Long-term monitoring of stormwater runoff and water quality in urbanized watersheds of the greater Phoenix metropolitan area, ongoing since 2008

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. The impacts of urbanization on aquatic systems are
well-studied, and frequently encapsulated in the ‘urban stream
syndrome’ (Walsh et al. 2005) that describes, among others, increased
nutrient loading and stream flashiness. However, there is some
evidence that aridland cities behave differently (Grimm et al. 2004,
2005), and the complex dynamics among catchment characteristics, storm
attributes, and runoff in highly urbanized settings of the Southwest
remains poorly understood. To enhance our understanding of stormwater dynamics and watershed
functioning in aridland, urban environments, the Central
Arizona–Phoenix Long-Term Ecological Research (CAP LTER) program began
monitoring stormwater runoff at the outflow of the Indian Bend Wash
(IBW) 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 sampler) is 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 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 10 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 by Hale et al.
(2014a, 2014b). 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. Four new sites that drain into
the Salt River were added in the winter of 2016. Three of the new
sites are located near downtown Phoenix in the Rio Salado habitat
restoration area, and the fourth catching runoff from numerous
freeways and entering the Salt River channel above the Tempe Town
Lake. Grimm N.B., Arrowsmith J.R., Eisinger C., Heffernan J., MacLeod A.,
Lewis D.B., et al. (2004) Effects of urbanization on nutrient
biogeochemistry of aridland streams. In: Geophysical Monograph Series.
(Eds R.S. DeFries, G.P. Asner and R.A. Houghton), pp. 129–146.
American Geophysical Union, Washington, D. C. Grimm N.B., Sheibley R.W., Crenshaw C.L., Dahm C.N., Roach W.J. and
Zeglin L.H. (2005) N retention and transformation in urban streams.
Journal of the North American Benthological Society 24, 626–642. Hale R.L., Turnbull L., Earl S., Grimm N., Riha K., Michalski G., et
al. (2014a) Sources and Transport of Nitrogen in Arid Urban
Watersheds. Environmental Science & Technology 48, 6211–6219. Hale R.L., Turnbull L., Earl S.R., Childers D.L. and Grimm N.B.
(2014b) Stormwater Infrastructure Controls Runoff and Dissolved
Material Export from Arid Urban Watersheds. Ecosystems 18, 62–75. Roach W.J. and Grimm N.B. (2011) Denitrification mitigates N flux
through the stream–floodplain complex of a desert city. Ecological
Applications 21, 2618–2636. Roach W.J., Heffernan J.B., Grimm N.B., Arrowsmith J.R., Eisinger C.
and Rychener T. (2008) Unintended consequences of urbanization for
aquatic ecosystems: a case study from the arizona desert. BioScience
58, 715–727. Walsh C.J., Roy A.H., Feminella J.W., Cottingham P.D., Groffman P.M.
and Morgan II R.P. (2005) The urban stream syndrome: current knowledge
and the search for a cure. Journal of the North American Benthological
Society 24, 706–723.