Present status of soil moisture estimation by microwave remote sensing

The spatiotemporal distribution of soil moisture is a key variable for hydrological and meteorological applications that influences the exchange of water and energy fluxes at the land surface/ atmosphere interface. Accurate estimate of the spatiotemporal variations of soil moisture is critical for numerous large- scale environmental studies. Recent technological advances in satellite remote sensing have shown that soil moisture can be measured by a variety of remote sensing techniques, each with its own strengths and weaknesses which minimizes the ill- posed conventional problems. Technical and methodological advances such as multi- configuration radar and forthcoming SAR constellations are increasingly mitigating the shortcomings of SAR with respect to soil moisture estimation at the field and catchment scale. This paper presents a comprehensive review of few selected inversion methods of soil moisture, with focus on technique in passive microwave and active microwave measurements, in addition to the factors which affect the microwave return. The theoretical and physical principles and the status of current basic retrieval methods are summarized. Limitations existing in current soil moisture estimation algorithms and the major influencing factors including radar configurations (polarization, incidence angel and frequency of bands) and soil surface characteristics on backscattering coefficient have been addressed and also discussed.