1 Spatial Modelling of the Terrestrial Environment - Georeferencial

72 Spatial Modelling of the Terrestrial Environment
Surface water content (%)
Vegetation optical depth
40
25
bare soil
0.8
0.5
possibles
truth
retrieved
10
0.2
40
crop
0.8
25
0.5
10
0.2
40
tall grass
0.8
25
0.5
10
0.2
40
shrubs
0.8
25
0.5
10
0.2
40
area–weighted mean
0.8
25
0.5
10
0.2
0 50 100 150 200 250 300 350
Time(DOY)
0 50 100 150 200 250 300 350
Time(DOY)
Figure 4.5 Patch-specific soil water content and vegetation optical depth for a synthetic 5
patch pixel with 20% bare soil; 20% water; 20% grass; 20% crop and 20% shrub. Also shown
are the weighted averages. c○ 2002 IEEE, reproduced with permission
symbols, with the black line linking those with the smallest error in brightness temperature.
A wide range of possible solutions indicates there is little information in the microwave
brightness temperatures about the value of the soil moisture/optical depth at a particular
time, whereas a narrow range indicates there is much more information in the brightness
temperature observations. The results suggest that, at higher optical depths, the microwave
brightness temperature is less sensitive to the soil moisture than at lower optical depths as
expected. When the preferred values are compared with the true values, i.e., the values for
which the original area-average brightness temperatures were calculated, it is gratifying to
see a significant level of agreement between the two. If these results were used for data
assimilation, potential errors could be evaluated and included in the assimilation process.
This study assumes that the optical depth is independent of both look-angle and polarization.
However, if the relationship between optical depth and look-angle/polarization could be
parameterized, a similar, but more realistic, study could be made.
4.4 Conclusion
Passive microwave remote sensing is an important technique for the retrieval of soil moisture
with application at a wide range of scales and in a variety of disciplines. However, at present,