3.7 Observation of Overland Flow in the Hillslope Plot 73
3.7.1 In situ overland flow experiments 73
3.7.1.1 Experimental results of overland flow 74
A total of 34 overland flow experiments were conducted on the hillslope plot varying the inflow intensities in the range of 59 to 406 mm/hr. Out of these 10, 11, and 13 experiments were conducted under sparse, moderate, and dense vegetation conditions, respectively. Duration of the runoff events were between 15 and 120 minutes. Table 3.9 enumerates details of the overland flow experiments conducted and the different estimated parameters. In each of the runoff events time of concentration and the resulting outflow at the downstream channel were recorded.
Fig. 3.28(a-f) shows the typical nature of the outflow hydrographs obtained from the experiments conducted under different conditions. The initial time lag in the hydrographs represent the time of concentration (tc). Within a very short time the
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Table 3.9 Details of the overland flow experiments conducted
Experiment No.
Vegetation Condition
Constant Inflow Intensity
(mm/hr)
Runoff Duration
(min)
Steady Preferential
Infiltration Rate (mm/hr)
Runoff Coefficient
1 128 25 65 0.49
2 59 42 33 0.44
3 115 30 55 0.52
4 167 20 77 0.54
5 361 15 139 0.61
6 85 32 43 0.49
7 136 25 67 0.51
8 72 45 37 0.49
9 158 35 74 0.53
10
Sparse
79 60 36 0.54
11 121 25 62 0.49
12 406 19 177 0.56
13 298 30 130 0.56
14 276 60 135 0.51
15 103 30 48 0.53
16 229 30 105 0.54
17 80 30 52 0.35
18 241 30 90 0.63
19 339 24 150 0.56
20 174 25 80 0.54
21
Moderate
270 30 90 0.67
22 132 51 112 0.15
23 190 120 167 0.12
24 300 15 175 0.42
25 310 27 245 0.21
26 87 62 86 0.01
27 214 63 172 0.20
28 95 42 87 0.08
29 305 60 214 0.30
30 89 36 84 0.06
31 91 48 83 0.09
32 134 57 98 0.27
33 97 51 93 0.04
34
Dense
184 41 145 0.21
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(a) Experiment No. 1 (b) Experiment No. 8
(c) Experiment No. 14 (d) Experiment No. 20
(e) Experiment No. 22 (f) Experiment No. 33 Fig. 3.28(a-f) Outflow hydrographs from the hillslope plot for the overland flow
experiments conducted under different conditions
0 0.0005 0.001 0.0015 0.002 0.0025
0 5 10 15 20
T ime (min) Discharge (m3/s)
0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012
0 5 10 15 20
T ime(min) Discharge (m3/s)
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045
0 10 20 30 40 50 60
Time (min) Discharge (m3 /s)
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035
0 5 10 15 20 25
Time (min) Discharge (m3 /s)
0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014
0 10 20 30 40
T ime (min) Discharge (m3/s)
0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007
0 10 20 30 40 50
Time (min) Discharge (m3 /sec)
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outflow becomes almost steady, only with some minor fluctuations, which in general occur in experimental observations. This phase represents a constant and steady infiltration condition of the soil, referred as preferential infiltration rate (fb). The steady discharge in the outflow hydrographs and constant soil moisture profile conditions during this stage, measured by the profile probe sensor, also indicates the attainment of steady recharge/infiltration condition over the hillslope plot. The values of fb have been computed from the in situ field experiments knowing the inflow and outflow hydrographs (Table 3.9). This fb is basically the spatially averaged steady infiltration rate over the hillslope plot.
Fig. 3.29 represents the relationship between tc and inflow intensity (i) under different degrees of vegetation in the experimental plot. It shows that tc is dependent on both inflow intensity and degree of vegetation. Vegetation roots are known to create new preferential flow paths as well as establish connectivity between the existing soil macropores to make them hydrologically active. Therefore, the rooting characteristics of surface vegetation under different conditions are closely associated with the seasonal dynamics of preferential pathways in soil (Beven and Germann, 1982; Ziegler et al., 2004; Scanlan and Hinz, 2007; Sarkar et al., 2008b;
Shougrakpam et al., 2010). It can be observed from the figure that the tc curves representing sparse and moderate vegetation conditions are close to each other but under dense vegetation the curve is distinctly separated. As between different seasons there were almost no apparent change in the physical conditions of the plot except vegetation density, one possible reason can be that under sparse and moderate vegetation densities the response of active macropore network in the subsoil, as indicated by the preferential infiltration rates (Table 3.9), were similar. But, under dense vegetation higher root density may have created more active preferential flow
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network within the subsoil. Lower values of runoff coefficients were also observed under dense vegetation condition compared to sparse and moderate vegetations (Fig.
3.30). As a result, higher values of tc were evident for dense vegetation with similar power relation with inflow intensity.
Fig. 3.29 Variation of time of concentration (tc) with inflow intensity (i)
Fig. 3.30 Observed runoff coefficients for the overland flow experiments
0 1 2 3 4 5 6
0 100 200 300 400 500 Inflow Intensity (mm/hr)
Time of Concentration (min)
Sparse Moderate Dense
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0 50 100 150 200 250 300 350 400 450
Inflow Intensity (mm/hr)
Runoff coefficient
Sparse Moderate Dense
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The experimental results revealed that in most of the cases the preferential infiltration rate is higher than the average saturated hydraulic conductivity of the soil matrix (50 mm/hr), which was approximated from the USDA (United States Department of Agriculture) soil textural classes and bulk density of soil samples collected from the plot (Schaap et al., 1998). This typical observation leads to the conclusion that preferential infiltration was more dominating over matrix flow in the hillslope plot. Fig. 3.31 depicts the relationships of fb and i for different degrees of vegetation. The relationships found are linear and vary with degree of vegetation.
Possibly, dense vegetation on the hillslope allowed a higher preferential infiltration rate. This may be attributed mainly to the active macropore network, developed by plant roots under high density of vegetation, which created a favorable hydraulic condition for macropore flow by establishing their connectivity. It is also interesting to observe that even under an extreme inflow intensity of 406 mm/hr; constant maximum preferential infiltration rate was not attained. It is an indication of high preferential infiltration characteristics of the plot. Tables 3.10-3.11 list the relationships derived on the basis of in situ overland flow experiments.
Fig. 3.31 Variation of preferential infiltration rate (fb)with inflow intensity (i)
0 50 100 150 200 250 300
0 100 200 300 400 500
Inflow Intensity (mm/hr) Preferential Infiltration Rate (mm/hr) Sparse
Moderate Dense
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Table 3.10 Relationships of tc with inflow intensity (i) Vegetation
Condition
Range of tc
(min)
Relationship of tc with Inflow
Intensity, (i) R2
Sparse 1.08 – 2.33 tc = 12.679 i -0.4043 0.93
Moderate 0.73 – 2.68 tc = 36.668 i-0.6059 0.61
Dense 2.15 – 5.38 tc = 60.713 i -0.5796 0.85
Table 3.11 Relationships of fb with inflow intensity (i) Vegetation
Condition
Range of fb
(mm/hr)
Relationship of fb with
Inflow Intensity, (i) R2
Sparse 33 - 139 fb = 0.3574 i + 14 0.98
Moderate 48 - 177 fb = 0.3924 i + 11.216 0.92 Dense 83 - 245 fb = 0.6052 i + 31.737 0.92
3.7.2 Observation of overland flow under natural storm events