LOIS in-stream water quality modelling. Part 2. Results and scenarios

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%).