World’s Soil Resources

(Sheffield and Wood, 2008; Dorigo et al., 2012). Figure 6.16(a) shows the spatial variability of soil water globally
from model simulations, ranging from high values in the wet tropics and northern boreal forests, to the desert
regions, such as North Africa, the Middle East, central Asia and Australia. Seasonally, soil water varies with
changes in precipitation (Figure 6.16 b) with the largest variations in the monsoonal regions of south and
southeast Asia, west and central Africa and the Amazon. From year-to-year, the El Niño Southern Oscillation
(ENSO) is the main driver of soil water variability globally (Sheffield and Wood, 2011), often leading to drought
conditions in the Amazon, south Asia, eastern Australia and southern Africa during El Niño years, and to
drought in the United States southwest and the Horn of Africa in La Niña years.
Longer-term changes in soil water are mostly driven by changes in precipitation (Figure 6.16 c and d). Global
warming may be playing a role in drying soil water in some regions, although this is a subject of debate. Over
the past 60 years, soil water has been generally wetting over the western hemisphere and drying over the
eastern hemisphere, mostly in Africa, East Asia and Europe. Trends over the past 20 years (Figure 6.16 e and
f) indicate intensification of drying in northern China and southeast Australia, and switches from wetting to
drying across much of North America, and southern South America, in part because of several large-scale and
lengthy drought events.
6.10.4 | Hotspots of pressures on soil moisture
Hotspots of pressures on soil water quantity and quality have emerged around the globe. These result from
changes in soil water driven by climate change and variability, coupled with human pressures on soil water
through, for example, agricultural intensification and extensification. We describe three hotspots: the North
China Plain, the Horn of Africa, and the southwestern United States.
The North China Plain has seen rapid expansion of agriculture driven by population growth and increasing
demand for food. This area is relatively dry with around 500 mm yr -1 of precipitation and so irrigation from
groundwater has become an important feature of agricultural intensification. However, groundwater has
been used at unsustainable rates, with the result that groundwater levels are dropping by over 1 m per year in
some parts (Kendy et al., 2003). Furthermore, precipitation has decreased over the past few decades (Figure
6.16 f). Coupled with intensive irrigation and fertilizer application, this has led to declines in soil water quality
through salinization and nitrogen leaching (Kendy et al., 2003).
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