Physically based modelling of sediment generation and transport under a large rainfall simulator

A series of large rainfall simulator experiments was conducted in 2002 and 2003 oil a small plot located in ail experimental catchment in the North Island of New Zealand. These experiments measured both runoff and sediment transport under carefully controlled conditions. A physically based hydrological modelling system (SHETRAN) was then applied to reproduce the observed hydrographs and sedigraphs. SHETRAN uses physically based equations to represent flow and sediment transport, and two erodibility coefficients to model detachment of soil particles by raindrop erosion and overland flow erosion. The rate of raindrop erosion also depended on the amount of bare ground tinder the simulator; this was estimated before each experiment. These erodibility coefficients Were calibrated systematically for summer and winter experiments separately, and lower values were obtained for the summer experiments. Earlier studies using small rainfall simulators in the vicinity of the plot also found the soil to be less erodible in summer and autumn. Limited validation of model parameters was carried out using results from a series of autumn experiments. The modelled suspended sediment load was also sensitive to parameters controlling the generation of runoff from the rainfall simulator plot; therefore. We found that accurate runoff predictions were important for the sediment predictions, especially from the experiments Where the pasture cover was good and overland flow erosion Was the dominant mechanism. The rainfall simulator experiments showed that the mass Of Suspended sediment increased post-grazing, and according to the model this was due to raindrop detachment. The results indicated that grazing cattle or sheep on steeply sloping hill-country paddocks should be carefully managed. especially in winter. to limit the transport of suspended sediment into watercourses. Copyright (c) 2006 John Wiley & Soils. Ltd.