Surface Evapotranspiration
Basic definitions:
Evapotranspiration: all processes by which water in liquid phase at or near
the earth’s surface becomes atmospheric water vapor
 “Evaporation” usually describes direct vaporization of water from open
water surfaces and bare soil
 “Transpiration” is evaporation from within leaves of vegetation
 Together these fluxes referred to as evapotranspiration (ET)
Notes:
 Estimating ET is important because (e.g.) : 1) long-term water balance
(need P and E estimates) determines available water; 2) most food supply
grown using irrigated agriculture (efficient irrigation requires knowledge of
transpiration by crops); 3) ET rate during interstorm periods controls soil
moisture evolution (impacts subsequent flooding); 4) ET couples the
surface water an energy budgets
 Direct measurement of actual evapotranspiration is much more difficult
(and expensive) than precip. or streamflow
Closest readily available (“easy”) measurement network are evaporation
pans (only measure open water [potential] evaporation)
 Therefore need to develop models for estimating evapotranspiration from
more readily available observations
Pan evaporation
climatology -- only
gives “potential”
(open water surface)
evaporation rate not
actual evapotranspir.
rate
Surface Energy Budget & Evapotranspiration
Surface energy budget/balance (SEB):
During interstorm periods net radiation is the primary forcing of the
surface; The surface energy balance can be written as:
Rn  LE  H  G
Where the net radiation (Rn) is partitioned into:
 LE = latent heat flux (energy associates with evapotranspiration)
= Lv * E [J/kg * kg/m2/s = W/m2]
 H = sensible heat flux (conduction of heat into air due to difference in
temperature between surface and overlying air
 G = ground heat flux (conduction of heat into ground due to difference in
temperature between surface and deeper soil)
Notes:
 Rn is mostly determined by the atmosphere (except for surface albedo
and outgoing longwave)
 Surface fluxes (LE , H, G) depend largely on the state (temperature &
moisture) of the surface; i.e. how much energy from net radiation goes into
latent heat vs. other fluxes depends on how moist the surface is.
 Due to high latent heat of vaporization of water, energy will
preferentially go into evap. if water is not limiting (latent heat flux will
correspond to “potential evap.” rate and rest will be partitioned into
sensible/ground heat fluxes)
 So discussion of evapotranspiration usually involves other fluxes
 E appears in both surface water AND energy budget
Plant Transpiration
Example of empirical stomatal resistance “stress” factors
Incoming SW rad. stress
Vapor press. deficit stress
Air temperature stress
Soil moisture stress
rsmin
rs
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rc 
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