World’s Soil Resources

Drought has plagued many parts of Africa because of high climate variability from year to year. Severe
droughts in the 1970s and 1980s led to the deaths of hundreds of thousands of people across the Sahel
(Sheffield and Wood, 2011). Recent droughts in the Horn of Africa have continued to affect millions of people
(Ledwith, 2011; UN, 2011), driven by an overall decline in rainfall that is expected to continue and may be linked
to anthropogenic warming of the Indian Ocean (Funk et al., 2008; Williams et al., 2011). Monitoring soil water
and its impacts on food security in the Horn of Africa is particularly difficult because of the lack of ground
measurements. Nonetheless, the use of satellite and modelling technologies has the potential to provide
drought and famine early warning (Anderson et al., 2012; McNally et al., 2013; Sheffield et al., 2014).
Soil water in the southwestern United States has been affected over the past two decades by frequent
severe drought events (2000-2002, 2007, 2009), culminating in a three year drought in California (2011-
2014) with state-wide impacts on agriculture (Howitt et al., 2014). A shortfall in irrigation water owing to a
depleted mountain snowpack was partly offset by increasing groundwater pumping. Recent analysis using
Gravity Recover and Climate Experiment (GRACE) satellites has confirmed the resulting massive losses of
groundwater since the 1980s from the aquifers underlying California’s agriculturally important Central Valley
(Famiglietti and Rodell, 2013). McNutt (2014) concludes that “... it is this underground drought we can’t see
that is enduring, worrisome, and in need of attention”.
6.10.5 | Conclusions
Soil water is vital for the health of terrestrial ecosystems and human well-being. Although only a small
fraction of the world’s water is stored in the soil, the fluxes of water through the soil are massive.
On the time-scale of years, the El Niño Southern Oscillation is the prime control on the global variability
in soil water. At longer time-scales, the global pattern of precipitation is the dominant driver in controlling
changes in soil water. This pattern may be influenced by climate change.
Global analysis of the changing patterns of soil water has revealed the emergence of three global hotspots
in terms of quantity and quality. These are the North China Plan, the Horn of Africa and the southwestern
United States. There will be great challenges to address in these hotspot regions and in other pockets where
declining soil water quantity and quality is threatening ecosystem health and human well-being.
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