Reply on RC2

Joel Bostic

doi:10.5194/bg-2023-40-ac2

Abstract

Abstract. Water-quality impacts of streamwater nitrate (NO3¯) on downstream ecosystems are largely determined by the load of NO3¯ from the watershed to surface waters. The largest NO3¯ loads often occur during storm events, but it is unclear how loads of different NO3¯ sources change during storm events relative to baseflow or how watershed attributes might affect source export. To assess the role of stormflow and baseflow on NO3¯ source export and how these roles are modulated by hydrologic effects of land-use practices, we measured nitrogen (δ15N) and triple oxygen (Δ17O) isotopes of NO3¯ and oxygen isotopes (δ18O) of water in rainfall and streamwater samples from before, during, and after 8 storm events across 14 months in two Chesapeake Bay watersheds of contrasting land-use. Storms had a disproportionately large influence on the export of unprocessed atmospheric NO3¯ (NO3¯Atm) and a disproportionately small influence on export of terrestrial NO3¯ (NO3¯Terr) relative to baseflow in the developed urban watershed. In contrast, baseflow and stormflow had similar influences on NO3¯Atm and NO3¯Terr export in the mixed agricultural/forested watershed. An equivalent relationship between NO3¯Atm deposition on impervious surfaces and event NO3¯Atm streamwater export in the urban watershed suggests that impervious surfaces that hydrologically connect runoff to channels likely facilitate export of NO3¯Atm during rainfall events. Additionally, larger rainfall events were more effective in exporting NO3¯Atm in the urban watershed, with increased rainfall depth resulting in a greater fraction of event NO3¯Atm deposition exported. Considering both projected increases in precipitation amounts and intensity and urban/suburban sprawl in many regions of the world, best management practices that reduce hydrologic connectivity of impervious surfaces will likely help to mitigate the impact of storm events on NO3¯Atm export from developed watersheds.

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