Hydrologic pathways and nutrient loading in the headwaters of the Western Lake Erie Basin

Study region: Western Lake Erie Basin (WLEB). The WLEB is a flat, artificially drained, and intensively managed agricultural region typical of the U.S. Midwest. Study focus: The study objective was to quantify hydrologic pathways using tracer-based hydrograph separation in a headwater watershed (19 km2) of the WLEB and assess relationships between water sources and fluxes and nutrient delivery. High-frequency measurement of two tracers (oxygen-18, 518O; electrical conductivity, EC) along with discharge and nutrient concentration (nitrogen, N; phosphorus, P) was conducted at the watershed outlet (2017-2020). New hydrological insights for the region: Results provide new insights into hydrologic and nutrient transport processes in artificially drained agricultural watersheds. Findings showed that 76 % discharge occurred during the winter and spring due to a threshold relationship with precipitation and soil moisture deficit. Hydrograph separation using 518O and EC provided similar estimates of event water (26 +/- 13 % and 26 +/- 11 %, respectively), but substantial differences (up to 21 %) were observed between tracers for individual storms. Annually, event water comprised 13-27 % of discharge. Nutrient loads during storm events were strongly correlated to discharge, as flowweighted mean concentrations (FWMCs) were consistent among storms and seasons. Both dissolved reactive P and ammonium-N FWMC were correlated to water sources. Findings suggest that decreasing discharge, decreasing the nutrient supply, and enhancing nutrient removal within ditch networks are needed to decrease nutrient loading drained watersheds in the WLEB.