1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Coupled Land Surface and Microwave Emission Models 73
there are no satellite-based sensors optimally configured for the retrieval of soil moisture
and there are still issues associated with the technology and retrieval methods to be resolved,
some of which are discussed in this chapter. In summary, the primary conclusions of this
chapter are as follows:
1. The impact of vegetation on the microwave emission from soil is significant and small
errors in its estimation can lead to significant errors in the retrieved soil moisture. Three
models accounting for the effect of vegetation were discussed. The most commonly
used method is a simple, one-parameter, optical-depth model, and several methods are
suggested for obtaining the required value of optical depth (a) using visible remote
sensing; (b) by comparison with more complex models; and (c) simultaneous retrievals
of both soil moisture and vegetation optical depth using multi-angle dual polarization
brightness temperatures. A further issue with the optical depth approach emerges, namely
that there is now evidence that the optical depth of vegetation is a function of, amongst
other things, the look-angle and polarization of the radiometer.
2. Although passive microwave remote sensing can provide estimates of near-surface soil
moisture, by itself, this is not enough to significantly improve the performance of land
surface models. Additional information about the whole soil profile is needed. Simple
statistically based methods and more complex assimilation methods have been proposed
to address this issue.
3. Land surface heterogeneity will impact the estimate of area-average soil moisture. Recently,
downscaling methods have been developed using either statistical techniques
or land surface modelling techniques with higher resolution information on vegetation,
soils and topography. Both the HYDROS and SMOS missions will provide simultaneous
observations that should allow some exploration of downscaling techniques.
Acknowledgements
Primary support for Dr. Eleanor Burke, while preparing this chapter, came from NASA
grant No. NAG5-8214.
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