Quantifying nutrient transfer pathways in agricultural catchments using high temporal resolution data

There are uncertainties in the definition of phosphorus (P) and nitrogen (N) transfer pathways within agricultural river catchments due to spatiotemporal variations such as water recharge and the farming calendar, or catchment soil and hydrogeological properties. This can have implications for mitigation policies. This study combined detailed pathway studies with catchment integrated studies to characterise N and P transfer pathways for four agricultural catchments with different land management, soil drainage and geology. A Loadograph Recession Analysis (LRA) method is introduced, to identify and quantify integrated delivery transfer pathways of total oxidised nitrogen (TON), total reactive phosphorus (TRP) and total phosphorus (TP). High temporal resolution river discharge and water quality measurements from a large runoff event (and recession) were used. In two catchments with well drained soils, below-ground delivery pathways of TON represented up to 97% of the total flow event load, and up to 63% of the TRP and TP load. In these catchments, hydrological quick flow pathways were only 2–8% of total flow but were efficient in delivering P (up to 50%). Two other catchments had poor to moderately drained soils where up to 55% of the hydrological pathways were quick flow. This quick flow delivered up to 88% of the event flow P load but background groundwater flows were apparently mixed with point source signals. Results suggest that, in catchments with permeable soils and geology, subsurface pathways will need to be considered for mitigation strategies for both diffuse N and P delivery and measures that target surface transfer pathways such as riparian buffer strips may be ineffective. In such catchments, long chemical recessions from storm events may prolong impacts on the ecological status of receiving rivers.

► We introduce a new method for quantifying N and P transfer pathways in agricultural catchments. ► High temporal resolution data of surface water was coupled to groundwater data. ► Transfer pathways of N and P transfer were characterised. ► Subsurface pathways of both N and P need consideration for mitigation strategies. ► Measures that target surface transfer pathways may be ineffective in some cases.