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Visual comparison of spatial patterns of annual suspended sediment loads estimated by two water quality modelling approaches

The Queensland Department of Environment and Resource Management is using the SedNet and E2 water quality modelling approaches to support government policy and natural resource managers in improving water quality. SedNet is designed to determine the long-term average annual sediment load, and does not deal with temporal variability. It includes hillslope erosion, gully erosion, and riverbank erosion, which enables land managers to undertake on ground works in areas of the landscape that generate disproportionate quantities of sediment. E2 is a daily time step model capable of modelling temporal variability in water quality as a result of management and/or climate changes. However, hillslope erosion, gully erosion, and riverbank erosion are not currently explicitly represented in E2 in which sediment generation is based on the concept of Event Mean Concentration (EMC) and Dry Weather Concentration (DWC) with user assigned values depending on factors such as land use, soil type, and topography. As a modelling framework, E2 is capable of housing alternative models for the same process. Both SedNet and E2 modelling approaches are based on node-link configuration of the stream network generated from pit- filled digital elevation models. This configuration allows the user to determine outputs from either model at any point of interest within the catchment. The objective of this study was to visually compare the spatial patterns of annual average sediment loads estimated by SedNet and E2 in relative terms. To our knowledge, a similar comparison of the two modelling approaches has not previously been done, and we believe that this work is a useful contribution in guiding model choice. The Burnett catchment, located in Southeast Queensland, Australia, has been chosen as the case study catchment for this work. In order to undertake a sub-catchment by sub-catchment comparison of model outputs, we intended to generate the same number of sub-catchments in E2 as those already generated in SedNet for a previous project. However, the version of E2 used for this study was unable to support as many sub-catchments as those in SedNet due to insufficient system memory. This issue has been addressed in the current version of E2 referred to as WaterCAST, which unfortunately was not available for this study. Despite the discrepancy in the number of sub- catchments generated, the similarity in the spatial patterns of average long-term suspended sediment loads shown in Figure 1 indicates that both modelling approaches generally identified the same generation hotspots within the Burnett catchment. Where there are differences (as shown in the highlighted areas), these could be explained by the difference in the structure and process representation of the two models. Given the fact that the two models are independent; the similarity in the spatial pattern of the resulting sediment generation is quite encouraging in terms of building confidence in the use of both modelling approaches.