From a drainage well to the catchment outlet - propagation of water quality signatures through a headwater agricultural catchment

Water quality signature of agricultural catchments is shaped by non-point and point sources contributing a wide range of solutes (e.g., nutrients, carbon, pesticides, emerging contaminants) and sediments to the drainage and stream network. In this presentation we evaluate storm event contribution of non-point sources in a small agricultural headwater (7.2 km², 58° 35' N and 16° 11' E ) dominated by clay soils, crop production and tile drainage. We used a combination of experimental and modelling data for points along the drainage and stream network, from individual drainage wells to the catchment outlet. For the 6 nested sampling locations, we used concentration-discharge (C-Q) metrics from both high-frequency in situ data and process-based model to aswer the following research questions: 1) does C-Q slope for solutes and sediments change signficantly along the drainage and stream network or is it a constant catchment-feature, 2) do the shape and direction of the C-Q relationships change significantly along the drainage and stream network in response to different contribution of flow pathways. Information about the contribution of different flow pathways was derived from a calibrated/validated HYPE model which differentiated surface, subsurface and tile drainage flow pathways. Our results showed that the contribution of different flow pathways changes from a tile drainage dominance for the drainage wells to a more balanced contribution of different flow pathways at the outlet . This led to more stable and chemostatic C-Q slopes at the outlet compared to steeper and more variable C-Q slopes at the drainage wells. We also showed that the changing contribution and timing of different flow pathways during storm events control the shape and the direction of the C-Q relationships for both solutes and sediments. Overall, our results showed that there was a surprisingly large variation in the C-Q relationships between different locations in the catchment despite its small size and fairly uniform land use.