Hierarchical Regulation of Nitrogen Export From Urban Catchments: Interactions of Storms and Landscapes

Abstract

In urban catchments of arid central Arizona, we investigate how the export of mineral and organic nitrogen (N) in storm runoff is regulated by interactions between local landscape characteristics and broader scale storm features. First, we test whether N export is more a function of (1) processes that affect N concentration in runoff or (2) the propensity of the catchment to convey rainfall as runoff. With data pooled across catchments, the mass of N in export (load) is determined by processes regulating runoff N concentration. There are exceptions when catchments are examined individually, where N load from some catchments is determined by the hydrologic responsiveness of the catchment. Second, we investigate the relationship between N export and catchment features. Loads per catchment area were greater from more impervious catchments, probably because impervious catchments held more N in a mobilizable phase and conveyed more rainfall as overland flow. Loads per area were lower from larger catchments, possibly owing to more N-retention hot spots in larger catchments. Catchments with the greatest N exports were those with commercial land use, and loads decreased as development became less prevalent or as residential replaced industrial land use. Third, we investigated how catchment features moderated direct responses of N export to storms. Export was less correlated with storm features in catchments that were larger, more pervious, and less industrial. Results support an "N build and flush" hypothesis, which purports that there is little biotic processing of N deposited to arid, urban surfaces with little organic matter. The rate and duration of deposition determine the size of the mobile N pool. Any amount of rainfall capable of generating overland flow would entrain nearly all mobilizable N and export it from the catchment. Nonetheless, these results suggest that, even with daunting seasonal and interannual variability in storm conditions, material export can be reduced by managing intrinsic catchment features.

Sources and Transport of Nitrogen in Arid Urban Watersheds

“…Larger watersheds also had lower runoff coefficients and therefore retained more DIN, as noted by Lewis and Grimm. 43 Sources of NO 3 − in Urban Stormwater. The isotopic composition of NO 3 − varied significantly across rainfall, soil, and impervious surfaces (Figure 3 − in rainfall were significantly higher than both impervious surface and soil sources (Figure 3).…”

Land cover controls on summer discharge and runoff solution chemistry of semi-arid urban catchments

“…Given that the Sonoran desert is nitrogen limited (Westerman and Tucker, 1978;Whitford, 2002), we expected to observe nitrogen retention at LD and therefore low NO 3 and NO 2 runoff concentrations. However, this was not the case and the literature supports a two part explanation for the high NO 3 concentrations across sites, which can be extended to all R2 constituents: (1) solutes deposited during dry conditions accumulated within the catchment due to soil water limitations for biogeochemical activity, resulting in mobilization of accumulated solutes during rainfall and subsequent runoff (Lewis and Grimm, 2007;McCrackin et al, 2008) and (2) biogeochemical processes such as mineralization, nitrification and phosphorous retention increase in response to soil wetting, leading to accumulation of solutes in the soil between storm events and which are flushed during subsequent runoff (Austin et al, 2004;Welter et al, 2005). Both of these processes are likely to occur in the uplands and ephemeral stream channels.…”

“…Some conceptual models suggest that both, urban runoff quantity and quality, are primarily controlled by percent impervious cover (Arnold and Gibbons, 1996;Paul and Meyer, 2001;Schueler, 1994), while others indicate that urban runoff quality and quantity are largely controlled by the extent of catchment connectivity and the characteristics of the stormwater drainage system (Carle et al, 2005;Hatt et al, 2004;Meierdiercks et al, 2010;Ogden et al, 2011;Walsh et al, 2009). However, few studies have specifically addressed how urbanization alters runoff responses in semi-arid regions (for example: Asaf et al, 2004;Ishaq and Alassar, 1999;Jiries et al, 2001;Lewis and Grimm, 2007) where runoff and subsequent streamflow occur only in response to rainfall. A recent study by Gallo et al (2012), for example, showed that short term (hours) partitioning of rainfall controls the temporal patterns of urban hydrologic responses in semi-arid study catchments in Tucson, AZ.…”

Nitrate retention capacity of milldam-impacted legacy sediments and relict A horizon soils

“…In contrast, legacy-sediment-rich fill terraces have been shown to dampen N removal pathways in the long-buried relict soils which they overlie, while also acting as potential sources of nitrate (NO − 3 ) to waterways (Weitzman et al, 2014). Climate-driven export of N from watersheds is known to occur (Howarth et al, 2006;Lewis and Grimm, 2007;Kaushal et al, 2008aKaushal et al, , 2010Duncan et al, 2015), with N stored during "dry" years or seasons and flushed from watersheds during "wet" years or seasons. Such drying-rewetting cycles have been linked to regionwide pulses of high NO − 3 concentrations in tributaries of the Chesapeake Bay .…”