Using Sea Water for Agriculture
Aim: marking a text structure, using graphics in note-taking.
1. Pre-reading:
Discussion on water supplies, fresh and sea water, arid areas in the world.
2. Scan the text 'Using Sea Water for agriculture' and label the margin brackets with the
expressions at the bottom.
3. Compare your margin notes with 'simple graphics'
4. Read the article step by step:
Paragraph 1,2 (focus on vocabulary)
Match the given expressions with their definitions
In the text, find the words that have the same meaning as the expressions on the
handout
Paragraph 3:
Label the diagram 'Cheapest method of evaporation' and describe it to your
neighbour.
.
"
Paragraph 4:
'Arizona methoď - flowchart
To show how the process continues, use the arrows .
5. Complete the spidergram with the key points
6. Recapitulation: types of note-taking
English for Biologists, MU
Based on: Study Reading, Glendinning, E.H, Holmstrom, B, CUP, 1997
Hana Němcova
Unit 6 The physical world
Control of evaporation. and particularly of transpiration of water through plants,
is obviously of crucial importance in all regions of the world where water is scarce.
It is being investigated most thoroughly in connection with the use of sea water for
agriculture. Sea water can actually be used as such for watering
certain plants, on certain soils.2
() But it seems unlikely that it can be at all widely 5
used for growing plants useful for food. and it is not at all certain how long it can
be carried on before the accumulation of salt in the lower parts of the soil makes it
unusable.
Most attempts to use sea water for agriculture depend on first removing the 10
excess salt. There are two basic methods of desalination. One depends on using
a membrane which will allow the water to pass, but will hold back the salts
(reversed osmosis). The other is distillation. that is to say water vapour or steam is
produced and this. which does not contain salts. forms fresh water when it is
condensed. The production of steam can be done by actually boiling the sea
water or. more gently, by encouraging evaporation from the surface of sea water 15
which is warmed but not raised to boiling point. Both the membrane-filtering
techniques and the boiling technique require large amounts of concentrated
energy. They are essentially industrial processes of a very energy-consuming
kind. The evaporation methods are much less demanding. and I will discuss
them first. 20
The cheapest way of evaporating sea water is to use the heat of the sun. The sea
water is run into shallow tanks of concrete or plastic. preferably with a black
bottom which absorbs the sun's heat. The tanks. which are usually built long and
narrow. are covered with a transparent roof with curved or sloping sides. The
water in the tanks is warmed, evaporates, and the water vapour condenses again 25
on the cooler glass roof and runs down the sides to be collected in a trough at the
bottom. Installations of this kind are already in use in many arid regions near the
sea, from the coasts of Chile to the Aegean islands. It is a very satisfactory process
provided one does not want too much water. It has mostly been used to
provide drinking water. The quantities required for agricultural irrigation would 30
require enormous areas of tanks.
A much more sophisticated low temperature evaporation scheme is being
developed in Arizona.21
The scheme involves using cold sea water which is
pumped into the installation to aid the condensation of the water vapour which
has been produced by hot sea water. Originally solar energy was used to heat the 35
sea water. but since any place that wanted to run such a scheme would certainly
be generating its own electricity. probably with a diesel engine. use was later
made of the 'waste heať in the cooling water of the engine.
They also introduced another improvement which is of very general
application. The fresh water was used on plants grown in plastic greenhouses. A 40
large sheet of plastic is attached to a low brick or stone wall. and a small pump
keeps the air pressure inside the plastic at about half a pound per square foot. above
the air pressure outside. so the plastic is inflated, in the form of a long low sausage.
The plastic is transparent to the sunlight which the plants need. while
the water, led to the plant roots and transpired through their leaves. is trapped 45
inside and not allowed to escape back into the general atmosphere; it can be used
again and again. Experimental plants of this kind are working in Arizona and
Mexico. and a quite big one, planned to provide food for a sizeable population, is
being built in the oil-rich Persian Gulf state of Abu Dhabi. There
are quite a large number of areas in the world in which arid deserts come near 50
enough to the sea coast for developments of this kind to make important
contributions to the worlďs food supply.
2
ad paragraphs 1 and 2
Match the given expressions with their definitions:
transpiration
reverse osmosis
distillation
excess
accumulation
desalination
investigation
evaporation
the separation of a liquid from a solid or another
liquid by vaporization followed by condensation
a change of state from liquid to vapour which can
occur at any temperature up to the boiling point
the evaporation of water vapour from plant leaves
via tiny pores
gradual increase in amount until there is a large
quantity in one place
a larger amount of something than is allowed or
needed
the salt removed from sea water so that it can be
used in homes and factories
separation of solute from a solution by causing the
solvent to flow through a membrane at pressures
higher than the normal osmotic pressure
an (official) attempt to find out the reasons for
something such as a scientific problem
In the text find the words that have the same meaning as the expressions below:
extremely important
there is not enough of it available
the top layer of the earth in which plants grow
continue doing something
have something inside or have something as a part
takes a lot of energy
needing a lot of ability, effort or skill