Geoscience Reference
In-Depth Information
expressed as a percentage of the quantity of irrigation water that will
be consumed in a nonsaline soil. It thus goes as supplement.
The concept is old (Rhoades 1974) and the leaching fractions have
progressively been reduced considering the experience gained and
improvement in technology (better distribution of the water from
sprinklers). The following formula is currently used:
Leaching fraction
=
EC
water
_________
EC
Maxforsoil
◊ 100
It is of the order of 10 to 40 per cent. But some formulae take into
consideration the texture because more water is needed in a clayey soil
(Vallès 1985; Bouteyre 1986). Also, very simply, it suggests to farmers
who irrigate by flooding that at least 12 inches (~30 cm) of water is
necessary to eliminate 80 per cent of the salts from the soil.
For treating the problem less empirically, it would be necessary to
obtain a true balance based on two equations:
For the salts:
H
i
C
i
-
H
d
C
d
= (
q
sat
C
t
-
q
0
C
0
)
P
where
H
is the height of the irrigation water (
i
) or of the drainage water
(
d
);
C
is the concentration of salts in the irrigation water (
i
), of the soil
at the start of the irrigation (0) and of the soil at the end (
t
);
q
is the
water content of the soil at the start of the irrigation (0) and at the end
of the application of water (
sat
= saturation);
P
is the thickness of the
soil to be leached.
For the water:
H
rain
+
H
i
-
PET
-
H
d
= (
q
sat
-
q
0
) ◊
P
where
PET
is the height of the water evapotranspired.
We have seen at the beginning of this chapter that the calculation
of SAR for irrigation water enables us to predict the adsorption of
sodium on the solid phase. Thus we have a means for avoiding certain
irrigation waters and the degradation they seem to be involved in.
Classification diagrams for water were established in 1954 by the U.S.
Salinity Laboratory Staff.
Paradoxically, a pure water is only good for irrigation if the soil is
free of salts or is minimally saline. If it is sodic and clayey, as we have
seen above, it can become impermeable. In this case, a strongly saline
water, for example seawater, may be used to eliminate most of the
Risk of degradation of structure
