Overland flow and tunnelflow generation on a semi-arid catchment: From field and experimental studies to simulation modeling

The major barriers to modeling hydrological processes in semi-arid and areas are a lack of understanding and model representations of some of the distinctive features and processes associated with runoff generation in those regions and a paucity of field data. This study incorporates field and experimental investigations into simulation modeling to explore stormflow generation processes on a semi-arid agricultural catchment of the Loess Plateau in China.; Overland flow and tunnelflow processes were first investigated in the field. The mechanisms responsible for runoff generation processes and their spatial variation within the catchment were further explored using field rainfall experiments. TOPOG, developed by CSIRO in Australia, was then modified by adding model representations of some of the predominant features and processes identified by field and experimental investigations. The modified models were used to continuously simulate both slowly changing hydrologic states during interstorm periods and fast-responding overland and tunnel flows during stormflow periods. Finally, model simulations, under a wide range of rainfall and spatially temporally varied land cover conditions, were validated by a comparison of observed and simulated stormflow discharges from both catchment outlet and internal plots.; Field investigations indicated that stormflow generation processes in this catchment are dominated by an infiltration-excess mechanism. Tunnel flows are almost entirely derived from the overland flow entering via inlets, but tunnel flow processes may be significantly disturbed by tunnel instability. Tunnel systems are likely to be initiated in catastrophic storm events, although the subsequent storms do expand these systems. Tunnel conduits are developed above materials of low permeability. The spatial variation in soil infiltration capacity is largely responsible for the spatial variability in runoff generation. The spatial variation in soil infiltration capacity is mainly caused by the spatial variation in crusting, which results from varied land management activities. The simulations showed that the model represents reasonably well stormflows generated by rainfall events with recurrence intervals >2 years, which account more than 60 per cent of runoff and 70 per cent of sediment leaving this area. The simulations are extended by the inclusion of some features associated with land management, such as terracing and the existence of a plow pan. Considerable variability in simulation accuracy was found among storm events and within the catchment.