3.6 Subsurface Soil Moisture Monitoring System 63
3.6.2 Study of soil moisture profile in the hillslope plot 66
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For oven dried samples (i.e. θ = 0) the sensor outputs (V0) were observed and finally the dry weight (W0 gm) of the samples were taken. As V0 is known, using Equation 3.5 the value of ε0 were calculated for all the soil samples. For oven dried samples moisture content (θ) is zero. Thus, we get
a0= ε0 (3.7) The volumetric water content of the samples can be determined from the following
equation:
v w
w V
W
W )
( − 0
θ = (3.8)
Now from Equation 3.6 we get
w
a w
θ ε
ε 0
1
= − (3.9)
Finally, knowing the values of the two coefficients aοanda1for each sensor depth the actual volumetric soil moisture content (θa) can be computed as
1
] 0
44 . 4 1 . 1 [
a
a V
a
−
= +
θ (3.10)
where V is the measured sensor output in mV at a particular sensor depth.
Following the above calibration procedure the sensors of the probe were calibrated for the five different depths of soil (Fig. 3.24). The average values of the coefficients aοanda1determined for the hillslope experimental plot soil are 2.4 and 8.6, respectively. These values were used to convert the measured sensor output at a particular depth to the actual volumetric soil water content.
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Fig. 3.24 Calibration of the profile probe sensors at different soil depths
the profile probe soil moisture sensor. Interesting observations could be made from the soil moisture distribution patterns in the hillslope soil. After establishing a wet antecedent moisture condition in the plot, runoff events were simulated using the sheet flow generation system. Fig. 3.25 (a-d) shows the temporal variations of the volumetric moisture contents of soil at different depths at the four selected locations (P1, P2, P3, and P4) of the plot. With a wet antecedent condition, the plot was subjected to a runoff event having inflow intensity 250 mm/hr for 30 minutes duration. The graphs clearly show that once a wet antecedent moisture condition is attained, the moisture content of soils at different depths remains almost constant during the runoff events as well as after the cessation of surface runoff. It can be observed that the constant soil moisture conditions over the plot have been attained very quickly during the runoff event. It also indicates that under wet antecedent condition, a steady infiltration/recharge condition has been attained in the hillslope plot within a very short time. It is a very important finding related to the present
0 2 4 6 8 10 12 14
0 200 400 600 800 1000 1200 Soil Depth (mm)
Values of the coefficients a0 and a1
a0 a1
Average a0 Average a1
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Probe Location (P1)
0 5 10 15 20 25 30 35 40
0 10 20 30 40 50 60
Time (min)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
Probe Location (P2)
0 5 10 15 20 25 30 35 40
0 10 20 30 40 50 60
Time (min)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
Probe Location (P3)
0 5 10 15 20 25 30 35 40
0 10 20 30 40 50 60
Time (min)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
(a)
(b)
(c)
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Fig. 3.25(a-d) Temporal variations in soil moisture contents at different depths (b.g.l.) in the hillslope plot during and immediately after runoff experiment
investigation. Here the main focus is to capture and understand the flow processes under initially wet soil condition. The unsaturated flow processes related to the buildup of soil moisture content from its initial dry condition is beyond the scope of this study.
Soil moisture content measurements were also done for prolonged periods after the runoff experiments. Fig. 3.26(a-d) shows the depth-wise variations of soil moisture contents in the hillslope plot recorded for a long duration of about 35 hours since the completion of runoff experiment. These figures also suggest that once a wet antecedent moisture condition is attained, soil moisture contents remains almost stable for a long duration. Therefore, once the soil was wetted, it remained at field capacity for 2-3 days even if there was no artificial runoff or natural rainfall. This is a significant finding, because from the early monsoon showers the topsoil of the hillslopes is expected to be wet and then it remains at field capacity for a significant period. Therefore, the subsequent storm events are extremely critical for the generation of rapid subsurface stormflow from these hillslopes.
Probe Location (P4)
0 5 10 15 20 25 30 35 40 45
0 10 20 30 40 50 60
Time (min)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
(d)
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Probe Location (P1)
0 5 10 15 20 25 30 35 40
0 5 10 15 20 25 30 35 40
Time (hr)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
Probe Location (P2)
0 5 10 15 20 25 30 35
0 5 10 15 20 25 30 35 40
Time (hr)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
Probe Location (P3)
0 5 10 15 20 25 30 35 40
0 5 10 15 20 25 30 35 40
Time (hr)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
(a)
(b)
(c)
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Fig. 3.26(a-d) Temporal variations in soil moisture contents at different depths (b.g.l.) in the hillslope plot for prolonged duration after runoff experiment
The spatio-temporal variations in the soil moisture profile along the centre line of the hillslope plot can also be studied from the profile probe measurements taken before, during, and after the runoff experiments. Fig. 3.27(a-h) shows the distribution of soil moisture at different time steps along the central transect of the plot for a simulated inflow intensity of 305 mm/hr continued for 40 minutes duration. The figures clearly depict that after a wet antecedent condition has been established, the moisture content in the top soil layer does not vary. The middle layer soil (300-400 mm) has relatively low but stable moisture content. Temporal variations of the soil moisture profiles indicate that the infiltrated water bypasses this layer to reach the bottom layer where the build up of water table takes place over the impermeable bed and causes lateral diversion of water in the form of subsurface stormflow. Such bypassing flow patterns within the soil combined with rapid buildup and recession of water table in the hillslope soil profile strongly indicates the existence of highly active lateral preferential pathways in the subsoil.
Probe Location (P4)
0 5 10 15 20 25 30 35 40
0 5 10 15 20 25 30 35 40
Time (hr)
Volumetric Moisture Content (%)
At 100 mm depth At 200 mm depth At 300 mm depth At 400 mm depth At 1000 mm depth
(d)
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(a) Initial soil moisture distribution (b) Water table buildup at t = 18 min.
(c) Subsurface flow initiation at t = 22 min. (d) Soil moisture profile at t = 42 min.
(Overland flow ceased)
(e) Recession of water table at t = 68 min. (f) Recession of water table at t = 85 min.
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(g) Recession of water table at t = 90 min. (h) Soil moisture profile at t = 120 min.
Fig. 3.27(a-h) Measured subsurface soil moisture profiles of the hillslope plot at different time steps
3.7 Observation of Overland Flow in the Hillslope Plot