Integration of a road erosion model, WARSEM, with a catchment sediment delivery model, CatchMODS

Road surface erosion is increasingly recognised as a potentially important source of suspended sediment delivered to streams and other water bodies, particularly in forested catchments. As a result, several models have been developed to estimate erosion rates and sediment delivery from unsealed roads. However, few studies have integrated such models or their outputs into catchment-scale sediment models so that a more complete simulation of sediment yields from a catchment can be achieved. This paper describes the integration of a road erosion model, WARSEM, into a catchment-scale sediment delivery model, CatchMODS, using two coastal catchments in southeastern Australia as case studies. The integration process involved the testing of the comparability of sediment delivery rates predicted by the WARSEM and CatchMODS models using geochemical sediment tracing techniques, and the incorporation of the WARSEM outputs into the CatchMODS framework. This study provides comprehensive information on the sources of suspended sediment in the Moruya-Deua and Tuross River catchments, and also, and more importantly, describes a method that can be applied to similar studies that aim at integrating road erosion modelling for the purpose of catchment-scale erosion and sediment delivery assessment. The annual erosion and sediment delivery rates from unsealed roads in the study catchments were modelled using WARSEM, and the sediment yields from hillslope, gully and streambank erosion were estimated using CatchMODS. Geochemical sediment tracing was employed to test the comparability of CatchMODS and WARSEM using the Goodenough Gully subcatchment as a case study. The results suggest that the level of agreement between the modelling and tracing results is 97% and that the models are comparable. Subsequently, the WARSEM outputs were directly integrated into the sediment routing equation in CatchMODS without modification. The results of the integrated sediment model suggested that road erosion accounts for 9% and 10% of the total sediment yields in the Moruya-Deua and Tuross River catchments, respectively. The integration of WARSEM and CatchMODS can be a useful tool for catchment managers to investigate the potential impacts of comprehensive management options including road, landuse and riparian management on sediment yields at a catchment scale