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.

城市化会显著改变流域（watershed）生态系统过程。土地利用变化与人类活动导致营养物质及其他物质的输入量增加，而土地覆被变化则改变了水文过程与物质的相应运移路径。上述变化会对流域过程及下游生态系统均产生影响。针对城市化对水生生态系统的影响已有大量研究，相关成果常被归纳为“城市河流综合征（urban stream syndrome）”（Walsh等，2005），该综合征涵盖了营养负荷升高、河流水文骤变等特征。不过，有研究表明干旱区（aridland）城市的表现有所不同（Grimm等，2004、2005），而美国西南部高度城市化区域内的集水区特征、风暴属性与径流之间的复杂动态机制仍有待深入阐明。

为加深对干旱区城市环境中雨水径流动态与流域功能的理解，亚利桑那中部-凤凰城长期生态研究计划（Central Arizona–Phoenix Long-Term Ecological Research, CAP LTER）于2008年启动了印第安本德冲刷渠（Indian Bend Wash, IBW）出水口的雨水径流监测工作。IBW是大凤凰城都会区的主要排水渠，汇水范围覆盖斯科茨代尔市大部分区域，同时也是盐河（Salt River）的支流。作为柔性工程（soft engineering）的典范，IBW流经斯科茨代尔市的河段主要由一系列人工湖、公园、步道、高尔夫球场、棒球场及其他非结构性设施构成，兼具为市民提供户外休闲场所与作为洪泛区的双重功能。Roach等（2008）与Roach及Grimm（2011）已详细阐述了这一新型生态系统独特的生物地球化学特征。

雨水采样工作设于IBW汇入盐河上游约0.6公里处，该点位与美国地质调查局（United States Geological Survey, USGS）的固定测流站（USGS 09512162 INDIAN BEND WASH AT CURRY ROAD, TEMPE, AZ）重合，可同步获取流量数据。研究采用ISCO® 6700型自动泵采样器（automated pump sampler），在多数产流风暴的水文过程线（hydrograph）内采集离散雨水样品。IBW出水口监测所获得的数据与技术积累，为后续美国国家科学基金会（National Science Foundation, NSF）资助的规模更大的雨水研究项目奠定了基础（DEB-0918457，NSF生态系统项目，2009-2013年）。

依托亚利桑那州雨水氮研究（Stormwater Nitrogen in Arizona, SNAZ）项目，研究团队在亚利桑那州斯科茨代尔与坦佩市布设了10个层级嵌套的城市雨水集水区，并为其安装了自动雨水采样器（ISCO® 6700型自动泵采样器）。其中11个点位加装了ISCO® 720型气泡式水位模块（bubbler modules），用于量化水位（进而推算流量），同时布设了ISCO® 674型翻斗式雨量计（tipping-bucket rain gauges）。这10个研究集水区的雨水基础设施类型跨度较大，涵盖了从老旧住宅区的高度工程化雨水设施到沙漠地区的非工程化冲刷渠，但土地利用类型均以住宅用地为主。与IBW出水口的采样方案一致，研究团队于2010年秋季至2012年夏季期间，在10个研究集水区的出水口处，针对多数产流风暴采集了离散雨水样品。此外，团队还在部分风暴事件中于部分点位采集了降雨样品，以辅助评估径流中物质的来源。该研究的成果已由Hale等（2014a、2014b）详细报道。

多数采样点位的监测工作在SNAZ项目资助周期结束后终止，但CAP LTER仍持续开展IBW出水口的长期径流监测。研究团队于2016年冬季新增了4个汇入盐河的监测点位：其中3个位于凤凰城市中心的里奥萨拉多（Rio Salado）栖息地恢复区，第四个点位收集多条高速公路的径流，并在坦佩镇湖上方汇入盐河河道。

参考文献：
1. Grimm N.B.、Arrowsmith J.R.、Eisinger C.、Heffernan J.、MacLeod A.、Lewis D.B.等（2004）《城市化对干旱区溪流营养生物地球化学的影响》，载于《地球物理专刊系列》（R.S. DeFries、G.P. Asner与R.A. Houghton编辑），第129–146页。美国地球物理联合会，华盛顿哥伦比亚特区。
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