World’s Soil Resources

Figure 6.14: The soil’s hydraulic conductivity, K (cm day -1 ) in relation to the matric potential, ψ (MPa). As the matric potential becomes
more negative the soil’s water content drops (see Figure 6.16) which increases the tortuosity and slows the flow of water. Source:
Hunter College. 3
The interactions between the structure and physical, chemical and biological components of the soil control
the myriad soil functions and processes that are essential for healthy soils, ecosystems and human well-being.
The soil acts as buffer and filter. Indeed, our soil is the world’s largest water filter. And through this buffering
and filtering, soil controls the quantity and quality of the world’s liquid freshwater.
6.10.2 | Quantifying soil moisture
Soil water varies on multiple time and space scales, driven by climate, weather variability, land cover,
topography and soil type and structure (Figure 6.15). Measuring variations in soil water is challenging especially
at large scales where the cost of direct measurement would be very high. Long-term measurement networks
have historically been limited to a few locations globally (Robock et al., 2000). However, with the recognition
of soil water as an essential climate variable and the realization that in-situ measurements are necessary for
the calibration and validation of remote sensing, the number of operational monitoring networks is increasing
(Dorigo et al., 2011). There are also short-term experimental campaigns with multi-scale soil water sampling
(Crow et al., 2012). For example, the Soil Climate Analysis Network (SCAN) in the United States provides soil
water measurements for 174 sites across the United States, with some measurements dating back to 1992.
New technologies such as the COsmic-ray Soil Moisture Observing System (COSMOS) cosmic-ray neutron
probes (Zreda et al., 2012) have enabled more efficient and larger measurement footprints of the order of
several hundreds of square meters.
3 http://www.geo.hunter.cuny.edu/tbw/soils.veg/lecture.outlines/soils.chap.5/soils_chapter.5.htm
Status of the World’s Soil Resources | Main Report Global soil status, processes and trends
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