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