LOIS in-stream water quality modelling. Part 1. Catchments and methods.
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 2,088 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.