Water scarcity and abundance, water productivity and their relation to poverty - Water Scarcity, Livelihoods and Food Security - page 19

Taken together, these analyses suggest that blue water is only physically
scarce in selected areas. Problems with water quality and lack of investment in
water storage technology are far more pervasive than scarcity. None of the
analyses consider green water. We conclude that institutions and governance
are central to dealing with issues of water control, water quality and infra-
structure investment.
The basin level—green water is often abundant
At the level of a river basin, freshwater can be scarce and abundant at the same
time, depending on whether we are talking about blue water or green water.
Green water and rainfall may be abundant when blue water is scarce. Even in
dry basins, green water can be deemed abundant when only a small proportion
of precipitation or actual evapotranspiration (AET) goes through agriculture.
Blue water makes up rivers, lakes and groundwater. Green water includes
water stored in the soil to be transpired in the process of vegetative produc-
tivity. There is a complicated interaction between green and blue water involv-
ing precipitation, temperature, topography, soil type, vegetation cover and
processes that control runoff and deep drainage (Chartres and Varma, 2011).
Analysis of future demand and supply for freshwater for the most part focuses
on blue water, but agriculture uses three to four times more green than blue
water.
Evapotranspiration (ET) is the sum of plant transpiration and evaporation
from soils and open water to the atmosphere. Meteorologists differentiate
between potential ET (PET), which is the atmosphere's ability to evaporate
water, and AET, which is the amount of water that does evaporate from all
sources. Agricultural uses, including pasture, often account for only a small
proportion of AET, so that a lot of green water does not go through
agriculture at all (Table 2.1). In the moderately dry Volta Basin, for example
(average precipitation 973 mm/yr), only about 10 percent of precipitation goes
through agricultural systems, accounting for 11 percent of AET.
More productive use of rainwater can therefore help to resolve the global
crisis of freshwater scarcity. We need to focus on the blue-green water nexus
(Falkenmark and Rockström, 2010), that is, on green water as well as on blue
water since transpiration from vegetation is a major water use.
The BFPs
The CPWF BFPs research provided information on the distribution of water
across different environments and land uses. From 2005 to 2009, the BFPs
researched water availability, water balances, WP, the relationships between
water, poverty, and other factors in ten river basins. 3 The BFP basins cover a
wide range of geographic settings on three continents with considerable cross-
basin and within-basin variability in size, topography, land use, extent of irriga-
tion, population density, income levels, poverty, precipitation, temperature,