Glossary

Plain-language definitions of the terms used across this site. Each is a structured DefinedTerm for machine readability.

Energy Budget Method: An evaporation estimate based on accounting for all energy entering and leaving a water body — net radiation, heat stored in the water, and heat exchanged with the air — and attributing the remaining energy to the latent heat of evaporation.; /how-to-calculate-evaporation
Evaporation: The process by which liquid water molecules with enough kinetic energy escape from a water surface into unsaturated air, becoming water vapor. From open water bodies it is driven mainly by the vapor-pressure deficit between the surface and the air.; /what-is-evaporation /methods-to-reduce-evaporation
Evapotranspiration (ET): The combined loss of water to the atmosphere from evaporation (off soil and water surfaces) and transpiration (from plants). It is the standard measure of total water loss from vegetated land surfaces.; /what-is-evaporation /how-to-calculate-evaporation
Fetch: The unobstructed distance that wind travels across a water surface. A longer fetch lets wind sweep away more moist air and build waves, generally increasing wind-driven evaporation over large open water bodies.; /what-is-evaporation /methods-to-reduce-evaporation
Floating Cover: A physical barrier that floats on a water surface to block sunlight and air contact, reducing evaporation. Types include modular tiles or balls, continuous geomembrane sheets, and suspended or floating-solar systems.; /methods-to-reduce-evaporation /methods/floating-modular-covers /methods/shade-balls
Hargreaves-Samani Method: A temperature-only method that estimates reference evapotranspiration from maximum and minimum air temperature plus extraterrestrial radiation (derived from latitude and date), without needing humidity or wind data.; /how-to-calculate-evaporation
Latent Heat of Vaporization: The amount of energy required to convert liquid water into vapor at constant temperature — roughly 2,260 kilojoules per kilogram near 100 °C, and somewhat higher at typical ambient temperatures. This energy is absorbed from the surroundings as water evaporates.; /what-is-evaporation /how-to-calculate-evaporation
Mass-Transfer (Aerodynamic) Method: An evaporation estimate based on wind speed and the vapor-pressure deficit between the water surface and the air, using an empirically calibrated transfer coefficient. It treats evaporation as wind-driven removal of water vapor from the surface.; /how-to-calculate-evaporation
Monolayer (chemical cover): An ultra-thin, often single-molecule-thick film of a chemical such as cetyl or stearyl alcohol spread over a water surface to slow evaporation. Commercial products include WaterSavr.; /methods-to-reduce-evaporation /methods/chemical-monolayers
Pan Evaporation & Pan Coefficient: Pan evaporation is the daily water loss measured from a standardized open pan (such as the US Class A pan). The pan coefficient is a scaling factor (often around 0.7) applied to that reading to estimate evaporation from a larger natural water body.; /how-to-calculate-evaporation
Penman-Monteith Equation: A physically based equation that combines an energy balance with an aerodynamic (vapor-transfer) term to estimate evapotranspiration. The FAO-56 form (Allen et al. 1998) is the international reference standard for reference-crop ET.; /how-to-calculate-evaporation /methods-to-reduce-evaporation
Priestley-Taylor Method: A radiation-driven simplification of the Penman approach that estimates evaporation from available energy using an empirical multiplier (commonly about 1.26), dropping the explicit wind/aerodynamic term.; /how-to-calculate-evaporation
Relative Humidity: The amount of water vapor in the air expressed as a percentage of the maximum the air could hold at that temperature. Lower relative humidity means drier air and a larger vapor-pressure deficit, which increases evaporation.; /what-is-evaporation /how-to-calculate-evaporation
Transpiration: The release of water vapor from plants, mainly through tiny pores (stomata) in their leaves. It is biologically controlled water loss from vegetation, distinct from direct evaporation off an open water surface.; /what-is-evaporation
Vapor-Pressure Deficit (VPD): The difference between the saturation vapor pressure at the water surface and the actual vapor pressure of the overlying air. A larger deficit means the air is drier relative to the surface, so evaporation proceeds faster; it is the primary driving force behind evaporation.; /what-is-evaporation /how-to-calculate-evaporation