Abstract
Abstract. Streams are natural features in urban landscapes that can provide ecosystem services for urban residents. However, urban streams are under increasing pressure caused by multiple anthropogenic impacts, including increases in human population and associated impervious surface area, and accelerated climate change. The ability to anticipate these changes and better understand their effects on streams is important for developing and implementing strategies to mitigate potentially negative effects. In this study, stream flow was monitored during April–November (2011 and 2012), and the data were used to apply the Storm Water Management Model (SWMM) for five urban watersheds in central Iowa, USA, representing a gradient of percent impervious surface (IS, ranging from 5.3 to 37.1%). A set of three scenarios was designed to quantify hydrological responses to independent and combined effects of climate change (18% increase in precipitation), and land cover change (absolute increases between 5.2 and 17.1%, based on separate projections of impervious surfaces for the five watersheds) for the year 2040 compared to a current condition simulation. An additional set of three scenarios examined stream response to different distributions of land cover change within a single watershed. Hydrological responses were quantified using three indices: unit-area peak discharge, flashiness (R-B Index; Richards–Baker Index), and runoff ratio. Stream hydrology was strongly affected by watershed percent IS. For the current condition simulation, values for all three indices were five to seven times greater in the most developed watershed compared to the least developed watershed. The climate change scenario caused a 20.8% increase in unit-area peak discharge on average across the five watersheds compared to the current condition simulation. The land cover change scenario resulted in large increases for all three indices: 49.5% for unit-area peak discharge, 39.3% for R-B Index, and 73.9% for runoff ratio, on average, for the five watersheds. The combined climate and land cover change scenario resulted in slight increases on average for R-B Index (43.7%) and runoff ratio (74.5%) compared to the land cover change scenario, and a substantial increase, on average, in unit area peak discharge (80.1%). The scenarios for different distributions of land cover change within one watershed resulted in changes for all three indices, with an 18.4% increase in unit-area peak discharge for the midstream scenario, and 17.5% (downstream) and 18.1% (midstream) increases in R-B Index, indicating sensitivity to the location of potential additions of IS within a watershed. Given the likelihood of increased precipitation in the future, land use planning and policy tools that limit expansion of impervious surfaces (e.g. by substituting pervious surfaces) or mitigate against their impacts (e.g. by installing bioswales) could be used to minimize negative effects on streams.
Highlights
The hydrology of urban streams is responsive to human activities in the surrounding landscape (Arnold and Gibbons, 1996; Walsh et al, 2005; Wenger et al, 2009)
Calibration and validation results indicated that Storm Water Management Model (SWMM) was able to simulate hydrological processes in the small watersheds we studied, and was sensitive to the gradient in initial % impervious surface (IS) among them (Fig. 3)
We used SWMM to simulate hydrological responses of five headwater streams to increases in precipitation and urban land cover based on projections to the year 2040
Summary
The hydrology of urban streams is responsive to human activities in the surrounding landscape (Arnold and Gibbons, 1996; Walsh et al, 2005; Wenger et al, 2009). Wu et al.: Using SWMM to predict urban headwater stream responses to streams in more natural settings, urban streams are located in landscapes associated with less infiltration and more surface runoff, often leading to greater peak discharge and shorter peak discharge lag times (Anderson, 1970; Arnold and Gibbons, 1996). Black et al, 2011; Walsh et al, 2001) have commonly been reported for urban streams. Accelerated climate change (Denault et al, 2006), land use and land cover change (Grimm et al, 2008), and combinations of such changes (Nelson et al, 2009; Tong et al, 2012) are thought to be among the major driving factors leading to rapid degradation of urban stream systems
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