Effects of urban stormwater infrastructure on surface hydrology and nutrient export in dissolved and particulate forms

Over recent decades urbanization has occurred rapidly, especially in arid regions of the USA. During this period, major changes in ecosystem structure have occurred, including changes in land cover and the construction of stormwater infrastructure which in some instances increases flow connectivity and in others decrease it. In this study we investigate the effects of urban stormwater infrastructure on surface hydrological processes and the contribution of dissolved and particulate forms of nutrients to total nutrient export from urban catchments within the Central Arizona Phoenix LTER study area. Nested catchments were instrumented to monitor flow and water quality in the Indian Bend Wash Catchment, Scottsdale, AZ, ranging in spatial scale from 6 ha, to >17000 ha. At the smallest spatial scales, catchments of comparable size and land-use cover represent different types of stormwater infrastructure, while at spatial scales of >100 ha the stormwater infrastructure draining catchments is heterogeneous.

Results show that catchments with highly connected stormwater infrastructure such as pipes require a low amount of rainfall to generate a runoff response at the catchment outlet. Therefore, within these catchments, frequent flushing of both dissolved and particulate forms of nutrients occurs. The combination of high flow velocities and frequent flushing renders the transport of materials within these highly connected catchments supply limited. In contrast, in catchments with highly disconnected stormwater infrastructure (such as retention basins and to a lesser extent, pervious washes), a larger amount of rainfall is required to generate a runoff response at the catchment outlet that is less flashy than highly connected counterparts. Across all sites, at high discharges the proportion of nutrients in particulate forms increases because of the increased capacity of flow to entrain and transport particulate organic material and sediment. The total export of dissolved and particle-bound materials from catchments increases with spatial scale, but export of materials per unit area actually decreases with an increase in spatial scale. We surmise that more frequent flow events occurring at small spatial scales redistribute particulate materials within catchments, which are temporarily stored and exported from the catchments during high-magnitude, infrequent flow events at that occur over larger spatial scales. While studies of urban hydrology tend to focus in impervious surface cover, results from this study have demonstrated that urban stormwater infrastructure has a controlling effect on surface hydrological processes, which further affects the amount and type of nutrient export within urban catchments.