World’s Soil Resources

At continental scales, the only practical means of estimating soil water is from satellite sensors or
simulation models. Satellite-based measurements of soil water are generally based on measuring microwave
emissions that vary because of the sensitivity of the soil dielectric constant to its wetness. These approaches
use radiative transfer models to simulate the transfer of radiation emitted from the soil through the vegetation
canopy and atmosphere to the satellite sensor. However, measurements have generally been restricted to the
top centimetre of the soil column because of the penetration depth of microwave signals for current sensors
(> 6 GHz). They are also restricted to sparsely vegetated regions. The recently launched Soil Moisture Ocean
Salinity (SMOS) (Kerr et al., 2001) and Soil Moisture Active Passive (SMAP) (Entekhabi et al., 2010) satellite
missions improve on this by using L-band (1-2 Ghz) sensors that have penetration depths of the order 5 cm
and are less restricted by dense vegetation. Estimates from land surface models have also contributed to
understanding the variation of soil water at large scales (Sheffield and Wood, 2008). These simulation models
are driven by observations of precipitation, temperature and other meteorology and simulate the surface
hydrological cycle with soil water as a prognostic state variable. Recent efforts have developed long-term
simulations of soil water at regional to global scales (Sheffield and Wood, 2007, 2008; Haddeland et al., 2011),
although uncertainties exist because of missing process representation in the models and because of errors in
model structure, parameters and the meteorological forcings.
6.10.3 | Status and trends
Understanding variations in soil water is critical for a range of applications including drought risk
management, agricultural decision making, and understanding and attributing climate change impacts.
Currently, long-term (multi-decadal) time series of soil water which have been developed from models
and satellite retrievals are being used to understand variability and long-term changes in soil water
Figure 6.15 Factors controlling soil water spatial variability and the scales at which they are important. Source: Crow et al., 2010)
Status of the World’s Soil Resources | Main Report Global soil status, processes and trends
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