An insight into the runoff generation processes in wet sub-tropics: Field evidences from a vegetated hillslope plot

•Physical runoff generation processes in a vegetated hillslope plot have been explored.•Plot scale runoff simulation experiments were used to characterise runoff processes.•Natural rainfall–runoff response and soil moisture profile data were analysed.•The role of antecedent soil moisture condition on runoff generation has been recognised.•The dominant runoff generation mechanism of the hillslope plot has been identified.

Understanding the physical processes of runoff generation from vegetated hillslopes in wet tropics and sub-tropics is critical for rainfall–runoff modelling. Limited field data often lead to poor process conceptualisation in hydrological models. Plot-scale runoff experiments are popularly used to characterise the runoff generation processes in hillslopes. In this study a vegetated hillslope plot, which typically represented the dominating soil and land cover class of the undisturbed natural hillslopes of north-east India, was selected to conduct runoff simulation experiments and to monitor rainfall–runoff processes over a long period of time (2005–2011). The empirical relationships representing the infiltration and surface runoff generation characteristics of the plot were evaluated for temporal deviations. Temporal vegetation dynamics of the plot showed significant control over the runoff generation processes. The natural rainfall–runoff response and measured soil moisture profile of the hillslope plot indicated strong threshold driven mechanism of runoff generation. The majority of storm runoff from the hillslope plot occurred through the subsurface route. Surface runoff events were rare and showed very low runoff coefficients. Rainfall depth, duration, maximum intensity, and antecedent soil moisture conditions primarily controlled the hillslope runoff response. The probability of runoff generation under wet antecedent conditions was high. The rainfall excess hydrographs predicted using the experimentally established empirical function for the hillslope plot compared well with the observed hydrographs only under similar wet antecedent soil moisture conditions.