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Abstract: The aim of this study was to evaluate four metrics to define the spatially variable (regionalised) hillslope sediment delivery ratio (HSDR). A catchment model that accounted for gully and streambank erosion and floodplain deposition was used to isolate the effects of hillslope gross erosion and hillslope delivery from other landscape processes. The analysis was carried out at the subcatchment (~40km2) and the cell scale (400m2) in the Avon-Richardson catchment (3300km2), south-east Australia. The four landscape metrics selected for the study were based on sediment travel time, sediment transport capacity, flux connectivity, and residence time. Model configurations with spatially-constant or regionalised HSDR were calibrated against sediment yield measured at five gauging stations. The impact of using regionalised HSDR was evaluated in terms of improved model performance against measured sediment yields in a nested monitoring network, the complexity and data requirements of the metric, and the resulting spatial relationship between hillslope erosion and landscape factors in the catchment and along hillslope transects. The introduction of a regionalised HSDR generally improved model predictions of specific sediment yields at the subcatchment scale, increasing model efficiency from 0.48 to >0.6 in the best cases. However, the introduction of regionalised HSDR metrics at the cell scale did not improve model performance. The flux connectivity was the most promising metric because it showed the largest improvement in predicting specific sediment yields, was easy to implement, was scale-independent and its formulation was consistent with sedimentological connectivity concepts. These properties make the flux connectivity metric preferable for applications to catchments where climatic conditions can be considered homogeneous, i.e. in small-medium sized basins (up to approximately 3000km2 for Australian conditions, with the Avon-Richardson catchment being at the upper boundary). The residence time metric improved model assessment of sediment yields and enabled accounting for climatic variability on sediment delivery, but at the cost of greater complexity and data requirements; this metric might be more suitable for application in catchments with important climatic gradients, i.e. large basins and at the regional scale. The application of a regionalised HSDR metric did not increase data or computational requirements substantially, and is recommended to improve assessment of hillslope erosion in empirical, semi-lumped erosion modelling applications. However, more research is needed to assess the quality of spatial patterns of erosion depicted by the different landscape metrics. [Copyright &y& Elsevier]

Abstract: A catchment-scale framework was developed to assess the contribution of sediment sources from farm management actions, gully and streambank erosion on the suspended sediment loads delivered to rivers and associated wetlands and floodplains for two catchments (Avon Richardson, 2885 km2 and Avoca, 4550 km2) in Victoria, south-eastern Australia. After considering commonly available data sets, outputs from the point-scale model (HowLeaky2008) were coupled to a catchment scale framework (CatchMODS). Spatially constant, linear scaling factors were used to link point-scale water surplus to streamflow and gross soil loss to hillslope erosion. The model was calibrated against discharge and suspended sediment loads estimated at water quality monitoring gauging stations. Following calibration, estimates of annual and monthly streamflow and 10-year average annual sediment loads were in good agreement with observations. Catchment-scale outputs, particularly sediment loads, were sensitive to scaling factors. The high sensitivity coupled with limited data hindered tight identification of sediment scaling parameters, therefore sediment outputs were uncertain, particularly in the Avoca catchment. Propagation of uncertainty in parameter estimation to model estimates was assessed qualitatively. The boundaries of model estimations were assessed by retaining predictions of behavioural parameter sets, defined as parameter sets that resulted in efficiencies of sediment load and specific sediment yield estimations not more than 5% lower than the efficiency of the optimal parameter set. Under current management conditions, mean annual suspended sediment load at the Avon-Richardson catchment outlet was estimated to be 3350 (3300–3700) t y−1, of which hillslope erosion contributed 65% (60–80%) and gully erosion 35% (20–40%). In the Avoca catchment, annual suspended sediment load was estimated to be 4000 (3500–5100) t y−1, of which hillslope erosion contributed 17% (5–24%), gully erosion 72% (55–93%), and streambank erosion 11% (1–21%). In the Avon-Richardson catchment management scenarios showed that alternative farming systems focussed on retaining vegetation cover throughout the year would yield a 50 per cent reduction of suspended sediment load, estimated at 1700 t y−1. In contrast, fencing and revegetation of connected gullies was estimated to yield the largest reduction in suspended sediment load (1770 t y−1, 44% of current load) in the Avoca catchment. The framework provides an improved tool to make more informed decisions about how much suspended sediment loads can be reduced in response to farm management actions, gully and streambank protection. Its primary strength lies in the ability to propagate farm management impacts to the catchment scale. Other valuable features for use by natural resource management agencies include a high level of transparency, availability of user-friendly interfaces, and a modular structure that provides flexibility and adaptability to new systems. [Copyright &y& Elsevier]

Abstract: Knowledge of erosion rates and sediment yields from gullies is critical for the effective prioritisation of management efforts aimed at reducing the impact of gully-derived sediments on water quality. This paper describes a rapid assessment method for estimating sidewall gully erosion rates for already established gully networks. It combines a conceptual-empirical model of the cycle of gully initiation and stabilisation with an easy-to-apply technique for assessing gully extent and dimensions. The method is illustrated using a case study of the Avon-Richardson Catchment of north-central Victoria, Australia. It is estimated that a total of 4.6×106 tonnes of sediment have been generated in that catchment from gully erosion since the latest major phase of gullying in the mid 1800s. This is equivalent to 8.9tha−1 y−1 if sediment generation was evenly distributed spatially and temporally across the 3300km2 catchment over 160years prior to 2010. Current rates of gully erosion are estimated to be 4700ty−1 (equivalent to 1.4tha−1 y−1). The results are comparable with observed rates of sidewall erosion in the Avon-Richardson Catchment. Further testing is required to build confidence in its applicability to assess sediment yields and for assisting regional authorities to plan remediation efforts. [Copyright &y& Elsevier]

Sediment monitoring, tracing and modelling are widely used to identify suspended sediment sources. Although each method has inherent limitations and uncertainties, their integration provides opportunities to form collective knowledge and encourages robust management strategies. This paper presents a Weight-of-Evidence approach to integrate multiple Lines-of-Evidence for identifying suspended sediment sources. Three sources of evidence were used: i) stream flow and suspended sediment monitoring at river gauges; ii) geochemical sediment tracing at river junctions; and iii) catchment-scale suspended sediment modelling of hillslope, gully, streambank and unsealed road erosion. We applied this approach on two data-poor catchments in Australia. Some reaches were consistently identified as major sources of sediment from all Lines-of-Evidence. However, inconsistencies between the types of evidence in other areas highlighted the high uncertainty in identifying suspended sediment sources in these areas and the need for further investigation. The integration framework maximised the use of scarce information, enabled explicit consideration of uncertainties for catchment management and identified where future monitoring and research should be targeted. [Copyright &y& Elsevier]