Water scarcity and abundance, water productivity and their relation to poverty - Water Scarcity, Livelihoods and Food Security - page 30

thought that there would not be enough water to grow the food that would be
needed. It was therefore reasonable to focus on WP as one way to address the
crisis, and incorporate it as a main objective of the CPWF. The CPWF
proposal emphasized the importance of WP (CPWF Consortium, 2002),
which was still regarded as being of great importance for the rest of the decade
(Rijsberman, 2004; Molden, 2007).
Initial concept
“Productivity of water is related to the value or benefit derived from the use
of water” (Molden, 1997; Molden et al., 2003). Starting from the point of
view of irrigation, water is classified on its utility, as to whether it is depleted
(removed from the system as by crop ET, flows to a sink, or becomes so
polluted as to be unusable), or whether it is outflow, which may or may not
be committed to some downstream use. The basin WP estimate will include
the WP of the downstream use if the outflow is used consumptively. In the
context of irrigation, WP is straightforward with the denominator as depleted
water and the numerator being either the yield of the crop, the saleable value
of that yield, or some other relevant measure such as energy content (yield of
calories).
Authors have broadened the WP concept to include rainfed agriculture,
grazing animals and aquaculture. Each of these presents difficulties in deciding
what to use as the denominator. For example, of the rain that falls on a crop,
some evaporates in situ , a fraction enters the soil and a variable part is runoff,
which is likely to become blue water. The fraction that enters the soil is either
taken up by plants and transpired, or percolates to depth where it may replenish
an aquifer, which may contribute to the blue water of stream flow. Given these
possibilities, what fraction of the precipitation should we use as the denomi-
nator in calculating WP? The answer depends on the scale of comparison,
often using annual or seasonal rainfall or some estimate of ET and ignoring
runoff and downstream use. Because the conditions in different basins are rarely
the same, it is usually not valid to compare WP between basins, although it
can be used with caution to indicate relative efficiency of water use. The same
arguments apply to intra-basin comparisons.
Different measures of WP
All terrestrial water originates from precipitation; even that stored in aquifers
came from historic precipitation. Hydrologists are principally concerned with
the utilization of blue water in managed irrigation systems. In rainfed systems,
the denominator should be ET, but this is difficult to estimate even at the level
of an experimental plot. It becomes more problematic as the scale increases to
the field, or the farm, but at the broader landscape or basin level, it can be
estimated by remote sensing (Vidal and Perrier, 1990; Ahmad et al, 2008). At
the level of a basin, authors typically use precipitation, either annual, or for