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

Authors: Williams, Mark; Penn, Chad; King, Kevin

Submitted to: Journal of Hydrology: Regional Studies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2025
Publication Date: 3/8/2025
Citation: Williams, M.R., Penn, C.J., King, K.W. 2025. Hydrologic pathways and nutrient loading in the headwaters of the Western Lake Erie Basin. Journal of Hydrology: Regional Studies. https://doi.org/10.1016/j.ejrh.2025.102275.
DOI: https://doi.org/10.1016/j.ejrh.2025.102275

Interpretive Summary: The movement of water through the landscape often controls when, where, and how nutrients such as nitrogen (N) and phosphorus (P) to delivered downstream to water bodies such as Lake Erie. In this study, we measured water and nutrient transport in a small agricultural watershed of the Western Lake Erie Basin from 2017 through 2020 to better understand how the source of water influences nutrient concentrations and loads. Results showed that watershed discharge was controlled by both precipitation amount and soil moisture and, as a result, most (76%) of the discharge occurred during the winter and spring seasons. Most of the water discharge was comprised of "old water" or water that was stored in the soil profile or groundwater prior to a storm event. Nitrogen and phosphorus loading occurred primarily during storm events, with larger storms resulting in greater nutrient loads. Concentrations of dissolved P and ammonium-N were also related to the the source of water, with greater concentrations observed when "new water" or precipitation accounted for a larger percentage of discharge. Findings indicate that decreasing discharge through increased water storage, decreasing the nutrient supply via nutrient management, and enhancing nutrient removal within ditch networks are needed to decrease nutrient loading from headwater watersheds to Lake Erie.

Technical Abstract: Water flow pathways, residence times, and fluxes can have a substantial influence on nitrogen (N) and phosphorus (P) transport. The objective of this study was to quantify hydrologic pathways using tracer-based hydrograph separation in a drained, agricultural headwater watershed (19 km2) of the Western Lake Erie Basin and assess relationships between water sources and fluxes and nutrient delivery. High-frequency monitoring of two tracers (oxygen-18, d18O; electrical conductivity, EC) along with discharge and nutrient concentration was conducted at the watershed outlet (2017-2020). Discharge exhibited a threshold relationship with precipitation and soil moisture deficit and, as a result, most (76%) discharge occurred during the winter and spring. Hydrograph separation using d18O 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 from 13% to 27% of discharge. Nutrient loads during storm events were strongly correlated to discharge, as flow-weighted mean concentrations (FWMCs) were consistent among storms and seasons. Both dissolved reactive P and ammonium-N FWMC were correlated to water sources, with greater concentration observed with increasing event water. High-frequency datasets of discharge, water quality, and tracer fluxes provided improved insight into watershed hydrologic and nutrient transport processes and can be used to improve and evaluate process representation in simulations models. Findings indicate that decreasing discharge through increased water storage, decreasing the nutrient supply via nutrient management, and enhancing nutrient removal within ditch networks are needed to decrease nutrient loading from headwater watersheds to Lake Erie.