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the growing season where there is more than one crop a year. In these cases,
WP needs to be interpreted with caution (Ogilvie et al., 2012).
Where water does not limit crop production, as in southern Nigeria, for
example, factors other than water, such as soil fertility, control WP. The
paradox is that apparent WP is higher in rainfed systems with lower precipi-
tation than it is in areas where rainfall is adequate or more abundant. The
paradox arises because the divisor is received precipitation. Crops in wetter
areas in general use a lower proportion of the rainfall than those in drier areas.
Moreover, if farmers use risk-avoidance strategies, such as low sowing densities,
that give some yield in bad years but cannot give high yields in good years,
average long-term WP will be low (Terrasson et al., 2009). For this reason WP
of crops in rainfed systems, especially those where only a small fraction of
rainfall goes through agriculture, also needs to be interpreted with caution.
WP based on crop yield in irrigated systems is straightforward, although we
need to be careful when we consider higher-value crops, where the higher
value may not offset lower yields. Then there is the converse example where
low-yielding crops, such as cotton in the Gezira, are grown upstream,
potentially limiting the water available for more valuable, higher-yielding crops
downstream in the Nile valley and its delta.
Water productivity of herbivores is difficult because they consume only a
small fraction of the available herbage (Peden et al., 2009). In well-managed
tropical pastures, utilization by cattle is rarely more than 30 percent. It is at
least tenfold less in extensively grazed rangeland. In estimating WP in aqua-
culture, losses to evaporation and infiltration of the ponds are the denominator,
unless the outflow is too contaminated to use downstream. WP in capture
fisheries is debatable, because water lost to evaporation or infiltration would be
lost anyway, but if preservation of the fishery resource prevents another use
such as hydropower, there is a cost in foregone development.
Utility of WP
As the CPWF progressed, and with more analysis, the objective remained to
improve WP, “more crop per drop,” but the limitations of the approach as an
end in itself became clearer. It is relatively straightforward to measure crop WP
with a combination of satellite data “to estimate both crop production and
consumptive water use” (Cai et al., 2012). It is more difficult to estimate WP
of livestock systems and capture fisheries, both of which need “development of
concepts and methodology” (Cai et al., 2012).
Limitations of WP
Because of the complexity in measuring it and interpreting the data, we
conclude that WP has limited usefulness as an objective. Nevertheless, WP is
a useful diagnostic tool, which with other data can identify bright spots of high
productivity and hot spots of low productivity per unit of water depleted in
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