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A method of model evaluation is presented which utilises a comparison with a benchmark model. The proposed benchmarking concept is one that can be applied to many hydrological models but, in this instance, is implemented in the context of an in-stream water quality model. The benchmark model is defined in such a way that it is easily implemented within the framework of the test model, i.e. the approach relies on two applications of the same model code rather than the application of two separate model codes. This is illustrated using two case studies from the UK, the Rivers Aire and Ouse, with the objective of simulating a water quality classification, general quality assessment (GQA), which is based on dissolved oxygen, biochemical oxygen demand and ammonium. Comparisons between the benchmark and test models are made based on GQA, as well as a step-wise assessment against the components required in its derivation. The benchmarking process yields a great deal of important information about the performance of the test model and raises issues about a priori definition of the assessment criteria. [ABSTRACT FROM AUTHOR]

The catchment and river modelling undertaken within the UK Land Ocean Interaction Study programme represents the most extensive consistent exercise of this type undertaken in the UK. The calibrated model provides a quantitative assessment of the water and chemical fluxes transported by the rivers Derwent, Yorkshire Ouse, Wharfe, Aire, Don and Trent into the Humber estuary. These fluxes have been partitioned according to source (diffuse or point) for each basin, and a number of determinands. Total nitrogen from diffuse sources ranged from 92% on the largely agricultural Derwent to 38% from the industrial Don catchment. Even on catchments dominated by diffuse sources, point sources can contribute most during summer months. A quantitative assessment of the effect of in-stream processes suggests that denitrification can remove up to 40% of the total nitrogen entering the river system. A climate change scenario representing the 2050s revealed major changes in flow patterns, with all rivers showing a faster return to zero soil moisture deficit in the autumn and increased winter runoff, but a mixed picture of enhanced and reduced summer flows. Many associated fluxes may also increase, but long-term mean concentrations reduce because of the increased flows. The effect of the scenario on a river ecosystem classification was minor. Fertilizer reduction scenarios showed that even a large reduction in N applied as fertilizer (50%) resulted in much smaller reductions in the load delivered to the Humber estuary (6–16%). [Copyright &y& Elsevier]

One achievement of the UK Land–Ocean Interaction Study (LOIS) was to link dynamic biogeochemical models of different domains, e.g. rivers, estuaries and coastal waters, and to use the linked model to investigate possible changes from the current status that might occur in the future, for example as the result of climate change. The Climate, Hydrochemistry and Economics of Surface-water Systems (CHESS) project has taken the LOIS methodology forward by exploring possible impacts of climate change on the water quality of European rivers, with the purpose of informing future catchment management. This was achieved by the application of a standard modelling framework to a set of five European catchments located in Finland (River Vantaa), United Kingdom (Yorkshire Ouse), Belgium (Dender), Italy (Enza) and Greece (Pinios). Baseline conditions were simulated using existing meteorological data from the period 1961–1990, and in all cases the modelling framework was able to reproduce key features of the flow and water quality regimes of the study catchments. The modelling framework comprised two models. The Soil Water Assessment Tool (SWAT) was used to simulate water and chemical fluxes, primarily nutrients and sediment, generated from diffuse areas and thereby provide sub-catchment inputs to an in-stream water quality model, the Quality Evaluation and Simulation Tool for River Systems (QUESTOR). QUESTOR integrated the diffuse runoff along the channel network, together with point source discharges from industry and sewage treatment works, and water abstractions for public supply, industry and agriculture. The modelling framework has been used for the baseline conditions, along with a set of six climate scenarios. These comprised four scenarios derived from different general circulation models (GCMs) representing the 2050s, and three scenarios from the same GCM representing the 2020s, 2050s and 2080s, with one scenario in both groups. Results have been explored using a range of measures to reflect the different stakeholder interests within and beyond the catchment. The results reflect the variability in present day conditions (e.g. climate, soils, agriculture and industry), and the variability in the climate scenarios, both between-catchment and between-scenario. Specific examples are: the loss of the spring snowmelt causing high flows on the Vantaa; a decrease in the already very low summer flows on the Pinios; and significant seasonal changes in nutrient losses, even though the annual change is small. [Copyright &y& Elsevier]

The catchment and river modelling undertaken within the UK Land Ocean Interaction Study (LOIS) programme represents the most extensive consistent exercise of this type undertaken in the UK. The calibrated model provides a quantitative assessment of the water and chemical fluxes transported by the rivers Derwent, Yorkshire Ouse, Wharfe, Aire, Don and Trent into the Humber estuary, i.e. a total drainage area of almost 18 000 km2 (almost 15% of the area of England) with 2088 km of modelled river. The model simulated flow, temperature, pH, suspended sediment, dissolved oxygen, biochemical oxygen demand, particulate organic nitrogen, ammonium, ammonia, nitrite, nitrate, zinc (dissolved and particulate), lindane and atrazine. For all rivers, except the Trent, simulations were for the period 1985–1995; the simulation period for the Trent was 1998–1995. The modelling used routine monitoring data collect by the England and Wales Environment Agency, for both calibration and validation, with other data sets mainly collated by the LOIS River-Atmosphere Coastal Study (RACS) data centre (e.g. land use, soils, river network). A stepwise model development started with a basic water quality model driven by observed in-stream water quality, and resulted in the development of a LOIS specific version of the in-stream model Quality Evaluation and Simulation Tool for River-systems that could be used to explore scenarios. The performance of the model was demonstrated by the reproduction of long-term statistics describing loads and concentrations, together with graphical output in the form of time series and distributions. The simulations were subsequently used to drive a linked catchment-river-estuary-coast-sea model covering the full domain of the RACS area. [Copyright &y& Elsevier]