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This paper reviews available models for estimating surface erosion and sediment delivery to streams from unsealed roads. It summarises current progress and identifies directions for ongoing research and model development. The paper provides a framework for assessing road erosion and sediment delivery models and it includes an overview of road erosion and sediment delivery processes and how they are commonly represented in models. Seven road models are reviewed in terms of their representations of erosion and sediment delivery processes, assumptions, application and limitations. While simple models are thought to be more useful and easily applied for land management purposes, more complex models provide a basis for building and consolidating scientific knowledge. This article reveals some of the limitations and needs of existing road erosion models. These include limitations of their ancestor hillslope erosion models, the imbalance between representation of erosion processes versus sediment delivery, a lack of representation of subsurface flow interception and the lack of model testing and uncertainty analysis. One of the most fundamental limitations to developing improved models of road erosion and delivery is access to data of an appropriate standard.

This paper reviews knowledge of nitrogen and phosphorus generation from land use and export to waterways, including studies relevant to Australia. It provides a link between current and future modelling requirements, and the context for incorporation of this knowledge into catchment models for use by catchment managers. Selected catchment models used by catchment managers are reviewed, and factors limiting their application are addressed. The review highlights the importance of dissolved N and P for overland flow and groundwater pathways, for sheep, beef and dairy grazing land use. Consequently, the effectiveness of riparian buffers to remove N and P may not be adequate. Consideration of the effects of rainfall and hydrology, dissolved P and N losses from pastures and event-based catchment-scale loads are therefore important factors that should be incorporated into catchment models. The review shows that it is likely that nutrient losses under Australian dairying conditions have many similarities to worldwide studies. Catchment models need to represent the importance of event-based loads, intensively farmed land use, management and forms of nutrients. Otherwise there is a likelihood of either underestimating nutrient losses, or potentially overestimating the effectiveness of riparian buffers.

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Modeling techniques for estimating pollutant loadings to water bodies range from simple export coefficient and regression models to more complex mechanistic models. All export coefficient models and many complex mechanistic models rely on pollutant export coefficients to estimate pollution sources and transport in large watersheds. Typically, pollutant export coefficients are determined by monitoring small catchments or field plots to isolate individual landuse contributions. However, pollutant export coefficients derived from small catchment and field plot scale studies cannot be confidently used in catchment-scale water quality modeling. The objective of this paper is to present a framework to estimate the export coefficients of pollutants from commonly available in-stream water quality monitoring data. A combination of readily and freely available statistical, spatial and hydrological tools and a multiple regression methodology is proposed to estimate pollutant export coefficients. A case study from the Fuji River catchment, Japan is presented where export coefficients of organic matters and nutrients are estimated. Most of the estimated pollutant export coefficients are significant at a equal to 0.05 and the landuse categories used in the multiple regression models explained more than 85% variability in loadings. These results are encouraging especially given the pressing need to identify appropriate management practices to improve the water quality within the catchment. It is recommended to investigate further the required number of water quality monitoring stations, sampling frequencies and sampling duration of water quality constituents to enhance the robustness and usefulness of the proposed methodology.

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