Soil moisture is generally defined as the water contained in the unsaturated soil surface of the Earth, derived from rainfall, from snowmelt, or by capillary attraction from groundwater. Soil moisture content (SMC) is a significant component of climatological, hydrological, and ecological systems. Classic estimates of global soil moisture are approximately 70 × 103 km3 (0.005% of the Earth’s total volume of water; Jones 1997), with a renewal time of 280 days (Wetzel 1983). It has long been recognized as a key state variable of the global energy and water cycle due to its control on exchanges of energy and matter and physical processes, in particular, the partitioning of available energy at the Earth’s surface into latent (LE) and sensible (H) heat exchange with the atmosphere. SMC also directly impacts the exchanges of trace gases on land, including carbon dioxide (Seneviratne et al. 2010), and strongly influences feedback between the land surface and climate, which, in turn, influences the dynamics of the atmosphere boundary layer and thus weather and global climate (Patel et al. 2009).