Estimating subsurface nitrogen accumulation-depletion in catchments by input-output flow analysis

We use input-output analysis of nitrogen flows between various sources-sectors and natural waters in the Swedish Norrström drainage basin for investigating and bounding the implication range of some uncertainty sources for results of subsurface nitrogen accumulation-depletion in this basin. We quantify different possible base and extreme assumptions of nitrogen discharges and transport pathways from agriculture to surface and groundwater in the basin. The results are robust in showing considerable nitrogen accumulation-depletion flow interactions taking place between the basin's mobile water and accumulated nitrogen pools in soils, sediments and/or relatively immobile subsurface water zones for all different scenario assumptions. Similar scenario robustness is also found in resulting relative contributions of different active nitrogen source-sectors to nitrogen flows in natural water systems. In the Norrström basin, and possibly more generally, nitrogen accumulation-depletion flows to and from accumulated legacies for the future or from the past appear therefore to be more important for water quality than current nitrogen discharges from active source-sectors.