Hydrological and Biogeochemical Investigation of Semi-Arid Urban Catchments: Assessing Opportunities and Performance of Green Infrastructure

Abstract

In arid and semi-arid environments such as the southwestern United States, water availability is the limiting factor in the stability and growth of environmental and human systems. This project addresses a fundamental challenge of cities located in semi-arid urban environments, where water managers must address the paradoxical issue of both drought and flooding. To address these challenges, cities such as Tucson, Arizona are implementing solutions such as green stormwater infrastructure in a decentralized manner throughout neighborhoods as street-scale basins and curb cuts with the goals of harnessing additional sources of water for societal use, reducing flood control issues, offsetting potable water use, and reducing urban heat island impacts. Taking advantage of the City of Tucson, Arizona as a living laboratory, this observational study investigates the opportunities and influence of green stormwater infrastructure on stormwater runoff and biogeochemical cycling on the urban subwatershed (neighborhood) scale via the development and analysis of runoff and hydrochemical datasets typically underrepresented in semi-arid urban environments, using a paired watershed study design. This project also invokes a coupled natural-human systems lens to characterize and evaluate the motivation, potential, and evolution of green stormwater infrastructure in Tucson. This study highlights the difficulties in assessing small-scale decentralized features influence on runoff responses in semi-arid urban watersheds with many complex and interacting landscape factors. The magnitude of differences of runoff response between the paired watersheds compared to the range of current green stormwater infrastructure implementation levels indicates there is likely a higher order control on this runoff response such as unlined channel reach between monitoring sites. Despite the short duration of storm runoff and the small watershed size, it appears that biogeochemical transformations like rapid soil microbial activity and respiration alter solution chemistry during episodic runoff. In addition, solute sourcing to runoff within the watershed area to the stream reach appear to contribute to changes in solution chemistry through solute additions. Investigations into the natural-human system of GSI in dryland cities describe a system in which a range of perceived benefits motivate a diverse set of stakeholders to collaborate and experiment with GSI implementation, leading to iterative changes in ecosystem provisioning and social learning.