9 Soil Moisture
209
which consists primarily of vertical stalks. The effect of frequency on penetration
depth can be seen in the lower part of Fig. 9.6. It is readily apparent that the penetration depth increases with a decrease in frequency or an increase in wavelength.
With radar the effect of the vegetation canopy adds more complexity to the
problem. Now to determine soil moisture, one must determine the soil roughness
effects and the effects of the vegetation canopy, which is not a trivial exercise.
All kinds of vegetation contain water along with some plant structure. Both of
these vegetation parameters are responsive to microwaves. Therefore, the radar
response from vegetated areas will have the integrated effect of vegetation and
soil. Most of the microwave models have been constructed by replacing the vegetated regions with a random medium whose statistical characteristics are related to
physical quantities of the medium.
9.4 Hydrologic Examples
Within the past few years there have been a number of very successful demonstrations of measuring soil moisture with both passive and active microwave systems.
The successes with the passive systems have been limited to aircraft campaigns but
the SAR results have been obtained from aircraft, the Space Shuttle and satellites.
The examples of successful use of passive microwave radiometers for measuring
soil moisture have been associated with large scale field campaigns designed to
examine the role of soil moisture in hydrology and land-atmosphere interactions.
These field campaigns have been conducted in a fairly typical way as far as the
remote sensing was concerned. During the experiment period, extensive ground
data and hydrologic data were collected. Soil moisture measurements consisted of
neutron meter, TDR, and a large number of gravimetric samples. In addition, a
number of more traditional hydrologic measurements were made such as rainfall,
streamflow, meteorological variables and ground based energy and flux stations.
Two AMES Research Center based NASA aircraft have typically participated in
the experiments, the DC-8 and C-130. The DC-8 carried the three frequency polarimetric Synthetic Aperture Radar (AIRSAR) and the C-130 carried the NS001
thematic mapper simulator, the Thermal Imaging Mapper, the Electronically
Steered Thinned Array Radiometer (EST AR) or its predecessor the Push Broom
Microwave Radiometer (PBMR)and a USDA laser profiler.
Some of the more successful campaigns are briefly described below:
The First ISLSCP Field Experiment (FIFE) was carried out over a 15 x 15 km
grassland site near Manhattan, Kansas. This area became the focus for an extended
monitoring program of satellite, meteorological, biophysical, and hydrological data
acquisition from early 1987 through October 1989. During FIFE the airborne Lband, four-beam Push Broom Microwave Radiometer (PBMR) was used to map
the spatial distribution of soil moisture over a small (37.7 ha) drainage basin. By
using low elevation, overlapping flight lines during a drying period, the spatial
patterns of soil moisture were mapped as shown in Fig. 9.7 (Wang et aI., 1989).
These patterns resembled those developed from a simple draining slab model and
209
which consists primarily of vertical stalks. The effect of frequency on penetration
depth can be seen in the lower part of Fig. 9.6. It is readily apparent that the penetration depth increases with a decrease in frequency or an increase in wavelength.
With radar the effect of the vegetation canopy adds more complexity to the
problem. Now to determine soil moisture, one must determine the soil roughness
effects and the effects of the vegetation canopy, which is not a trivial exercise.
All kinds of vegetation contain water along with some plant structure. Both of
these vegetation parameters are responsive to microwaves. Therefore, the radar
response from vegetated areas will have the integrated effect of vegetation and
soil. Most of the microwave models have been constructed by replacing the vegetated regions with a random medium whose statistical characteristics are related to
physical quantities of the medium.
9.4 Hydrologic Examples
Within the past few years there have been a number of very successful demonstrations of measuring soil moisture with both passive and active microwave systems.
The successes with the passive systems have been limited to aircraft campaigns but
the SAR results have been obtained from aircraft, the Space Shuttle and satellites.
The examples of successful use of passive microwave radiometers for measuring
soil moisture have been associated with large scale field campaigns designed to
examine the role of soil moisture in hydrology and land-atmosphere interactions.
These field campaigns have been conducted in a fairly typical way as far as the
remote sensing was concerned. During the experiment period, extensive ground
data and hydrologic data were collected. Soil moisture measurements consisted of
neutron meter, TDR, and a large number of gravimetric samples. In addition, a
number of more traditional hydrologic measurements were made such as rainfall,
streamflow, meteorological variables and ground based energy and flux stations.
Two AMES Research Center based NASA aircraft have typically participated in
the experiments, the DC-8 and C-130. The DC-8 carried the three frequency polarimetric Synthetic Aperture Radar (AIRSAR) and the C-130 carried the NS001
thematic mapper simulator, the Thermal Imaging Mapper, the Electronically
Steered Thinned Array Radiometer (EST AR) or its predecessor the Push Broom
Microwave Radiometer (PBMR)and a USDA laser profiler.
Some of the more successful campaigns are briefly described below:
The First ISLSCP Field Experiment (FIFE) was carried out over a 15 x 15 km
grassland site near Manhattan, Kansas. This area became the focus for an extended
monitoring program of satellite, meteorological, biophysical, and hydrological data
acquisition from early 1987 through October 1989. During FIFE the airborne Lband, four-beam Push Broom Microwave Radiometer (PBMR) was used to map
the spatial distribution of soil moisture over a small (37.7 ha) drainage basin. By
using low elevation, overlapping flight lines during a drying period, the spatial
patterns of soil moisture were mapped as shown in Fig. 9.7 (Wang et aI., 1989).
These patterns resembled those developed from a simple draining slab model and