A key aspect of N loss from urban systems is the link between landscape heterogeneity and variability in non-point (NPS) nitrogen (N) flux. High variability in N loss has been documented from urban systems. However, typical NPS studies characterize landscape heterogeneity by land use and weakly explain variability in stream N. Focusing on land cover, rather than land use, may better explain observed variability in N loss because land cover elements may better indicate drivers of N loss. In semi-arid urban systems, storm flow accounts for the majority of stream discharges, and residential irrigation contributes significantly to flows in the dry season. To address how landscape heterogeneity and hydrologic variability interact to affect water quantity and quality in urban streams, we examined the relationship between land cover, stream flows, and watershed N loss during wet and dry seasons in metropolitan Sacramento, CA. We analyzed fine-scale variation using the land cover classification HERCULES (High Ecological Resolution Classification for Urban Landscapes and Environmental Systems) in 8 residential watersheds. HERCULES classifies high-resolution aerial photographs into 5 elements: buildings, pavement, herbaceous and woody vegetation, and bare soil. We sampled the streams for discharge, NO3, and Total N during baseflow and storm events from 2010-2012.
Results/Conclusions
We found that N concentrations and export were higher during the rainy season compared to the dry season. Also, a seasonal flush was observed for N, with loads decreasing from the early part through the end of the rainy season. The proportion of pavement in the watersheds, a commonly used indicator of urban intensity, did not strongly correlate with increased levels of stream N, but did correlate with the magnitude and timing of flows during storms. N levels were positively correlated with herbaceous vegetation cover during the wet season, but negatively correlated with herbaceous vegetation and positively correlated with building cover during the dry season. Urban base flows were comparable between the wet and dry seasons, although residential lawn irrigation is the main source of baseflows during the dry season, while precipitation drives baseflows during the wet season. Different magnitudes of flow and water sources between storms and base flows may account for the observed change in relationship between herbaceous land cover and N export. Management to reduce NPS N loss from urban watersheds may be directed toward identifying sources and sinks associated with specific land cover, and also account for important seasonal dynamics which change drivers of N loss.