453
Remote Sensing and Modeling of Global Evapotranspiration
In addition, Mu et al. (2007b) found that VPD alone can capture interannual variability of the full water stress from both the atmosphere and soil for almost all of
China and the conterminous United States, although it may fail to capture the full seasonal water stress in dry regions experiencing strong summer monsoons.
19.3.4 Soil evaPoRation
To calculate soil evaporation, the potential evaporation (λE SOIL_POT ) is first calculated
using the P–M method (Equation 19.5). The total aerodynamic resistance to vapor
transport (r tot ) is the sum of surface resistance (r s ) and the aerodynamic resistance for
vapor transport (r v ) such that r tot = r v + r s (Van de Griend 1994). A constant r totc (107
s∙m –1 ) for r tot is assumed globally based on observations of the ground surface in tiger
bush in southwest Niger (Wallace and Holwill 1997), but it is corrected (rcorr) for
atmospheric temperature (T) and pressure (P) (Jones 1992), with standard conditions
assumed to be T = 20°C and P = 101,300 Pa.
rcorr
T
Pa
r tot
=
+
×
=
1 0
273 15
293 15
101300
1 75
.
.
.
.
r r
rcorr
r
totc
totc
×
= 107 0
.
(19.13)
where r v (in seconds per meter) is assumed to be equal to the aerodynamic resistance (r a ;
in seconds per meter) based on Equation 19.5, since the values of r v and r a are usually
very close (Van de Griend 1994). The aerodynamic resistance (r a ) is parallel to both the
TABLE 19.1
BPLUT for the RS-ET Algorithm
Parameter
ENF
EBF
DNF DBF
MF
WL
Wgr
Csh Osh Grass Crop
T min _open
(°C)
8.31
9.09 10.44 9.94
9.50 11.39 11.39 8.61 8.80 12.02 12.02
T min _close
(°C)
–8.0
–8.0
–8.0
–6.0
–7.0
–8.0
–8.0
–8.0 –8.0
–8.0
–8.0
VPD_close
(Pa)
2500 3900 3500 2800 2700 3300
3600 3300 3700 3900
3800
VPD_open
(Pa)
650
930
650
650
650
650
650
650
650
650
650
Note: ENF, evergreen needleleaf forest; EBF, evergreen broadleaf forest; DNF, deciduous needleleaf
forest; DBF, deciduous broadleaf forest; MF, mixed forest; WL, woody savannas; Wgr, savannas;
Csh, closed shrubland; Osh, open shrubland; Grass, grassland, urban and built-up, barren, or
sparsely vegetated; and Crop, cropland.
Remote Sensing and Modeling of Global Evapotranspiration
In addition, Mu et al. (2007b) found that VPD alone can capture interannual variability of the full water stress from both the atmosphere and soil for almost all of
China and the conterminous United States, although it may fail to capture the full seasonal water stress in dry regions experiencing strong summer monsoons.
19.3.4 Soil evaPoRation
To calculate soil evaporation, the potential evaporation (λE SOIL_POT ) is first calculated
using the P–M method (Equation 19.5). The total aerodynamic resistance to vapor
transport (r tot ) is the sum of surface resistance (r s ) and the aerodynamic resistance for
vapor transport (r v ) such that r tot = r v + r s (Van de Griend 1994). A constant r totc (107
s∙m –1 ) for r tot is assumed globally based on observations of the ground surface in tiger
bush in southwest Niger (Wallace and Holwill 1997), but it is corrected (rcorr) for
atmospheric temperature (T) and pressure (P) (Jones 1992), with standard conditions
assumed to be T = 20°C and P = 101,300 Pa.
rcorr
T
Pa
r tot
=
+
×
=
1 0
273 15
293 15
101300
1 75
.
.
.
.
r r
rcorr
r
totc
totc
×
= 107 0
.
(19.13)
where r v (in seconds per meter) is assumed to be equal to the aerodynamic resistance (r a ;
in seconds per meter) based on Equation 19.5, since the values of r v and r a are usually
very close (Van de Griend 1994). The aerodynamic resistance (r a ) is parallel to both the
TABLE 19.1
BPLUT for the RS-ET Algorithm
Parameter
ENF
EBF
DNF DBF
MF
WL
Wgr
Csh Osh Grass Crop
T min _open
(°C)
8.31
9.09 10.44 9.94
9.50 11.39 11.39 8.61 8.80 12.02 12.02
T min _close
(°C)
–8.0
–8.0
–8.0
–6.0
–7.0
–8.0
–8.0
–8.0 –8.0
–8.0
–8.0
VPD_close
(Pa)
2500 3900 3500 2800 2700 3300
3600 3300 3700 3900
3800
VPD_open
(Pa)
650
930
650
650
650
650
650
650
650
650
650
Note: ENF, evergreen needleleaf forest; EBF, evergreen broadleaf forest; DNF, deciduous needleleaf
forest; DBF, deciduous broadleaf forest; MF, mixed forest; WL, woody savannas; Wgr, savannas;
Csh, closed shrubland; Osh, open shrubland; Grass, grassland, urban and built-up, barren, or
sparsely vegetated; and Crop, cropland.