Figure 3 shows the strong negative relationship 
between the brightness temperature and soil 
moisture. The correlation coefficient has been 
very high, supporting the fact that brightness 
temperature increases as soil moisture content 
decreases. Therefore, in figure 2, the lower 
temperature suggests the wet condition of the 
soil at the start of the experiment, whereas the 
higher temperature indicates dry condition 
towards the end of the experiment. The ESTAR 
derived soil moisture values have been validated 
by comparing them with the field measured soil 
moisture values (figure 4). It is clear from this 
figure that, the soil moisture derived from 
ESTAR are in good agreement with the field 
measurements. 
40+ 
Ww 
© 
1 
A 
201 
Soil moisture (95) 
p 
e 
jer 
  
  
0 TEENS 1 et + deir Ru dere 
180 200 220 240 260 280 
ESTAR Brightness temperature (°C) 
O Ground truth. y - 126.26 -0.44 x. 1? - 0.68 
» ESTAR y = 100.94 - 0.35x r -0.97 
Figure 3. Relationship between brightness 
temperature and volumetric soil moisture. 
Strong correlations between the two variables are 
observed. 
  
  
  
  
50 
$ 404 Line of equivalence 
o 
3 oo 
$31 og99 ó 
E Oo o 
= 204 o 8 
a … 0 
e | o9 9 oT o o9 
= 101 © Q O00 o 
a O 
o 
0 ppt 
0 10 20 30 40 50 
Observed soil moisture (%) 
Figure 4. Verification of the soil moisture 
derived from ESTAR microwave instrument. 
48 
Figures 5(a)-(c) show two dimensional 
contour maps of multi-temporal soil moisture 
information derived from ESTAR brightness 
temperature. Although eight layers of soil 
moisture data have been derived for each day of 
the experiment, only three maps are shown in 
figure 5 for illustration purposes. The spatial 
and temporal pattern soil moisture observed in 
these maps closely follow that from maps of 
brightness temperature because of the direct 
negative relationship between the two. The 
watershed can be partitioned into eastern and 
western regions characterized by high soil 
moisture content of more than 35% (i.e., areas 
of lower brightness temperature in figure 2), and 
the central region of relatively lower soil 
moisture content of about 15-20%. Therefore, at 
the start of the experiment, surface soil (0-5 cm) 
was at near saturated condition in the most part 
of the watershed. The soil experienced a 
dry-down condition during the experiment 
because there was no rainfall. The soil moisture 
available at the start of the experiment was lost 
by evapotranspiration and sub-surface drainage. 
At the end of the experiment, the range of soil 
moisture content varied from a about 2096 (wet 
soil) in the eastern and western regions to about 
5% (very dry soil) in the central region. The soil 
moisture layers stored in the GIS are employed 
in overlay operations to generate maps showing 
changes in soil moisture (figure 6). During the 
experiment, soils in the eastern and western parts 
of the watershed registered a total loss about 
20%, and those in the central region lost about 
10% of soil moisture (figure 6c). These 
temporal observations of surface soil moisture 
are interesting from the perspective of 
determination of soil properties. 
Figure 7 shows the map of soil texture 
for the Little Washita watershed. The watershed 
is dominated by silt loam and loam on both the 
eastern and western regions, which are 
partitioned by a wide area of fine sandy loam 
and sand. It is interesting to observe the pattern 
of spatial distribution of soil texture in 
conjunction with the patterns of spatial and 
temporal variation of brightness temperature 
(figure 2), soil moisture (figure 5), and changes 
in soil moisture (figure 6). It is obvious from a 
comparative study of these figures that the 
spatial and temporal distribution of both 
brightness temperature and soil moisture closely 
follow the distribution of soil texture in the 
watershed. Areas identified by loam and silt 
loam are characterized by higher changes of 
total soil moisture, while those of sand and fine 
sandy loam are characterized by remarkably 
lower amounts of change. Coarse grained sandy 
soils drained quickly while clayey soils, 
characterized by low hydraulic conductivity, 
retained water and later drained during the 
3865 = 
Northing (km) 
3860 
  
Northing (km) 
  
3870 &- 
3865 [4 
Northing (km) 
3860 
  
  
Figure 5. T 
brightness t 
follow the d 
of sand, loai