Environmental Engineering Reference
In-Depth Information
Table 26.1 Water demands
Source of demand
Tonnes of water needed per tonne of produce/tissue
Domesticated animals/humans
1
Industrial
Paper
250
Nitrogen
600
Agricultural
Sugar cane
1000
Wheat
1500
Rice
4000
Cotton
10000
as much water through products and services as in food and drink. Figure 26.15 compares
the water consumption of Spain, France, Greece and Italy.
With precipitation so seasonal, the value of groundwater supplies cannot be
overestimated. The volume of water stored in rocks depends on the percentage of empty
spaces, i.e. their
porosity
. Not all ground water is available, however, and the proportion
that can drain under gravity is called the
specific yield
. Ground water is not stationary but
flows through the rock. The rate of flow depends upon the porosity and the degree to
which the parts are interconnected, i.e. the permeability. Rocks which are both porous
and permeable, i.e. can both store water and allow water to flow through them, are called
aquifers. Alluvial aquifers consisting of geologically young alluvial, terrace or fan
deposits are important aquifers and follow all river valleys and deltas. Sedimentary rocks
such as sandstone and limestone tend to have smaller pores, but fracturing and fissuring
can contribute greatly to specific yield. Permeable basalt can be an important aquifer
locally, but other igneous and metamorphic rocks such as granites, schists and gneisses
may have no primary porosity but depend upon weathering to enlarge joints to provide
some secondary porosity and permeability. Table 26.2 summarizes the porosity and
permeability values of typical Mediterranean aquifers.
In cool temperate regions, groundwater and surface water sources both contribute to
society's demand for water. Although the precise balance will vary, on average each is
contributing about 50 per cent to supplies for drinking, for industry and for agriculture.
Ground water contributes also to the surface flow of streams, because they are partially
fed by drainage from aquifers. Rivers also increase in discharge from source to mouth as
they receive more surface run-off downstream.
