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A key problem in unifying sediment assessment and management approaches is in defining the hierarchy of decisions within a management framework
. A basin-scale framework should be comprised of two principal levels of decision making; the first for basin-scale evaluation (site prioritisation) and the second for site-specific assessment (risk ranking). High priority, high risk sites and sites prioritised for management for socio-economic objectives should then be evaluated for management options. Although it is site-specific risks and objectives that will be managed, solutions may involve actions in other parts of the river basin (e.g., source control). A basin-scale assessment involves the balancing of a Conceptual Basin Model (CBM, which considers the mass flows of particles and contaminants, screening level assessment of sediment quality and archived data), and basin-scale objectives (BOs) to generate a Basin Use Plan (BUP). The Basin Objectives should define the ecological, regulatory and socio-economic goals for both the river basin (and its outlet to estuaries and the sea) and specific parcels of sediment. The development of a Basin Use Plan balances the CBM and the BOs, and should then result in a site prioritisation for further management that best meets the objectives of all stakeholders. On the other hand, site-specific assessment and management is characterised by tiered assessment and the determination of site-specific risk. Management options are driven by site-specific impact on BOs, site-specific risk, technical and economic feasibility and regulations. The proposed conceptual approach to basin-scale sediment management provides a possible frame-work for addressing the complexities inherent in managing sediments at both a basin-wide and site-specific scale
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An evaluation of the performance of a physically-based distributed model of a small Mediterranean mountain catchment is presented
. This was carried out using hydrological response data, including measurements of runoff, soil moisture, phreatic surface level and actual evapotranspiration. A-priori model parameterisation was based as far as possible on property data measured in the catchment. Limited model calibration was required to identify an appropriate value for terms controlling water loss to a deeper regional aquifer. The model provided good results for an initial calibration period, when judged in terms of catchment discharge. However, model performance for runoff declined substantially when evaluated against a consecutive, rather drier, period of data. Evaluation against other catchment responses allowed identification of the problems responsible for the observed lack of model robustness in flow simulation. In particular, it was shown that an incorrect parameterisation of the soil water model was preventing adequate representation of drainage from soils during hydrograph recessions. This excess moisture was then being removed via an overestimation of evapotranspiration. It also appeared that the model underestimated canopy interception. The results presented here suggest that model evaluation against catchment scale variables summarising its water balance can be of great use in identifying problems with model parameterisation, even for distributed models. Evaluation using spatially distributed data yielded less useful information on model performance, owing to the relative sparseness of data points, and problems of mismatch of scale between the measurement and the model grid.
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