Seasonal solute dynamics across land uses during storms in glaciated landscape of the US Midwest

Abstract

Summary Considering the importance of solute exports during storms in annual nutrient budgets at the watershed scale, it is critical to understand the impact of seasonality and differences in land use on watershed hydrological and biogeochemical response to storm events. This study investigates the hydrological response to storms and chloride, nitrate and dissolved organic carbon (DOC) export dynamics during one spring storm and two summer storms in an agricultural catchment (watershed A) and a mixed agricultural/urban catchment (watershed M). Drier antecedent moisture conditions in summer were associated with lower runoff ratios during summer storms than during the spring storm studied. Watershed M also had a much flashier hydrologic behavior than watershed A, suggesting that moderate differences in land use significantly affected each watershed hydrological response to storm events. DOC concentrations were not significantly different between watersheds A and M; however, nitrate and chloride concentrations and export rates were, respectively, higher and lower in watershed A than M. Regardless of land use, nitrate concentrations were also consistently higher during the spring storm than during the two summer storms studied. Although DOC concentrations varied seasonally, precipitation characteristics appeared to be the primary controls on DOC concentration during storms. Generally, chloride and DOC concentrations, respectively, decreased and increased along with discharge during storms. No clear concentration patterns relative to discharge were observed for nitrate in the agricultural watershed. Nitrate concentrations tended to increase following the peak in discharge in the mixed land use watershed. Analysis of stream DOC specific UV absorbance (SUVA) indicated a sharp increase in stream DOC aromaticity during storms regardless of land use and seasons, suggesting a shift in the source of DOC to the stream during storms from low aromaticity DOC at baseflow to highly aromatic DOC during storms. Overall, although many variables can contribute to differences in solute flushing patterns between the watersheds studied, this study indicates that moderate differences in land use (85% agriculture in the agricultural watershed, and 33% agriculture/33% urban/17% pasture/13% forest in the mixed land use watershed), storm characteristics and seasonality (spring vs. summer) can significantly impact watershed response to precipitation and patterns of chloride, nitrate and DOC exports during storms at the watershed scale. Owing to the importance of solute export dynamics in streams during storms in annual solute budgets, we argue that more studies investigating the impact of seasonality and differences in land uses on watersheds’ hydrological and biogeochemical responses to storms should be conducted in a variety of geomorphic settings.