Glossary

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

Advection: The horizontal transport of heat or dry air by wind — for example, warm dry air blowing off surrounding land onto a water body. It adds energy that radiation-only methods miss.; /calculate/priestley-taylor /calculate/mass-transfer /glossary/fetch
Aerodynamic Resistance: A measure of how strongly the air resists the transfer of water vapour and heat away from a surface, set mainly by wind speed and surface roughness. Lower resistance (more wind) means faster evaporation.; /calculate/penman-monteith /calculate/mass-transfer /glossary/fetch
Albedo: The fraction of incoming solar radiation reflected by a surface. Open water has a low albedo (about 0.06), so it absorbs most incoming sunlight — one reason open-water net radiation, and thus evaporation, differs from that of brighter vegetated or soil surfaces.; /calculate/penman-monteith /glossary/net-radiation /what-is-evaporation
Bowen Ratio: The ratio of sensible-heat flux to latent-heat flux at a surface. Measuring it lets the energy-budget method split available energy between heating the air and evaporating water.; /calculate/energy-budget /glossary/sensible-heat-flux /glossary/latent-heat-flux
Eddy Covariance: A micrometeorological technique that measures evaporation directly by correlating high-frequency fluctuations in vertical wind speed and humidity. It provides a benchmark against which simpler estimation methods are judged.; /how-to-calculate-evaporation /research/2007-tanny-direct-measurement-small-reservoir-evaporation
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.; /calculate/energy-budget /how-to-calculate-evaporation /glossary/bowen-ratio
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
Extraterrestrial Radiation: The solar radiation arriving at the top of the atmosphere, determined purely by latitude and day of year. It is the radiation input used by temperature-only methods such as Hargreaves-Samani.; /calculate/hargreaves-samani /glossary/net-radiation
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.; /calculate/hargreaves-samani /how-to-calculate-evaporation /glossary/reference-evapotranspiration /glossary/extraterrestrial-radiation
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 /calculate/energy-budget /glossary/latent-heat-flux
Latent-Heat Flux: The energy carried away from a surface as water evaporates — evaporation expressed in energy units. Dividing it by the latent heat of vaporisation converts it to a depth of water lost.; /calculate/energy-budget /glossary/latent-heat-of-vaporization /glossary/sensible-heat-flux
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.; /calculate/mass-transfer /how-to-calculate-evaporation /glossary/aerodynamic-resistance
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
Net Radiation: The balance of radiation at a surface — incoming solar and longwave minus reflected solar and emitted longwave. It is the primary energy source for evaporation in radiation-driven methods.; /calculate/energy-budget /calculate/priestley-taylor /glossary/albedo
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.; /calculate/pan-evaporation /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.; /calculate/penman-monteith /how-to-calculate-evaporation /methods-to-reduce-evaporation /glossary/psychrometric-constant
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.; /calculate/priestley-taylor /how-to-calculate-evaporation /glossary/advection
Psychrometric Constant: A coefficient (gamma) relating water-vapour partial pressure to air temperature, used to weight the energy and aerodynamic terms in the Penman and Penman-Monteith equations.; /calculate/penman-monteith /calculate/priestley-taylor /glossary/penman-monteith-equation
Reference Evapotranspiration: The evapotranspiration from a standardised, well-watered grass surface — the reference used by FAO-56. It is not the same as open-water evaporation and must be adapted before it represents a reservoir.; /calculate/penman-monteith /calculate/hargreaves-samani /glossary/evapotranspiration
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
Saturation Vapour Pressure: The vapour pressure at which air is fully saturated with water vapour at a given temperature; it rises steeply as temperature increases.; /glossary/vapor-pressure-deficit /calculate/mass-transfer /what-is-evaporation
Sensible-Heat Flux: The transfer of heat between a surface and the air driven by their temperature difference — energy that warms the air rather than evaporating water. In the energy budget it competes with the latent-heat term for available energy.; /calculate/energy-budget /glossary/latent-heat-flux /glossary/bowen-ratio
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 /calculate/mass-transfer /glossary/saturation-vapour-pressure