Session 3 : Estimation of real evapotranspiration
320
L’objectif de cette étude est de tester la performance de la méthode sur un site semi-aride
de l’Arizona. Des comparaisons entre les flux d’énergie mesurés et estimés par
télédétection sont présentées. Finalement, le principe d’une approche nouvelle et robuste
utilisant la télédétection est esquissé.
INTRODUCTION
Recently, increased emphasis has been placed on understanding the interaction between regional
climate and the hydrological cycle in arid and semi-arid regions (Kustas et al., 1991; Goutorbe et
al., 1993). Accurate partitioning of available energy into sensible and latent heat flux is crucial to
the understanding of surface-atmosphere interaction. However, this is very difficult in arid and
semi-arid regions because neither the soil nor the vegetation totally dominates the exchange of
water and heat with the atmosphere. The relative contributions to total sensible and latent heat
flux from the soil and plant components may vary throughout the day and throughout the season.
Thermal infrared remotely sensed surface are increasingly being used in operational models to
evaluate the spatial variation in the energy balance components. While this approach has been
found to be successful over surfaces with near full vegetation cover, its performance has been
questioned over sparsely vegetated surface. The problem has been that over partial cover
conditions, the assumption that consists of assimilating aerodynamic surface temperature to
remotely sensed surface temperature is not valid. Over such surfaces, the difference between
radiative and aerodynamic temperatures can reach 10 to 15
o
C (Chehbouni et al., 1996a).
The objective of this analysis is to investigate the extend to which remotely sensed data in the
visible, near-infrared, and thermal infrared in conjunction with ancillary meteorological data and
one dimensional energy balance model for surface fluxes estimation in arid and semi-arid area.
Data collected during Monsoon’90 experiment have been used. The advantage and limitations of
this approach are investigated. Finally, a principle of new and robust approach of using remotely
sensed data will be outlined.
MODELLING
Available energy
Net radiation (R n ) , which represents the balance of short and long wave radiation reaching and
leaving the surface can be expressed as :
R
R
T
Tr
n
s
s a
a
= −
+
−
(
)
(
)
1
4
4
α
ε ε σ
σ
where Rs is the incoming short-wave radiation, σ is the Stephan-Boltzmann constant (Wm -2
K -4 ), and ε s is the surface emissivity; ε a is the sky emissivity defined as: ε a = 1.24(e a /T a ) 1/7 ,
where e a and T a are air vapour pressure and air temperature respectively. T r is radiative surface
temperature and α is the surface albedo which was derived from red and NIR surface reflectance
as :
α = 0.526 red + 0.474 NIR
320
L’objectif de cette étude est de tester la performance de la méthode sur un site semi-aride
de l’Arizona. Des comparaisons entre les flux d’énergie mesurés et estimés par
télédétection sont présentées. Finalement, le principe d’une approche nouvelle et robuste
utilisant la télédétection est esquissé.
INTRODUCTION
Recently, increased emphasis has been placed on understanding the interaction between regional
climate and the hydrological cycle in arid and semi-arid regions (Kustas et al., 1991; Goutorbe et
al., 1993). Accurate partitioning of available energy into sensible and latent heat flux is crucial to
the understanding of surface-atmosphere interaction. However, this is very difficult in arid and
semi-arid regions because neither the soil nor the vegetation totally dominates the exchange of
water and heat with the atmosphere. The relative contributions to total sensible and latent heat
flux from the soil and plant components may vary throughout the day and throughout the season.
Thermal infrared remotely sensed surface are increasingly being used in operational models to
evaluate the spatial variation in the energy balance components. While this approach has been
found to be successful over surfaces with near full vegetation cover, its performance has been
questioned over sparsely vegetated surface. The problem has been that over partial cover
conditions, the assumption that consists of assimilating aerodynamic surface temperature to
remotely sensed surface temperature is not valid. Over such surfaces, the difference between
radiative and aerodynamic temperatures can reach 10 to 15
o
C (Chehbouni et al., 1996a).
The objective of this analysis is to investigate the extend to which remotely sensed data in the
visible, near-infrared, and thermal infrared in conjunction with ancillary meteorological data and
one dimensional energy balance model for surface fluxes estimation in arid and semi-arid area.
Data collected during Monsoon’90 experiment have been used. The advantage and limitations of
this approach are investigated. Finally, a principle of new and robust approach of using remotely
sensed data will be outlined.
MODELLING
Available energy
Net radiation (R n ) , which represents the balance of short and long wave radiation reaching and
leaving the surface can be expressed as :
R
R
T
Tr
n
s
s a
a
= −
+
−
(
)
(
)
1
4
4
α
ε ε σ
σ
where Rs is the incoming short-wave radiation, σ is the Stephan-Boltzmann constant (Wm -2
K -4 ), and ε s is the surface emissivity; ε a is the sky emissivity defined as: ε a = 1.24(e a /T a ) 1/7 ,
where e a and T a are air vapour pressure and air temperature respectively. T r is radiative surface
temperature and α is the surface albedo which was derived from red and NIR surface reflectance
as :
α = 0.526 red + 0.474 NIR