Linking Runoff Source Areas and Nitrogen Fluxes Across Topographic and Land Use Gradients

Highlights Topography and soil depth influence runoff generation and nitrogen fluxes in urban areas. Urban areas act as ammonium sinks but nitrate sources to streams. Storm flows increase while baseflows decrease in urban areas. Fertilized lawns show lower runoff but higher nitrogen loss in wet locations. ABSTRACT. Land use in urban areas can alter natural hydrologic and chemical processes and make distinguishing the water quality impact of these land uses difficult. To quantify how hydrological and chemical processes are altered in urban areas, runoff was collected from 213 storm events over a six-year period, from three land uses and nine topographic positions, and analyzed for runoff, ammonium (NH 4 + -N), and nitrate (NO 3 - -N) flux using linear mixed effects models. Monitored land uses included fertilized lawns (FL), low maintenance (LM) areas, and urban wooded (FR) areas. Stream gages, installed at the entrance to the urban area and the watershed outlet, monitored the impact of the urban area on integrated stream water quality. Analysis revealed that the FL land use had higher N loss in areas with convergent topography, shallow soil, and higher wetness. In contrast, the FL had lower N loss than the LM or FR land uses in areas with deeper soil, less contributing area, and less runoff. Streamflow in the urban area increased under storm conditions and decreased under dry conditions compared to the undeveloped upper area of the watershed. NH 4 + -N loads measured at the stream gauges indicated that the urban area, in aggregate, acted as an NH 4 + -N sink, as evidenced by higher normalized loads from the forested upper watershed than after the stream flowed through the urban area. In contrast, NO 3 - -N loads increased substantially in the urban area of the watershed. These results show that urban land uses can both contribute to and mitigate nitrogen losses depending on site conditions. Keywords: Nitrogen loading, Runoff, Soil depth, Streamflow, Topographic index, Urban watershed.