161 
A classical expression for LEp is given by the same authors : 
LE P = - 
P' Rn+ }Ea 
(8) 
P' + /(l + rcp/ra) 
where P’ is the derivative of the saturated vapor pressure function P (Pa.K'l), y is the psychrometric constant 
(Pa.K' 1 ), Ea is the evaporative potential of air given by 18 (P-ea)lra (W.m' 2 ), and rep is the canopy 
resistance at maximal évapotranspiration (s/m) (Jackson et al., 1981). 
Combining (4) and (7), we can express the values of these temperatures as : 
= Ta + -{Rn-G) 
pCp 
(9) 
= Ta+ — {Rn-G-LEp ) 
pCp 
(10) 
The proposed method consists in determining, on a given Landsat TM image, well-identified 
areas where surface temperature remain minimal (irrigated fields, wet forest...), respectively maximal (bare 
soil, stubble, dry forest, rangeland...), during the whole campaign of Landsat data acquisition. These surfaces 
are assumed to remain "wet" ( LE=LEp ), respectively "dry" ( LE=0 ), all this time. Their temperatures, called 
"warm" and "cold" in section 2.1., are then determined using equations (9) and (10), meteorological data, an 
estimation of their aerodynamic resistances. 
2.3. Study sites 
We chose a set of various study sites where we have been conducting previous research 
associated with Landsat TM images and ground measurements, including surface temperature and 
meteorological data. In order to be representative of a large amount of conditions, we tried to find various 
conditions of vegetation and climate. Their characteristics are shown on Table 1. 
Study Site 
Vegetation 
TMCCT 
Data 
Date 
Ground measurements 
Path/Row 
Format* 
Number of targets 
Cold Warm 
Met data 
Les Maures, France 
Forest 
195/30 
PNN 
29/07/92 
1 
1 
X 
43°13'N 6°22'E 
Cork oak 
14/08/92 
0 
0 
X 
Gharb, Morocco 
Irrigated crops 
201/36 
PNN 
10/07/87 
1 
1 
X 
34°30'N 6°04'W 
Sugar cane 
Maricopa Agric. 
Irrigated crops 
37/37 
A 
23/07/85 
0 
1 
X 
Center, Arizona 
Cotton, Alfalfa 
08/08/85 
2 
1 
X 
33°04'N 111°59'W 
05/04/86 
1 
2 
X 
21/04/86 
0 
2 
X 
24/06/86 
1 
1 
X 
Walnut Gulch, Az. 
Rangeland 
35/38 
A 
23/04/92 
0 
1 
X 
31°43'N 110°00'W 
Irrigated crops 
10/06/92 
0 
1 
X 
30/09/92 
0 
1 
X 
Table 1 : Description of study sites