The amount of water stored in the unsaturated soil zone is generally referred to as soil moisture. It is one of the key geophysical variables for understanding the Earth’s hydrological cycle. It is classed as an essential climate variable (ECV) of the Global Climate Observing System, GCOS (GCOS-107, 2006). Soil moisture determines the partitioning of incoming water into infiltration and run-off. It directly affects plant growth and other organic processes connecting the water cycle to the carbon cycle. Run-off and base flow from the soil profile determine river flows and flooding, connecting hydrology with hydraulics. Soil moisture also has significant impact on the partitioning of water and heat fluxes (latent and sensible heat), connecting the hydrological cycle with the energy cycle. Together, soil moisture, temperature and their impacts on the water, energy and carbon cycle (Fig. 1, below) play a major role in climate change projections (IPCC, 2007; Seneviratne et al., 2010).
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