Hysteresis during storm events impacts stream sediment and nutrient load calculations

• HARP metrics quantify diverse Q-C relationships across ten water constituents. • Land use and local mechanisms influence Q-C hysteresis during storm events. • Biased sampling and strong hysteresis can significantly affect load accuracy. • HARP-based mechanisms help reduce uncertainty in storm-driven load estimates. Understanding flow (Q)-concentration (C) interactions is essential for providing effective catchment management strategies to reduce loads of contaminants from land to waterways. We tested if a non-linear Hysteresis Area, Residual, and Peak (HARP) analysis tool could improve Q-C relationships in streams and provide superior nutrient load estimates during storm events compared with linear or univariate models. Q-C patterns and load estimates were tested with data from three stations in a subtropical stream in South East Queensland, Australia, from 2007 to 2022. HARP revealed distinct site- and constituent-specific characteristics associated with hysteresis. Clockwise hysteresis patterns predominated at two stations where the subcatchment was dominated by intensive cropping, reflecting rapid nutrient mobilization during rising flows, while the station where subcatchment had greater forested area showed anticlockwise patterns, indicating a delayed nutrient response to rising flow. Strong inter-station differences in HARP parameters for nitrate-nitrogen (NO 3 -N) were likely due to stream interactions with groundwater, while ammonium-nitrogen (NH 4 -N) and phosphate-phosphorus (PO 4 -P) showed no significant differences among stations. Substantial errors in sediment and nutrient load estimates may occur when hysteresis effects are overlooked during storm events, particularly where there is an asymmetric hysteresis pattern between the rising and falling limbs of the hydrograph. Load estimates depend on both the sampling strategy and calculation methodology, with inaccuracies compounded when sampling is unevenly distributed during storms with strong hysteresis, and modified by subcatchment land use and groundwater contributions.