THE INFLUENCE OF MAN

In certain cases, h u m a n activities

may

modify the mineral content of water. Irrigation with fresh water at flood time-to

be

studied in the following chapter -reduces the salt content, whereas the

pumping

*of the water-tables increases it.

I

spoke above of the well-known case of fresh water which floats on top of

highly

mineralized water and is removed

by pumping in

excess of the n e w influx.

There is another case, namely the concentration of a water-table

by

considerahle pumping, combined with irrigation vertical to the water-table which is being pumped.

This

makes a n almost continuous cycle, since the water which is being

pumped

is also the water which ‘percolates through and

in

part returns to the water-table, a movement which comhines with the movement of the water out of the water-table itself.

Physical and chemical properties of ground water in the arid countries

In the case of certain water-tables

in

which, although they are situated at sea level, a vertical cross-section of the water-table has failed in the first place to reveal a mineral content which increases lower down, it has been found that the mineral content of the water cons- tantly increases round the pumping well

(with

a n increase in NaC1 and CaSO,). In the course of 10 years, salinity increased from 300 mg. of NaCl to 700

in

some cases and 1,300

in

others.

It is, however, difficult to understand

why

the cons- tant flow of the phreatic water-table (which is between

about

5 m.

and

20

m. per day). should fail to carry downstream the water which is concentrated

by

re- entering the cycle.

Finally, irrigation carried out with water of even a low mineral content m a y enrich the water-table so m u c h that it becomes impossible to cultivate the soil.

I

k n o w in particular of two such cases, the Pemegault summarize, although volumes could be written on this subject, which is basic

in

connexion with the utiliza- tion of water.

As

regards domestic use, water is drinkable

only

within limits. Schoeller has studied those limits

in

arid countries, and they must be considerably expanded

by

comparison with the limits in temperate countries,

in view

of the quality of water available

in

the arid coun- tries.

It is practicable to free the water from salt for con- sumption purposes, but it has not yet been possible to do so on a large scale for the purpose of irrigation, although the matter is under study.

As

regards irrigation water, the presence of certain salts or other elements

in

excess m a y prevent the cultivation of certain crops and even, on occasion, all cultivation.

In

descending order of importance, the presence of the following salts makes water toxic to plants:

MgSO,, MgCI,,

CaCI,, NaCl. Miege has produced a table of the percentages which are generally admissible for each crop.

Certain elements, while indispensable

in

small quan- tities, m a y become toxic as soon as certain amounts of them are exceeded: boron, for instance, is poi. oonous to plants

in

certain doses; fluorine, which is found

in

land rich

in

phosphate, causes a disease, well k n o w n under the n a m e of ‘‘Darrnoms’’ (fluorosis), when it is absorbed

by

plants or

when

too m u c h of it is present in water.

According to

E. Y.

Wilcox and

R. C.

Cole, there are three essential factors which determine the quality of water for irrigation:

The total amount of salts contained

in

the water;

The percentage of N a calculated on the basis of the referred to

in

discussing the chemical properties of water.

According to the studies m a d e

by

Nazir A h m a d

in

the Punjab, standard water for good irrigation is water containing

60

parts of salt to 100,000 parts of water.

This writer has devised a n index, following the formula given below, which quickly indicates the limits which should not be exceeded

in

practice, namely 120 parts of salt to

100,000

parts of water.

This

index is negative

in

the case of water suitable for irrigation.

Salt index

=

(total N a

-

^24.5)

-

^[(total C a of CaCû,)

x

4.851.

T h e quality of irrigation water m a y , however, be greatly improved

by

increasing its speed of in-filtratioai, provided the salt content is not too high, even

if

the salt includes a considerable proportion of N a salts.

In

the arid counTries, the weathering of the terrain

in

itself reduces the speed of infiltration of water (Flechter, 1949), but cultivation processes m a y help percolation.

According to

L. D.

Doneen, the adclition of quantities of gypsum to irrigation water, sufficient to reduce the N a concentration to

50

per cent, increases the speed of water infiltration from 33 to

168

p, according to circumstances.

M. R.

Huberty and

A. F.

Pilisbury suggest the use of (NO,),Ca, while

R. F.

Eeitener and

J. E.

Christiarnsen suggest using organic matter and gypsum.

One fact is certain and should be further studied: the speed of infiltration of irrigation water m a y be increased, thus avoiding over-rapid evaporation,

by

the adüition of certain products, but without of course exceeding cer- tain limits.

