Passive Microwave Remote Sensing of Soil Moisture and Regional Drought Monitoring - Monitoring and Predicting Agricultural Drought: A Global Study - page 98

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46

that can be expected from these new sensors and exploratory studies using
previous sensor systems.

R egional-Scale Soil Moisture Dynamics Using Passive
M icrowave Sensors

W ashita '92 was a large-scale study of remote sensing and hydrology con-
du cted using an aircraft-based L-band mapping radiometer over the Little
W ashita watershed in southwestern Oklahoma, United States (Jackson et
al ., 1995). Passive microwave observations were made over a nine-day pe-
rio d in June 1992. The watershed was saturated with a great deal of stand-
in g water at the outset of the study. During the experiment, no rainfall
oc curred, and observations of surface soil water content exhibited a dry-
do wn pattern. Observations of surface soil water content were made at
sit es distributed over the area. Significant variations in the level and rate of
ch ange in surface soil water content were noted over areas dominated by
di fferent soil textures.
Passive microwave observations were made on eight of the nine days of
th e study period. The radiometer data were processed to produce bright-
ne ss temperature maps of a 740-km 2 area at a 200-m resolution on each
of the eight days. Using the single-channel soil water content retrieval al-
go rithm described in previous sections, these brightness temperature data
w ere converted to soil water content images. Grayscale images for each
da y are shown in figure 7.2. These data exhibited significant spatial and
te mporal patterns. Spatial patterns are associated with soil textures, and
te mporal patterns are associated with drainage and evaporative processes.
Th ese results clearly show that consistent information can be extracted
fr om low-frequency passive microwave data. They also illustrate the ex-
ist ence of the spatial and temporal variability that cannot be captured by
po int observations. The basic concepts developed in the Washita '92 exper-
im ent were evaluated in a follow-up to the study that expanded the spatial
do main to 10,000 km 2 and expanded the time period to one month. Re-
su lts presented by Jackson et al. (1999) verified that the watershed scale of
th e Washita '92 experiment could be extrapolated in both space and time
sc ales compatible with satellite observation systems.

[98],

Line
——
-2.2
——
Long
* PgEn

[98],

Surface and Profile Relationships

In formation on the spatial and temporal variation of the unsaturated zone
of the soil can be used to estimate recharge. This information can be of
sig nificant value if the data are provided as a spatially distributed prod-
uc t. Remote sensing satisfies the spatial and temporal needs; however, it
ca nnot be used to directly assess the entire depth of the unsaturated zone
w ithout some additional information. Establishing a link between the eas-
ily accessible surface layer and the full soil profile has long been a research
go al. A foundation for this endeavor is described by Jackson (1980). In
that study, under the assumption of hydraulic equilibrium within a soil