8 Evaporation
165
8.3 Remote Sensing of Land Evaporation: Applications and
Modelling Approaches
8.3.1 General
Land surface temperature is controlled by the relative magnitude of heat flux densities at the land- atmosphere interface (Eq. 8.4). It is therefore related to the latent
heat flux, even though this relationship is complex in a general case. Actual evaporation can be related to the heat balance using a general form ofEq. 8.1 (Menenti,
1984):
LE = Pacp[e* (z) - e(z)] + sarah (Rn + G,;) + S,Par,h G"
r(rav +r,·v) + Sarah +s,PaCpr,h
where:
Sa is the slope of the saturated soil vapour pressure curve in air;
Ss in soil;
e' (z) is saturated and e is actual vapour pressure at height z;
rah.v is aerodynamic resistance to heat and vapour transfer in air;
rs.h.v is resistance to heat and vapour transfer between the liquid water- moist air
interface and the physical boundary of the evaporating system (e.g. leaf or soil
surface);
y is psychrometric constant;
GE is soil heat flux at liquid water - moist air interface.
This equation describes the influence of: available energy (i.e. Rn-GE), vapour
pressure deficit and the various flow resistances in soil and air on evaporation. The
same latent heat flux density LE may be observed, therefore, for different (Rn-GE)values, given different combinations of the remaining variables such as vapour
pressure and flow resistances.
A further consequence is that LE is not a one-value function of surface temperature. This can be easily shown by re-writing the previous equation as (Jackson
et aI., 1988):
(r, +re) (Rn +G)-~(e' -e)
TO-Ta= Pacp
r
l+~+~
r re
(K)
(8.8)
where the notation has been simplified by indicating as re resistances in air and as
rj resistances to transport in the evaporating system. This equation describes concisely how radiative, boundary layer and surface (rj) conditions control surface
temperature. The internal resistance rj is higher for drier surfaces, but changes of
surface temperature are related in a more complex way to rj and to LE. For large rj
-values, for example, (To-Ta) does not depend on rj anymore and increases with Rn
165
8.3 Remote Sensing of Land Evaporation: Applications and
Modelling Approaches
8.3.1 General
Land surface temperature is controlled by the relative magnitude of heat flux densities at the land- atmosphere interface (Eq. 8.4). It is therefore related to the latent
heat flux, even though this relationship is complex in a general case. Actual evaporation can be related to the heat balance using a general form ofEq. 8.1 (Menenti,
1984):
LE = Pacp[e* (z) - e(z)] + sarah (Rn + G,;) + S,Par,h G"
r(rav +r,·v) + Sarah +s,PaCpr,h
where:
Sa is the slope of the saturated soil vapour pressure curve in air;
Ss in soil;
e' (z) is saturated and e is actual vapour pressure at height z;
rah.v is aerodynamic resistance to heat and vapour transfer in air;
rs.h.v is resistance to heat and vapour transfer between the liquid water- moist air
interface and the physical boundary of the evaporating system (e.g. leaf or soil
surface);
y is psychrometric constant;
GE is soil heat flux at liquid water - moist air interface.
This equation describes the influence of: available energy (i.e. Rn-GE), vapour
pressure deficit and the various flow resistances in soil and air on evaporation. The
same latent heat flux density LE may be observed, therefore, for different (Rn-GE)values, given different combinations of the remaining variables such as vapour
pressure and flow resistances.
A further consequence is that LE is not a one-value function of surface temperature. This can be easily shown by re-writing the previous equation as (Jackson
et aI., 1988):
(r, +re) (Rn +G)-~(e' -e)
TO-Ta= Pacp
r
l+~+~
r re
(K)
(8.8)
where the notation has been simplified by indicating as re resistances in air and as
rj resistances to transport in the evaporating system. This equation describes concisely how radiative, boundary layer and surface (rj) conditions control surface
temperature. The internal resistance rj is higher for drier surfaces, but changes of
surface temperature are related in a more complex way to rj and to LE. For large rj
-values, for example, (To-Ta) does not depend on rj anymore and increases with Rn