Managing Scarce Water Resources in Agriculture in West Asia and North Africa - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 449

Irrigation Water Use Efficiency (IWUE ET ) is a measure of the increase in the
crop production (biomass or marketable component) relative to the increase in water
consumed when irrigated, over the consumption under nonirrigated conditions as
follows (Burman et al. 1981):

IWUE ET = ( Y i - Y n )/( ET i - ET n ) (22.2)

where:
Y i = mass of marketable crop produced with irrigation
Y n = mass of marketable crop produced without irrigation
Et i = mass of water used in ET by the irrigated crop
Et n = mass of water used in ET by the nonirrigated crop

The concept of WUE has proved useful in both experimental and field studies of crop
water use in dryland and irrigated agriculture, however the term WUE is inappropri-
ate to the intended concept behind it because a maximum established (by theory or
observation) does not exist for reference and that is why WP is a more appropriate
term than WUE, but it is still not as widely used/accepted as WUE.
It should be well understood that the issue of water productivity is multidisci-
plinary and scale or level-dependent. In their detailed analysis “Accounting for Use
and Productivity of Water,” Molden et al. (2003) put forward an interesting question
(which crop and which drop?) when discussing the banner “More Crop per Drop”—a
slogan used recently by the Secretary General of the United Nations as a response
to the threatening crises of water scarcity. WP is addressed at different scales (plant,
farm, project, and basin levels) and a conceptual framework for better understanding
of WP and water accounting across scales is introduced. It has been pointed out that
the highest WP at one scale does not necessarily result in the highest WP at another
scale. Economic productivity and opportunity cost of water make the undertaking
far more complex.

t he W ater a CCoUnting f rameWork
It starts by defining the boundaries of each scale or domain of use and the inflow
and outflow of water across these boundaries. Components of water inflows and
outflows are classified into various water accounting categories. A clear distinc-
tion is made between water depletion and water consumption. Water depletion is
the use or removal of water (from a domain, particularly a basin) that renders it
unavailable for further use. Water may be depleted by evaporation, flows to sinks
(such as sea or saline groundwater), or incorporation into products (such as bot-
tling water).
Not all water diverted to a service or use is depleted. Water depletion is clas-
sified into beneficial and nonbeneficial. Beneficial water depletion is subclassified
into process depletion and nonprocess depletion. Process depletion is the amount
of water diverted and depleted to produce an intended good. For agriculture, it is
water transpired by crops. In industry, for example, it includes the amount of water
evaporated by cooling systems, or converted into a product. Nonprocess depletion