DETERMINATION OF UNSATURATED HYDRAULIC CONDUCTIVITY OF AN ALFISOL
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Date
1993
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ANDHRA PRADESH AGRICULTURAL UNIVERSITY
Abstract
A knowledge of functional relationships
existing between e, K(8) and '"I'm is important for
understanding the dynamics of water flow in the soil
system and its availability to the plant roots. Hence,
two experiments were conducted, to determine the
unsaturated hydraulic conductivity of an Alfisol, one
in the field (instantaneous profile method, Rose
et al., 1965; Watson, 1966) and the other in the
laboratory (Gardner, 1956) with the objectives of
measuring K(8) in the field as well as in the
laboratory and to compare the results obtained with the
two methods.
The instantaneous profile method was carried
in the field for a drainage period of 30 days upto a
depth of 105 cm, installing the tensiometers at 15 cm
interval, in the experimental plot. Evaporation and
lateral seepage (upto 75 cm depth) was checked using a
plastic sheet. 8 and'i'm in the profile, were regularly
monitored and the values were used to determine K(8).
In the laboratory, undisturbed soil cores from top
three depths i.e., upto 45 cm were collected and kept
successively in the pressure plate apparatus and were
. subjected to small pressure increments of 0.1 bar,
starting from 0.1 upto 0.8 bar, in order to determine
the K(8) function.
The results obtained are summarized below:
In the initial stages, the volumetric moisture
content (8) varied from 24.58 to 29.66 per cent in the
profile, 24 hours after irrigation was stopped. Over a
drainage period o~ 30 days, 8 decreased in the profile.
The reduction in 8 was rapid in the first eight days
and then was gradual for rest of the drainage period.
The reduction was large in the middle depths, compared
to 0-15 and 90-105 cm layers. The 'I'm, in the profile
reduced similarly to the pattern of reduction in 8.
Over 30 days, \flm varied from +7. 5 to -804 cm, in the
profile. The 0-15 and 90-105 cm depths showed the least
reduction in 8, with a similar reduction in 'Vm. The
75-90 cm depth recorded a drastic reduction in '+'m, with
a small change in 8. This layer was acting as a
transitional layer in the entire profile studied. The
other depths recorded a gradual reduction over a wide
range of 8.
Ove;4the dra!£age period, K(8) varied from 1.0
to 3.73 x 10 cm day , with an associated decreaEE in
e from 29.66 to 19.57 per cent in the profile. K(8)
decreased exponentially, with reduction in 8 . A sharp
reduction in K(8) was noticed in 75-90 cm depth, while
others recorded a gradual K(8) reduction. With
decrease in 'r'm in all the depths K(8) decreased
exponentie!ly. Oyir 30 days, K(8) varied from 1.0 to
3. 73 x 10 cm day , with an associated 'flm range from
+7.5 to -804 cm. Rapid decrease in K(8) was noticed in
0-15 and 90-105 cm layers, with least reduction in q,m,
while other depths recorded a gradual
2
K(8)_
1
reduction.
The 0(8) varied from 13.43 to 0.035 cm day , with an
associated 8 range from 29.66 to 19.57 per cent during
drainage period. Only 75-90 cm depth showed a sharp
reduction in 0(8). 0(8) decreased exponentially, with
8 reduction.
In the laboratory, all the functional
relationships of 8 vs. 'I'm, K(8) vs. 8, K(8) vs. 'fm and
8 vs. 0(8) showed a similar trend with the
corresponding relationships obtained with field method,
for the top three depths viz., 0-15, 15-30 and 30-45 cm
depth. Overall, the laboratory method recorded lower
K(8) and 0(8) values to the order of ten times compared
to the corresponding values with field method.
Specifically, the 0-15 and 15-30 cm layers, with
laboratory method recorded lower K(8) and 0(8) values,
those for the corresponding depths in the field, as
these layers initially had lower 8 values in case of
. laboratory. The 30-45 cm depth recorded more or less
similar K(8) and 0(8) values with both laboratory and
field methods, as it had similar 8 ranges.
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