World’s Soil Resources

6.10 | Global soil-water quantity and quality: status, processes and trends
The world relies on its freshwater for ecosystem health and human well-being and prosperity. Yet only 2.5
percent of the world’s water is fresh, and of that, 68.7 percent is in the form of ice. Groundwater comprises 30.1
percent of the freshwater, and just 0.4 percent of the world’s freshwater is in lakes, rivers and the soil.
6.10.1 | Processes
Soil water comprises only 0.05 percent of the world’s store of freshwater. However, the upward and
downward fluxes of water and energy through the soil are massive, and they are strongly linked. The flows are
upward in the form of water vapour, long-wave radiation and reflected short-wave radiation, and downward
in the form of liquid water and short-wave radiation (Figure 6.12). The soil-vegetation system is the first
receiver of the rain and energy that fall on our lands. The soil-vegetation system, which encompasses the
upper reaches of the groundwater or basement rock to just above the soil-vegetative layer, is the critical zone
for controlling terrestrial water quantity and quality.
Rodell et al. (2015) estimate the total annual precipitation onto continents to be 116 500±5 100 km 3 yr -1
– equivalent to approximately five-times the water stored in the Great Lakes of North America. Sixty percent
of this (70 600±5 000 km 3 yr -1 ) returns to the atmosphere through evapotranspiration. The remaining 40
percent (45 900±4 400 km 3 yr -1 ) leaves the continents as runoff, with the greatest proportion either running
off the surface of the soil or returning to streams via the groundwater flow system after passing through the
soil. Thus small changes due to human intervention and climate change that alter these fluxes can have very
large impacts on the store of soil water.
The quantity, quality and flow of water over and through soil affect the spatial and temporal availability
and usage of water. The quantity of soil water in a particular layer of soil can be determined by the soil-water
retention curve, the so-called ‘soil-water characteristic’ (Figure 6.13). This curve describes the relationship
between the amount of water a particular soil can hold and the energy, or matric potential, required to
overcome adhesive and cohesive forces to extract water from the soil. Soils of different textures have very
differing characteristic curves (Figure 6.13) and this affects the movement and storage of water in the landscape.
© European Space Agency
Figure 6.12 The flows of water and energy through the soil-vegetation
horizon
Status of the World’s Soil Resources | Main Report Global soil status, processes and trends
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