Environmental Engineering Reference
In-Depth Information
depends on technology and climate (especially for agricultural products), virtual water is con-
tingent to the place of production (Hoekstra and Chapagain, 2007). Table 9.6 compares virtual
water for different types of animal products that were produced in different countries. It is
evident from the data the consumption of water to produce the same product varies signifi-
cantly from one country to another. For instance, beef in Mexico takes more than twice of the
amount taken by the world average.
The concept of virtual water is valuable but not free of shortcomings. The main limitation is
the fact that all sources of water are treated as having the same value (“The concept of 'virtual
water,' ” 2008). For the calculation of virtual water, there is no distinction whether the water
comes from precipitation, a river, or an aquifer. Under this assumption, a product coming from
two different countries can contain the same virtual water, but if one country depends on rain
and the other on irrigation with water taken from an aquifer, then the impact is completely dif-
ferent and this metric cannot capture that difference. This problem is in part solved by the
concept of green, blue, and gray water footprint, which will be discussed in the next section.
Water footprint
Water footprint is to water what carbon footprint is to carbon dioxide. Water footprint is a
metric that compounds the direct and indirect consumption of water by an entity in a period
of time, weight, number, or volume of product. The concept of direct and indirect water use
is similar to direct and indirect carbon dioxide emissions covered in Chapter 11. A food
processor's direct water consumption is the water used at the facility, whereas the indirect
consumption is the water that was used to produce the agricultural products and ingredients
used as raw materials by the processor.
Water footprint calculation is based on three kinds of water: green, blue, and grey.
Green water
is rainwater accumulated in the soil as moisture. The green water footprint
accounts for the use of green water to produce goods or services (Hoekstra, 2008).
●
Blue water
is surface and groundwater “consumed to produce goods or services.”
Consumption implies water that is evaporated and incorporated into the product. It does
not include water that has been extracted and then returned to the body of water from
which it was withdrawn (Ercin et al., 2011).
●
Grey water
is polluted water associated with the production of good and services. Grey
water footprint is calculated as the volume of water that is needed to dilute polluted water
to a level that “quality of the water remains above agreed water quality standards”
(Hoekstra, 2008).
●
Water footprint of a nation
Hoekstra and Chapagain (2007) present a methodology to calculate the water footprint of a
nation, which has two components: the “internal” and the “external” water footprint. The
internal water footprint is calculated as the total water used by the country minus the virtual
water exported to other countries:
Agricultural water use (equal to the evaporative water demand of crops)
+
Water withdrawal in the industrial sector
+
Water withdrawal in the domestic sector
−
Virtual water exported to other countries
=
Internal water footprint
