Summary

Urbanization dramatically alters watershed ecosystem processes. Land-use change and anthropogenic activities contribute to increased inputs of nutrients and other materials, while changes to land cover alter hydrology and the corresponding movement of materials. These changes have ramifications for both watershed processes and downstream systems. While the impacts of urbanization on aquatic systems are well-studied, there is some evidence that aridland cities behave differently (Grimm et al. 2004, 2005) from more mesic systems. As such, the complex dynamics among catchment characteristics, storm attributes, and runoff in highly urbanized settings of the Southwest remain poorly understood.

To enhance our understanding of stormwater dynamics and watershed functioning in aridland, urban environments, the Central Arizona–Phoenix Long-Term Ecological Research program began monitoring stormwater runoff at the outflow of the Indian Bend Wash in 2008. The IBW is a major drainage in the greater Phoenix metropolitan area, draining much of the City of Scottsdale, and a tributary to the Salt River. A model of soft engineering, the IBW as it runs through the City of Scottsdale is comprised largely of a series of artificial lakes, parks, paths, golf courses, ball fields and other non-structural elements designed with the dual roles of providing outdoor amenities to the City residents and as a floodplain. A unique biogeochemistry of this novel system is detailed by Roach et al. (2008), and Roach and Grimm (2011).

Stormwater sampling is conducted near the outflow of the IBW ~0.6 km above its confluence with the Salt River. The sampling location coincides with a permanent USGS gauging station (USGS 09512162 INDIAN BEND WASH AT CURRY ROAD, TEMPE, AZ) that provides corresponding discharge data. Automated stormwater sampling equipment (ISCO® 6700 automated pump samplers) are used to collect discrete stormwater samples throughout the hydrograph of most runoff-generating storms.

Data and expertise garnered by the stormwater monitoring near the outflow of the IBW helped pave the way for a more expansive stormwater research effort facilitated by a leveraged grant from the National Science Foundation (DEB-0918457, NSF Ecosystems, 2009-13). Through the Stormwater Nitrogen in Arizona (SNAZ) project, ten hierarchically nested urban stormwater catchments in Scottsdale and Tempe, AZ were instrumented with automated stormwater samplers (ISCO® 6700/6712 automated pump samplers). A subset of those 11 locations were fitted with bubbler modules (ISCO® 720 bubbler modules) for quantifying water height (and subsequently discharge), and tipping-bucket rain gauges (ISCO® 674). The ten study catchments differed in type of stormwater infrastructure, spanning a continuum from highly engineered stormwater infrastructure in older residential areas to non-engineered washes in the desert, but not in land-use type (land use in all study catchments is predominantly residential). As per sampling near the outflow of the IBW, discrete stormwater samples were collected from most runoff-generating storms at the outflow of the 11 study catchments from the fall of 2010 through the summer 2012. Rainfall samples were collected at a subset of the locations during several storms to provide data that would contribute to an assessment of sources of materials in runoff. Results of this study are detailed in publications by Hale et al. listed below. Sampling at most locations ceased at the end of the SNAZ award period, but the CAP LTER continues its long-term monitoring of runoff near the outflow of the IBW.

Methods

Sample collection and analysis

Stormwater samples are collected from the field sampling location within 12 hours of a storm, and transported to CAP LTER laboratory facilities at Arizona State University (ASU) for processing and analysis. The suite of analytes and the instruments used to perform analyses have changed during the course of the monitoring. Analysis and instrument details are included in the ‘analysis’ table that is available for download. In all cases, dissolved analytes reflect analysis following filtration through ashed (550℃, ≥ 1 h) GF/F (0.7-µm pore size) filters. Similarly, particulates reflect analysis of materials retained (retentate) following filtration through GF/F filters. Holding times and preservation methods are in accordance with guidelines established by the American Public Health Association (APHA. 1998. Standard Methods for the Examination of Water and Wastewater, 20th edn. APHA, NY). Identical methods are employed for the sampling and chemical analysis of rainfall samples, except that only a single, bulk sample of rainfall was collected for any given site/storm.

Discharge

Except at IBW where discharge is available from the USGS, water stage-height was measured with ISCO® 720 bubbler modules that were installed in concrete channels, box sections, or pipes to facilitate development of depth-discharge rating curves. Rating curves were developed using the Manning’s Equation to calculate discharge (Q) from the flow-stage measurements. At some locations, irregularities in the ground-surface profile or aberrant water-height values resulted in unrealistic discharge estimates. Both the raw discharge values as calculated from water height and the Manning’s Equation (discharge.discharge) and these ‘corrected’ discharge estimates (discharge.discharge_edited) are included in the discharge table that is available for download.