Germany-Wide High-Resolution Water Balance Modelling to Characterise Runoff Components as Input Pathways for the Analysis of Nutrient Fluxes

Abstract

The input of nutrients into surface waters and groundwater is directly linked to runoff components. Due to the different physicochemical behaviour of nitrogen and phosphorus compounds, the individual runoff components have different significance as input pathways. Within the scope of the Germany-wide project AGRUM-DE, spatially differentiated runoff components were modelled with the water balance model mGROWA at a resolution of 100 m. The modelled distributed runoff components include total runoff, surface runoff, drainage runoff, natural interflow, direct runoff from urban areas, and groundwater recharge. Although the mGROWA model operates in daily time steps, modelled runoff components can be aggregated to mean long-term hydrologic reference periods—for this study, 1981–2010. We obtained good model agreement through the comparison of measured discharge from 298 river gauges against the spatial means of the modelled runoff components over their corresponding catchment areas. Therefore, the model results provide reliable input for input pathway-specific modelling of actual nutrient inputs as well as scenario analyses expected from the application of nutrient reduction initiatives. This ensures that any differences in the model results stem exclusively from differences in nutrient supply (fertilisation of the soils) and not from climatic effects, such as the influence of wet or dry years.