Evaporation Rate Calculator
Evaporation from open water surfaces is driven by the energy available to vaporize water (primarily solar radiation) and the atmosphere's capacity to carry water vapor (vapor pressure deficit and wind speed). The simplified Penman approach estimates daily evaporation as the weighted combination of a radiation term and an aerodynamic term. This calculator uses daily average temperature (degrees C), relative humidity (%), wind speed (m/s), and solar radiation (MJ/m2/day) to estimate open water evaporation in mm/day. This is suitable for lake, reservoir, and pond water balance estimates.
Simplified Penman evaporation formula
e_s (kPa) = 0.6108 * exp(17.27 * T / (T + 237.3)) [saturation vapor pressure]
e_a (kPa) = e_s * RH / 100 [actual vapor pressure]
VPD (kPa) = e_s - e_a [vapor pressure deficit]
Radiation term (mm/day) = 0.0353 * Rs [Rs in MJ/m2/day]
Aero term (mm/day) = 0.26 * VPD * (1 + 0.54 * u2) [u2 = wind m/s at 2m]
E (mm/day) = 0.5 * Radiation term + 0.5 * Aero term [simplified weighting]
This simplified form uses equal weighting (50/50) for radiation and aerodynamic terms. The full Penman formula uses the psychrometric constant and slope of the saturation vapor pressure curve to derive theoretically correct weights. For engineering applications, use FAO-56 Penman-Monteith with measured radiation and humidity data.
Evaporation in context
- Lake Mead in Nevada/Arizona, the largest US reservoir, loses approximately 3 to 4 feet of water to evaporation each year, equivalent to over 800,000 acre-feet annually (Bureau of Reclamation).
- The US Class A evaporation pan, maintained by NOAA at hundreds of weather stations, is the standard reference measurement. Lake evaporation is approximately 70 percent of pan evaporation.
- Climate warming is expected to increase evaporation rates by 5 to 15 percent across much of the western US by 2050 (Bureau of Reclamation SECURE Water Act reports).
- Floating covers and reservoirs can reduce evaporation by 85 to 90 percent and are used in water-scarce areas.
Frequently asked questions
What is the Penman method for estimating evaporation?
The Penman (1948) method is the foundational physics-based approach for estimating open water evaporation. It combines a radiation term (energy available for evaporation) with an aerodynamic term (vapor pressure deficit and wind speed). The FAO Penman-Monteith equation is the internationally accepted standard for reference evapotranspiration.
What is the difference between evaporation and evapotranspiration?
Evaporation is water loss from open water surfaces such as lakes, ponds, and reservoirs. Evapotranspiration (ET) combines evaporation from soil and transpiration from plants. Reference ET (ETo) is defined for a standard short grass or alfalfa crop and is used in irrigation scheduling. This calculator estimates lake/open-water evaporation.
What drives the rate of evaporation?
The four main drivers are: solar radiation (energy input), air temperature (affects vapor pressure), relative humidity (the saturation deficit is the driving force for evaporation), and wind speed (removes humid air near the surface, maintaining the vapor gradient). Hot, dry, windy conditions produce the highest evaporation rates.
What are typical lake evaporation rates in the US?
Pan evaporation data from NOAA shows that annual lake evaporation ranges from about 20 inches in the northern US (Oregon, Wisconsin) to over 80 inches in the desert Southwest (Arizona, Nevada). Daily rates range from near zero in winter to 0.5 to 0.8 inches per day in summer in arid regions.
Why does this calculator use a simplified formula?
The full Penman equation requires many meteorological inputs (net radiation components, psychrometric constant, vapor pressure from dewpoint, aerodynamic resistance) that are complex to compute. This simplified version uses the key drivers in a form accessible to non-specialists. For engineering design, use FAO-56 Penman-Monteith with full station data.
Official sources
- NOAA: US Evaporation Data (NOAA).
- USGS: USGS Evaporation and the Water Cycle.
- FAO: FAO-56 Penman-Monteith Reference ET.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.