Laboratory studies on soil moisture flow and runoff generation in laterite soil
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Date
1998
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Department of Land and Water Resources and Conservation Engineering, Kelappaji College of Agricultural Engineering and Technology, Tavanur
Abstract
If land and water are to be put to the best use, a full understanding of soil-
water relation is necessary. Much of the research on retention and movement of water
in soil and the use of water by plants is done with this objective. Modelling of water
movement in soils requires knowledge of hydraulic conductivity as a function of
volumetric water content or soil water pressure head, and the soil water retention
curve.
Laterite soils are by far the most important group occurring in Kerala and cover
the largest area. A study was conducted to analyse the saturated and unsaturated flow
of water in laterite soil under selected precipitation intensities. The objective was to
establish a relation between hydraulic conductivity, moisture content and hydraulic
head. Runoff generation and relation between runoff and soil moisture status were also
considered.
A rainfall simulator and a soil trough were fabricated. The design of the rainfall
simulator was based on that of Bhardwa] et al (1992). The rainfall simulator comprised
of a drop forming mechanism mounted on a supporting frame. A float valve maintains
a constant head of water in the drop forming tank to get a desired rainfall intensity. The
soil trough had provision for collecting surface and subsurface outflow of water.
Provision was also made to incline the soil trough when a sloping plot was required.
The experimental set up was installed in the Soil and Water laboratory of KCAET,
Tavanur.
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The rainfall simulator was tested for different rainfall intensity by changing the
head of water in the tank Rainfall intensity increased as the head of water in the tank
was increased. At a head of 22 cm, a maximum rainfall intensity of 285.6 rnrn/h was
obtained. Drop size determination by flour - pellet method showed that the simulated
raindrop size remained almost constant The uniformity coefficient also increased with
increase in intensity. The basic soil properties were also determined. The particle size
distribution curve showed that the soil was coarse graded. The bulk density of the soil
was 1.53 g/cm3.
The piezometer readings in the soil trough showed that unsaturated flow takes
place through the soil under simulated rainfall. Experiments were done for different
rainfall intensities of 20, 40, 90, and 285.6 mm/h and also at 10% and 20% slopes with
40 and 90 mm/h intensity. The variation in moisture content with time was found to
increase with intensity of rainfall. Steeper slopes also increased the rate of depletion of
moisture with time. Instantaneous profile method was adopted to determine hydraulic
conductivity. An exponential relation was observed between hydraulic conductivity
and volumetric soil moisture content
Matric suction and volumetric moisture content were observed at different depths - 10,
20, 30 and 40 cm. The hydraulic conductivity - moisture content relationships were
established. Studies on runoff generation showed that inclination of the bed rock is a
major factor controlling subsurface outflow. Variation in intensity of rainfall did not
cause considerable variation in outflow. Surface runoff also was affected only by the
slope of the soil trough.
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