Hydrological Response of a preferential infiltration Dominated Natural Hillslope in Brahmaputra River Basin

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In Brahmaputra river basin, where the hillslopes are characterized by high degree of soil macroporosity and the area receives extreme rainfall events frequently during the monsoon seasons, rapid lateral preferential flow from the adjacent hilly areas often triggers devastating flash floods in the rivers. Using experimental data and physically based semi-distributed hydrological models, the present study has characterized the overland flow and subsurface stormflow behavior on a macropore dominated vegetated hillslope plot. The plot was instrumented with profile probe soil moisture meter and piezometers for monitoring spatial and temporal variations in soil water contents. Soil moisture storage, movement, and distribution patterns indicated rapid preferential flow behavior of the hillslope soil. It was found that once the soil was saturated, it remained at field capacity for the following 2-3 days. Such antecedent wet conditions are extremely favorable for rapid stormflow generation from hillslopes. Dye infiltration tests were also conducted with undisturbed soil columns collected from the hillslope plot. Form the digital image processing of the observed dye patterns high degree of macroporosity throughout the soil profile was evident. It was also found that 1-2 mm diameter macropores, developed mainly by plant roots, dominated the soil profile. Due to high macroporosity of the hillslope soil point scale infiltration measurements showed significant effect of scale over plot scale measurements. A simple sheet flow generation system was used to capture the overland flow characteristics on the hillslope plot with some approximation of natural rainfall condition in terms of variation of overland flow depth along the slope length. At the hillslope scale, the relationship between rainfall intensity and preferential infiltration rate was found to be linear, but there was power relation between ManningDs roughness coefficient and rainfall intensity. There were three major influences on overland flow: (i) preferential infiltration rate; (ii) rainfall intensity; and (iii) degree of vegetation. A physical based numerical solution of the subsurface flow equations was given to develop a subsurface stormflow model. The model could be simulated for different user defined macropore structures of the soil. The model was calibrated using the measured experimental data from the hillslope plot. The subsurface stormflow model captured the observed spatial and temporal variations of saturated profile observed over the hillslope plot reasonably well. It was found that lateral XI macropore flow dominated subsurface stormflow response from the hillslope. The results showed that the hydraulically effective or hydrologically active lateral macroporosity and peak rates of matrix and macropore flow were primarily controlled by the rate of recharge. Different topographical shapes and bedrock profiles were found to have significant effect on surface and subsurface flow hydrographs of different theoretical hillslope types. In non-macroporous soils subsurface stormflow was not evident. Even under wet antecedent conditions, hillslope soils having very high lateral macroporosity did not produce any saturation excess overland flow under high intensity storm events. The impact of disturbing the lateral macroporosity of hillslope soil was significant on ...
Supervisor: Subhashisa Dutt