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

Surface Evapotranspiration

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:

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

Rn =LE + H +G

Plant Transpiration

Example of empirical stomatal resistance “stress” factors

Incoming SW rad. stress Vapor press. deficit stress

Air temperature stress Soil moisture stress

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