There are many publications on soil moisture mapping with microwave radiometry. How useful is thermal infrared data for estimating approximate soil moisture? How accurate would such estimates be?
Hain, C. R., W. T. Crow, J. R. Mecikalski, M. C. Anderson, and T. Holmes (2011), An intercomparison of available soil moisture estimates from thermal infrared and passive microwave remote sensing and land surface modeling, J. Geophys. Res.,116, D15107, doi:10.1029/2011JD015633.
IR based retrieval methods complement MW-based methods. IR retrievals are more accurate in the presence of vegetation and can be performed at higher spatial resolution. But IR retrievals are only possible under clear skies, contrary to MW retrievals which can be performed in the presence of clouds and precipitation.
Actually, the same authors are very active in this field:
Hain, Christopher R., John R. Mecikalski, Martha C. Anderson, 2009: Retrieval of an Available Water-Based Soil Moisture Proxy from Thermal Infrared Remote Sensing. Part I: Methodology and Validation. J. Hydrometeor, 10, 665–683. doi: http://dx.doi.org/10.1175/2008JHM1024.1
Yilmaz, M.T., W.T. Crow, M.C. Anderson and C. Hain, "An objective methodology for merging satellite and model-based soil moisture products," Water Resources Research, 48, W11502, doi:10.1029/2011WR011682, 2012.
Hain, C.R., W.T. Crow, M.C. Anderson and J.R. Mecikalski, "An EnKF dual assimilation of thermal-infrared and microwave satellite observations of soil moisture into the Noah land surface model," Water Resources Research, 48, W11517, doi:10.1029/2011WR011268, 2012.