MODELLING OF HILLSLOPE RUNOFF PROCESSES

N. M. Shakya (Civil Engineering Department, Institute of Engineering, Nepal)
S. Chander (Civil Engineering Department, Indian Institute of Technology, Delhi, India)

Hillslope hydrology is concerned with the partition of precipitation as it passes through the vegetation and soil resulting in overland flow and sub-surface flow. The push to model these processes arose as a result of multiplication of field catchment studies based on dense instrumentation of small areas. The studies led to the following conclusions: a) Presence of preferential paths, termed as macroposes resulting in dominance of subsurface flow on hillslope. b) These macropores are interconnected and form spatial networks. c) The networks continuously feed and discharge water from the micropores of soil. d) The macropores conduct appreciable volume of water to the saturated zone.

These processes must now be recognized and incorporated into models in order to place hydrologic prediction on a more physically correct and less empirically footing. The present study is thus aimed in modelling hillslope flows with emphasis on subsurface stormflows that involve macropores. The study has identified the physical processes connected with the runoff process on a hillslope based on field observations. For this purpose the components which need to be specified are a) The nature of flows in matrix and macropore domains. b) The spatial and temporal characteristics of the macropore network. c) Interaction between the domains. d) Initiation of flows in the macropores.

The parameters of the model are derived from physical properties of the soil, nature of macropores and hillslope geometry. The conceptualization of model can then be used to examine physical processes involved in infiltration, runoff production and parameters relating various processes obtained from the soil properties of the hillslope. This has also resulted in the computation of development of ground water table, runoff hydrograph and moisture profile along hillslope. The results of the model have been compared with the studies carried out elsewhere on published data in respect of outflow hydrograph, moisture profile and water table formations. The model performs reasonably well in all cases resulting in quick response due to additional percolation through preferential paths. Both Horton and Dunne's variable source area generation mechanisms are explicitly incorporated in the model. It has therefore the potential for inclusion into existing models used for predicting watershed response more rationally, physically correct and less empirical footing under actual field conditions. The results of simulation suggests that highly porous soils have to be classified in terms of macroporosity along with other soil water characteristics so that the saturated hydraulic conductivity of matrix can be estimated with the help of observed saturated hydraulic conductivities of such soils.