A mechanistic assessment of nutrient flushing at the catchment scale

Quantifying nutrient flushing mechanisms at the catchment scale is essential for model development and prediction of land use change and climate change effects on surface water quality. To date, the description of nutrient flushing at the catchment scale has been largely mechanistically weak. This paper mechanistically assesses the flushing mechanism of DOC, DON and DIN at the hillslope and catchment scale during two storm events, in a small catchment (WS10), H.J. Andrews Experimental Forest in the western Cascade Mountains of Oregon. Using a combination of natural tracer and hydrometric data, and end-member mixing analysis we were able to describe the exact flushing mechanism at the site. This mechanism involved vertical preferential flow to the soil–bedrock interface and then lateral downslope flow, with a finite source of DOC and DON in the organic horizon. Both specific UV absorbance (SUVA, 254 nm) and continuously measured raw fluorescence measured with a fluorometer increased during the storm events, suggesting DOC was more aromatic during stormflow compared to baseflow conditions. SUVA patterns in lateral subsurface flow from the hillslope and stream water in combination with hydrometric data enabled us to infer that the contribution of deep soil water and groundwater was higher during the falling limb compared to the rising limb of the hydrograph and contributed to the dilution of DOC, DON and SUVA values during storm events. Overall this study showed the value of using a combination of hydrometric data, natural tracer data, and in particular DOC quality indices such as SUVA and fluorescence to mechanistically assess nutrient flushing at the catchment scale.