The distribution of flood water is a n admirable process which brings water to the plants, allows the land

in

25

The

the phreatic water-Lables to be gradually

freed

from salt, saline than water

in

the surrounding water-table, although the latter seems to feed those springs. This is Once the water-tables have been freed from salt, another €act which

I

have never seen mentioned

in

they can be used intensively and provide a source of any publication.

Similarly, draining of the water-table6 should leach

i

, I and enriches underground water-tables.

I I

life for the arid zones.

’

the sudden arrival of water

in

large quantities.

has a

dry

residue of 3-4 gm., are periodically tested

by

E X A M P L E F R O M T H E F O U A R A T the inhabitants

with

a view to reduction

in

salt.

Along the irrigation canals, which are derived from

A

drainage channel with a flow of about 200 ]./sec.

rivers in some regions, the phreatic water-table shows effects a general drainage of the terrain so that a distinct improvement

in

the water, and its mineral now, after 22 years of operation, the mineral content of content

may

be from 1 to 1.5 gm. less than it would the water has changed from

300

to 180 mg.

/i.

have bcen without irrigation. Archambault refers to certain artesian borings

in

Such distribution is possible in the case of a river South Tunisia

in

which the sdt content of the water with a

big

Bow, such as the Euphrates. has decidedly fallen (in mg.

/I.):

Here are s o m e practical data on this point: D o u z No. 1 bore: from 1,913 to 1,209;

In

the c a e of fresh flood water distributed at a n ave- ouz No. 1 bore: from 1,935 to 625;

rage rate af 10-15 million CU. m. over a surface of 25 sy. D o u z

No.

2 bore: from 1,917 to 1,920;

km., the reduction

in

mineral content is roughly as D o u z No. 2 bore: from 1,935 to 841;

follows

(in mg.):

west zone (without distribution), Kebili No. 6 bore: from 1,911 to 1,710;

5,030, 5,260, etc.; central zone

(with

distribution), Kehili

PTO.

6 bore: from 1,935 to 1 3,400,3,340, 3,330, etc.

;

east zone (without distribution), H e r e agab,

1

have found no refer

5,025, 4,090,4,550, etc. menon

in

any publication on research

in

the arid zones,

The

good effect of artificial distribution in reducing despite its great importance for the use of water

in

mineral content is obvious, and

I

should like to e m p h a - these countries.

size once more that such practices also have other Nevertheless, the best use of water is

by

the rational results, namely irrigation and reí3lin.g of the water- adaptation of crops to its mineral content.

This

is

table. being actively studied

in

all countries, and applied

Drainage, either

by

natural draining or

by

pumping, particularly in California. Thus certain percentages also very definitely improves the water. Thus, where of

Er

which are very harmful to citrus fruits m a y permit there is weathered shale, live springs are always less the cultivation of cereals.

Thus underground water m a y be freed from salt

by

Certain cases in Southern Moracco, where the water

the ground, and improve the water

in

the long run.

C O N C L U S I O N S

Before concluding, it would th e of water for irrigation. Almost entire irrele- the new points m a d e in this vance of the temperature of water so far as its questions which have arisen and should be discussed qualitative composition is concerned, but close at a future meeting. relationship between its quantitative composition

and the temperature, since the latter exercises

a

considerable effect here through evaporation.

Uselessness of very accurate analyses of irrigation water.

Importance of the proportion of salts, other than NaCl to NaC1

in

irrigation water, the proportion of NaCl being low.

Possibility of reducing salinity in water by distributing fresh water from floods.

Need to find certain secondary elements-bromides and miscellaneous metals-in water, especially to improve scientific knowledge of water, and also to determine the possibilily of putting it to practical useful to recapitulate

count, as well as the

N E W POINTS

Overriding importance, chemically, of water derived from limestone

in

arid countries, and also, though ta a lesser degree, of water from sandstone and sand.

Good general quality of water from alluvial soil, except where there is an abnormal concentration of salts.

Poor chCrnical quality o€ water from shale, marl, calca- reous marl and eruptive rocks.

Importance of average daily and annual temperature as a means of determining temperature of water lying

at slight depth. Small influence of temperature on use.

26

Physical and chemical properties of ground water in the arid countries

Need

to study means of treating irrigation water to reduce the saline content.

Need to disseminate the synthetic results of laboratory analyses, stating as

fully

as possible both the geologi- cal and physical conditions of the water analysed (e.g. water from sand

...,

depth

...,

metres

...,

without pressure..

.,

etc.), and what crops are grown with that water.

QUESTIONS STILL UNDER

^I

Dans le document HYDROLOGY SYMPOSIUM (Page 22-25)