1 Spatial Modelling of the Terrestrial Environment - Georeferencial

66 Spatial Modelling of the Terrestrial Environment
canopy boundaries suppresses internal reflections in the canopy and associated interference
patterns in the microwave emission (Lee et al., 2002a).
Figures 4.1c and d show the time series of modelled and measured brightness temperatures
for the same time periods as in Figure 4.1a and b but in the presence of a soy
bean canopy. During drying period 2 (Figure 4.1c), the fresh weight of the canopy was
3.45 kg m −2 and the height was 0.89 m, while during drying period 3 (Figure 4.1d), the
fresh weight of the canopy was 4.41 kg m −2 and the height was 1.15 m. The effect of
vegetation on the microwave emission from the soil is included using the extended Wilheit
(1978) model (Lee et al., 2002a). As in the bare soil examples, the soil near-surface water
contents range from 40% to 10% for drying period 2 (Figure 4.1c), and span a similar range
for drying period 3 (Figure 4.1d). However, this is not as readily apparent in the presence
of vegetation because the vegetation perturbs the emission from the soil.
4.2.3 Potential Near-Future L-Band Missions
The European Space Agency (ESA) has approved the Soil Moisture Ocean Salinity (SMOS)
mission (SMOS homepage) with a proposed launch date between 2005 and 2007. The
SMOS mission will be based on a dual polarization, L-band radiometer with an innovative
aperture synthesis concept (a two-dimensional interferometer) that can achieve an
on-the-ground resolution of around 50 km and provide global measurements of microwave
brightness temperature at a range of different angles. The proposed SMOS retrieval algorithm
is based on a non-coherent model of emission from soil and vegetation using the
simple model discussed in section 4.2.2 and will simultaneously retrieve the soil moisture
and the vegetation optical depth by exploiting the range of available look-angles (Kerr et al.,
2001; Wigneron et al., 2000). Wigneron et al. (2000) used simulated SMOS observations
(created by adding random and systematic errors to the model proposed for the SMOS
retrieval algorithm) and showed that the retrieval lost accuracy as the number of available
independent measures of microwave brightness temperatures at different angles decreased.
NASA has selected the HYDROspheric States mission (HYDROS) as a reserve mission.
It will consist of an L-band passive microwave radiometer with a resolution of around
40 km and an active microwave radar with a resolution of up to 3 km. Both instruments
will measure at multi-polarizations but constant look-angle.
4.3 Discussion
The use of coupled land surface and microwave emission models to explore potential solutions
to three of the limitations of L-band passive microwave remote sensing is discussed.
4.3.1 Accounting for Effects of Vegetation in Retrieval Algorithms
Retrieval algorithms typically use the simple optical depth model to account for the effects
of vegetation. If the only brightness temperature measurements available are at a single
polarization and look-angle, such as those measured during the Southern Great Plains
1997 (SGP97) field experiment, the algorithms require ancillary information to estimate
the optical depth. One commonly used method is to define the vegetation water content
and opacity coefficient for each class of vegetation within a land cover classification, and