Urban Semi-arid Catchments Hydrologic and Biogeochemical Influences of Stream Substrate and Green Stormwater Infrastructure

Urbanization has unique impacts on the non-perennial waterways present in arid and semi-arid climates. Here we investigate the impacts of urbanization and green stormwater infrastructure on the hydrology and water quality of several streams and across scales in southern Arizona. Hydrologic data from a broad set of undeveloped and urban catchments show a tripling of runoff (fraction of rainfall converted to discharge) due to urbanization at smaller spatial scales (roof to lot scale; ~10m2); at the small catchment scale (~106 m2) runoff also occurs more frequently and for a longer time-period. At larger spatial scales (> 109 m2), due to a combination of relatively less urban land cover and infiltration losses into sandy streambed sediments, streamflow differences on a per unit area basis are minimal. These hydrologic shifts are likely due to urban impervious cover increasing runoff, connected imperviousness creating a flow network facilitating longer flow duration and stream bed infiltration losses at the largest spatial scale. These observed increases in runoff have helped spur an interest in the deployment of green stormwater infrastructure (GSI) to meet water resource needs, provide urban shade, reduce flooding hazards, and improve water quality. Intensive observations of two streams in Tucson indicate that at current levels of deployment (<0.1% total watershed area) the impact of GSI on both streamflow volume and water quality is considered minimal. Modeling results indicate greater deployment (approximately 2x current levels) may lead to decreased streamflow volumes with unknown impacts on water quality. Statistical analyses of observational data align with these modelling results. Observatoinal data does show the potential influence of stream substrate on water quality. Sandy bedded streams as opposed to concrete-lined streams do appear to reduce streamflow volumes and promote carbon and nitrogen transformations during runoff including increases in ammonium and decreases in total nitrogen during flow events. These results offer a window into how urbanization alters the hydrology and biogeochemistry of semi-arid catchments and indicates that urbanization may increase streamflow and decrease the biogeochemical reactivity of the landscape. Critically the results also indicate that a broad view of green stormwater infrastructure should include both built systems and the natural sandy washes present in most arid and semi-arid climates.

This project was funded by an NSF CNH grant (NSF 1518376) and by the UWIN Urban RCN (NSF 1444758).