water density near 1000 kg/m³. For large basins, even a few mm/day can translate into thousands of liters per day.
If the deficit is negative, the air is effectively more humid than the surface condition supports. The model then indicates condensation potential rather than evaporation. This may occur at night, after rainfall, or when cool water is exposed to warm humid air.
Evaporation estimates support water-balance studies, makeup-water planning, and short operational forecasts. Use CSV exports for spreadsheets and time-series tracking, and PDF exports for field documentation. For professional reports, record the coefficient set, measurement heights, and the period represented by each estimate. This improves traceability across teams greatly.
The rate is an equivalent water depth lost per chosen time basis. When K is calibrated, mm/day can be compared across days and sites. Multiply by area to convert into volume loss.
Water surface temperature drives saturation vapour pressure at the interface. If you can only measure one temperature precisely, measure water temperature well, then use reliable air temperature from a nearby station.
Use local calibration if possible. Start with literature or project standards, then fit K (and optionally a, b) so modeled evaporation matches observed pan or flux measurements under similar conditions.
Negative values occur when the computed vapour pressure deficit is negative, meaning the air is effectively “wetter” than the surface condition. That indicates condensation potential rather than net evaporation.
Yes. Wind speed varies with height above ground. If your wind data comes from a different height than your calibration, adjust coefficients accordingly or convert wind speed to a common reference height.
The vapour pressure relationship changes with salinity and fluid properties. For seawater or process fluids, coefficients may need correction. Use specialized correlations if accurate salinity or composition effects are required.
CSV supports spreadsheets, dashboards, and time-series analysis. PDF provides a snapshot of inputs and outputs for field logs. Record coefficient choices and measurement sources to keep reports auditable.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.