85 4 Satellite Remote Sensing of Surface Soil Moisture Brian W. Barrett and George P. Petropoulos 4.1 Introduction Soil is arguably the Earth’s most valuable nonrenewable resource and undoubtedly the most biologically diverse part of the biosphere. Roughly half of a soil’s volume is com- posed of mineral and organic content, while the other half consists of pores. Soil moisture (or water) content (SMC) refers to the amount of water in these pores and generally refers to the water contained in the unsaturated soil zone (e.g., Hillel 1998). SMC is affected by the soil texture (determines water holding capacity), topography (affects runoff and iniltra- tion), land cover (inluences evapotranspiration), and climate (precipitation, wind, humid- ity, and solar illumination), and, as a result, SMC is highly variable both in space and time. Although soil moisture comprises only a tiny percentage (~0.001%) of the total global water budget, its importance and inluence in the hydrological cycle cannot be understated. It is a key parameter in the exchange of mass and energy at the land surface–atmosphere CONTENTS 4.1 Introduction .......................................................................................................................... 85 4.2 Optical Remote Sensing of Soil Moisture......................................................................... 88 4.3 TIR Remote Sensing of Soil Moisture ............................................................................... 89 4.4 MW Remote Sensing of Soil Moisture .............................................................................. 90 4.4.1 Passive MW Techniques ......................................................................................... 91 4.4.2 Active MW Techniques ........................................................................................... 94 4.4.2.1 Scatterometers ........................................................................................... 95 4.4.2.2 Synthetic Aperture Radar ........................................................................ 96 4.4.2.3 Soil Moisture Retrieval Using Semiempirical Scattering Models...... 99 4.5 Synergistic Methods in Soil Moisture Retrieval............................................................ 100 4.5.1 Optical and Thermal Data Synergies.................................................................. 100 4.5.2 MW and Optical Data Synergies ......................................................................... 103 4.5.3 Active and Passive MW Data Synergies ............................................................. 104 4.6 Operational Global Soil Moisture Products from Remote Sensing............................ 106 4.6.1 AMSR-E ................................................................................................................... 107 4.6.2 ASCAT ..................................................................................................................... 108 4.6.3 SMOS ....................................................................................................................... 108 4.6.4 WindSat ................................................................................................................... 109 4.7 Conclusions......................................................................................................................... 110 Acknowledgments ...................................................................................................................... 111 References..................................................................................................................................... 111