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in a preferential flow dominated hillslope to measure the subsurface flow velocities. It was reported that the subsurface flow velocities were more closely related to the 1-h rainfall intensity than to the antecedent moisture conditions. Very little water table response of the plot indicated that preferential flow operated independently from soil matrix and the majority of flow was carried through the preferential pathways during the storms. It was also found that the subsurface flow velocities were more for shorter length of slopes. It was concluded that the preferential flow network of the hillslope was an important factor to control the subsurface flow velocity. Anderson et al.
(2010) characterized the subsurface flow processes in a watershed by monitoring water table dynamics using piezometers. The study was focused to characterize water table-runoff relationship, to identify the existence of preferential flow, and to test the feasibility of identifying areas within the watershed that are dominated by lateral preferential flow. Tang et al. (2011) studied lateral subsurface flow generation to quantify its contribution in nutrient loading in streams. Hillslope hydrology and stream hydrology were simultaneously monitored and the subsurface flow was separated from observed storms by chemical mixing model. It was found that lateral subsurface flow mainly delivered nitrates to the stream. Therefore, it was suggested to put more attention on lateral subsurface flow generation processes of hillslopes to control non-point source surface water pollution.
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erosion, slope failure and landslides, surface and groundwater pollution in many watersheds around the world. For better assessment of the short and long term hydrological impacts of these issues it is extremely important to identify the dominating processes at the field level to have more clarity in understanding.
Considering the large spatial and temporal variability of the preferential flow features and the extent of physical complexity of the controlling flow processes, it is difficult to conceptualize and model preferential flow processes to full satisfaction. Most importantly, from the studies conducted in different parts of the world for the past few decades it has been quite clear that in general the dominance and extent of preferential flow processes show wide variations. In a given region, not only the dynamic physical structures of the macropores may be different, but also the degree of its influence on the flow processes may vary widely. Therefore, in the present scenario before we provide a generalized solution for preferential flow dominated hydrological response of watersheds, it is important to have practical assessment of these flow processes in different parts of the world. The reports and experimental evidences about preferential flow from plot scale or catchment scale studies conducted in different watersheds around the world should help to have a better insight to the problem in our hand.
In the above context, the Brahmaputra river basin of India, being one of the largest river basins of the world, present a wide scope of hydrological assessment of its dominating flow processes. The neighboring area of the basin is also known to receive the largest depth of rainfall in the world. Under very high rainfall conditions the vast undisturbed forested hillslopes of the river basin have significant hydrologic impact on the region. The issues like devastating flash floods, accelerated soil erosion, slope failure and landslides are very common in the river basin. But, the literature survey shows that no detailed hydrological investigation has been carried out in the
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river basin to assess the role of preferential flow on the hydrological response of its vegetated hillslopes. However, the outlook of the hydrologic condition prevailing in the basin gives strong indications of the existence of extreme preferential flow condition in the hillslopes. Therefore, the present study has been taken up in the Brahmaputra river basin with the specific objectives mentioned in the previous chapter to experimentally establish and model the hydrological response of a vegetated hillslope plot in the river basin.
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CHAPTER 3
EXPERIMENTAL INVESTIGATIONS
This chapter presents a description of the study area and the selected experimental hillslope plot in the Brahmaputra river basin. A detailed description of the experimental setup and the methodologies for the in situ experiments are provided herein. The typical characteristics of the hillslopes, climate, and hydro-geologic conditions prevailing in northeast India have been elaborated in order to justify the adoption of the experimental techniques. The descriptions of the laboratory setup and the experimental procedures have been outlined. Detailed instrumentation used in the hillslope plot and the data captured from natural and artificial storm events have been presented. The observed field data and the results obtained from the experimental investigations have been discussed in detail to draw suitable inferences about the hydrological response of the hillslope plot.
3.1 Hillslope Experimental Site
An in situ field plot of 18 m × 6 m size on a natural hillslope, shown in Fig.
3.1, was selected in the Brahmaputra river basin (Singh et al., 2004). The geographic location of the site is 26°12′ N latitude and 91°42′ E longitude with an elevation of about 55 m above mean sea level. Fig. 3.2 shows the location of the experimental plot in the Brahmaputra river basin. In order to quantify its topographic characteristics, a detailed topographic survey was conducted with a uniform grid of 0.25 m, using a total station of (±5 mm + 2 ppm) accuracy in all the directions. Fig. 3.3 shows the digital elevation map of the experimental site. The average slope in the main sloping direction is 20 percent whereas coefficient of variation of slope is 10.49% within the plot. Therefore, the microtopographic variations within the plot can be considered insignificant.
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Fig. 3.1 Hillslope experimental plot Fig. 3.2 Map of Brahmaputra River basin
Fig. 3.3 Digital elevation map of the experimental plot
*Not to scale
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