Water scarcity and abundance, water productivity and their relation to poverty - Water Scarcity, Livelihoods and Food Security - page 17

analysis of planetary boundaries; threats to water security from multiple
stressors at sub-national levels; and economic versus physical water scarcity in
basins.
Analysis of planetary boundaries defines boundaries, “within which we
expect that humanity can operate safely. Transgressing one or more planetary
boundaries may be deleterious or even catastrophic due to the risk of crossing
thresholds that will trigger non-linear, abrupt environmental change within
continental- to planetary-scale systems” (Rockström et al., 2009). These
boundaries include climate change, ocean acidification, stratospheric ozone,
global P and N cycles, atmospheric aerosol loading, land use change, biodiver-
sity loss, chemical pollution, and use of freshwater. Recent analysis concludes
that passing a boundary of about 4000 billion m 3 /yr of consumptive use of
blue water 1 will increase the risk of collapse of terrestrial and aquatic eco-
systems (Rockström et al., 2009). The analysis focuses on blue water, however,
it is not restricted to agricultural water nor does it address the question of
whether water use in agriculture will substitute for natural land use.
An analysis of threats to water security from multiple stressors at sub-national
levels takes a different slant (Vörösmarty et al., 2010). It uses spatial accounting
to assess threats to human water security, where a threat is exposure to stressors
at given location. There are four categories of stressors: catchment disturbance,
pollution, water resource development and biotic factors. Catchment distur-
bance includes cropland use, impervious surfaces, livestock density, wetland
and disconnectivity. Pollution includes such factors as soil salinization, loading
of excess plant nutrients, toxic materials and sediments, acidification and
thermal alteration. The analysis concludes that 80 percent of the global popu-
lation is exposed to high levels of threat.
Areas not exposed include parts of the Amazon, central Africa, the Malay
Archipelago, and parts of southeast China and Southeast Asia with low
populations and high rainfall. Rich countries make massive investment to offset
high stressor levels. All CPWF basins are located in areas with high levels of
threat to water security. 2 The analysis focuses on blue water in rivers; however,
it is not restricted to agricultural water and emphasizes quality more than
availability.
The basin level—blue water is (sometimes) scarce
Although much rainwater is unused by people, water scarcity is an important
topic. With regard to blue water, the CA (Molden, 2007) distinguished
between areas with no water scarcity, with economic water scarcity, and with
physical water scarcity (Rijsberman, 2006).
Physical water scarcity occurs when withdrawal of water approaches or
exceeds sustainable limits commonly set at 75 percent of the river flow. This
may be because of a lack of supply, high demand, or both. Economic water
scarcity occurs when there is inadequate investment in water-related infra-
structure, which limits access to water even where there is no local physical