Evaporation, Transpiration,Sublimation. Processes by which water changes phase- Liquid or solid to gas vapor. Learning Objectives: Evapotranspiration (ET) Learn what conditions are necessary for evaporation to occur Learn what factors control evaporation rates Learn how to measure ET
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Processes by which water changes phase-
Liquid or solid to gas vapor
Net radiation: Rnet is determined by measuring incoming & outgoing short- & long-wave rad. over a surface.
Rnet can – or +
If Rnet > 0 then can be allocated at a surface as follows:
Rnet = (L)(E) + H + G + Ps
L is latent heat of vaporization, E evaporation, H energy flux that heats the air or sensible heat, G is heat of conduction to ground and Ps is energy of photosynthesis.
LE represents energy available for evaporating water
Rnet is the primary source for ET & snow melt.Energy Budget
In a watershed Rnet, (LE) latent heat and sensible heat (H) are of interest.
Sensible heat can be substantial in a watershed, Oasis effect where a well-watered plant community can receive large amounts of sensible heat from the surrounding dry, hot desert.
Advection is movement of warm air to cooler plant-soil-water surfaces.
Convection is the vertical component of sensible-heat transfer.
Volumetric concentration cwv mol m-3
Vapor pressureea, also pH2OkPa
(the partial pressure of H2O vapor)
Relative humidity RH %
=(ea/es)* 100, where es
is saturation vapor pressure
Vapor pressure deficit VPD kPa
=es – ea
1. Soil moisture also important
2. Soil texture: affects soil moisture content and capillary forces
- provides shade- limits insolation (energy and heat)
- reduces windspeed at ground level
- increase vapor pressure through transpiration
The driving force of transpiration is the “vapor pressure gradient.” This is the difference in vapor pressure between the internal spaces in the leaf and the atmosphere around the leaf
VPD * LAI * leaf conductance
VPD Vapor pressure deficit
LAI Leaf area index
Flow of liquid water =
(Yleaf – Ysoil) * K
fine soils with ample soil-moisture storage, warm summers, cool winters, and little change in precipitation throughout the year
Effects of soil type
coarse soils with
limited soil-moisture storage,
warm, dry summers, cool, moist winters.
Model T RH or e Lat Elev Rad. Wind
Penman x x x x x
Priestly-Taylor x x
Jensen-Haise x x x
Blainey-Criddle x x
(mm/yr) cool winters, and little change in precipitation throughout the year
> 70% annual precipitation in the US cool winters, and little change in precipitation throughout the year
In General: ET/P is
~ 1 for dry conditions
ET/P < 1 for humid climates & ET is governed by available energy rather than availability of water
ET affects water yield by affecting antecedent water status of a watershed high ET result in large storage bin to store part of precipitationEvapotranspiration