89
Thermal Radiation and Energy Closure Assessment
and challenge the interpretation of some of the EC measurements. In particular, we
investigated characteristic eddy sizes and intermittency during extreme advective
events.
5.2 SURFACE ENERGY BALANCE
5.2.1 eneRgy Balance equation
The SEB at a surface is most recognized in its simplified form as a linear function
expressed as
R n – G – H – LE = 0,
(5.1)
where R n is the net radiation (incoming short- and longwave minus outgoing shortand longwave radiation) partitioned into the soil heat flux (G), sensible heat flux
(H), and latent heat flux (LE; all units are in watts per square meter) that the energy
released or absorbed during a phase change from liquid to vapor during evaporation
from soil and/or vegetation surfaces. Equation 5.1 does not account for photosyn5.1 does not account for photosyn.1 does not account for photosynthetic activity and heat storage for above-ground vegetation, as it is assumed for this
study to have been negligible relative to the turbulent fluxes of heat and water vapor
(H and LE, respectively).
For hydrologic studies, Equation 5.1 provides a means of quantifying the most
difficult component of the SEB, that is, LE, for the estimation of ET. The simplified
SEB model affords the ability to partition the available energy (R n – G) at a surface
into the turbulent fluxes of H and LE. Additionally, when using the EC technique to
measure direct H and LE fluxes, the ratio of the sum of the turbulent fluxes to the
available energy allows us to have a means of assessing the quality of the measurements of turbulent fluxes via the energy closure approach that will be presented later.
Quantifying the SEB for any surface requires sound instrumentation and measurement techniques that produce the most reliable and accurate estimates of the
energy balance (Equation 5.1). In this study, we focused on quantifying and under5.1). In this study, we focused on quantifying and under.1). In this study, we focused on quantifying and understanding the SEB for an irrigated cotton field in a semiarid environment because of
the unique case where irrigated (wet) surfaces are surrounded by vast dry surfaces
and thus represent an ideal condition to study the effects of advected warm dry air
moving over a wet surface and imparting additional energy in the form of a saturation deficit that enhances ET in addition to the available energy fluxes. This can
result in substantial increases of ET, making simple ET model estimates generally
fail to account for all factors.
Remote sensing applications to the canopy or surface temperature would provide
a direct incorporation of these temperatures into energy balance models to estimate
ET. Canopy temperatures can be placed directly into simpler forms of the energy
balance to estimate evaporation as
LE R G C
T T
r
n
p
c
a
a
= − −
−
ρ
(
) ,
(5.2)
Thermal Radiation and Energy Closure Assessment
and challenge the interpretation of some of the EC measurements. In particular, we
investigated characteristic eddy sizes and intermittency during extreme advective
events.
5.2 SURFACE ENERGY BALANCE
5.2.1 eneRgy Balance equation
The SEB at a surface is most recognized in its simplified form as a linear function
expressed as
R n – G – H – LE = 0,
(5.1)
where R n is the net radiation (incoming short- and longwave minus outgoing shortand longwave radiation) partitioned into the soil heat flux (G), sensible heat flux
(H), and latent heat flux (LE; all units are in watts per square meter) that the energy
released or absorbed during a phase change from liquid to vapor during evaporation
from soil and/or vegetation surfaces. Equation 5.1 does not account for photosyn5.1 does not account for photosyn.1 does not account for photosynthetic activity and heat storage for above-ground vegetation, as it is assumed for this
study to have been negligible relative to the turbulent fluxes of heat and water vapor
(H and LE, respectively).
For hydrologic studies, Equation 5.1 provides a means of quantifying the most
difficult component of the SEB, that is, LE, for the estimation of ET. The simplified
SEB model affords the ability to partition the available energy (R n – G) at a surface
into the turbulent fluxes of H and LE. Additionally, when using the EC technique to
measure direct H and LE fluxes, the ratio of the sum of the turbulent fluxes to the
available energy allows us to have a means of assessing the quality of the measurements of turbulent fluxes via the energy closure approach that will be presented later.
Quantifying the SEB for any surface requires sound instrumentation and measurement techniques that produce the most reliable and accurate estimates of the
energy balance (Equation 5.1). In this study, we focused on quantifying and under5.1). In this study, we focused on quantifying and under.1). In this study, we focused on quantifying and understanding the SEB for an irrigated cotton field in a semiarid environment because of
the unique case where irrigated (wet) surfaces are surrounded by vast dry surfaces
and thus represent an ideal condition to study the effects of advected warm dry air
moving over a wet surface and imparting additional energy in the form of a saturation deficit that enhances ET in addition to the available energy fluxes. This can
result in substantial increases of ET, making simple ET model estimates generally
fail to account for all factors.
Remote sensing applications to the canopy or surface temperature would provide
a direct incorporation of these temperatures into energy balance models to estimate
ET. Canopy temperatures can be placed directly into simpler forms of the energy
balance to estimate evaporation as
LE R G C
T T
r
n
p
c
a
a
= − −
−
ρ
(
) ,
(5.2)