Thursday, August 6, 2009

PS 74-184: Urban ecosystems: Alterations to peakflow, microclimate, and the natural environment

Jason A. Hubbart, University of Missouri


More than half of the world’s population now lives in urbanized ecosystems. Continued human population growth over the next several decades will place unprecedented demand on urban water resources, resulting in escalating water quality and supply challenges. Urbanization exacts significant impacts on stream ecosystems including increased magnitude and frequency of peak flows, altered microclimates and reduced biodiversity. Increased impervious surface area in urban settings reduces or eliminates soil infiltration and increases the amount of stormwater runoff delivered to stream channels. Stormwater routing networks in urban areas channelize runoff, reducing stormwater transit time. Stormwater flow serves as an important transport mechanism for non-point source pollutants, and impervious surfaces serve as conduits to flow, replacing soils and vegetation that would otherwise attenuate runoff and transport of pollutants. Stream channels typically broaden and deepen in response to increased volume, velocity and frequency of peak flows, leading to increased channel instability, accelerated erosion, and subsequent loss of habitat. Given these scenarios, urbanization can degrade freshwater resources, detrimentally impact in-stream and riparian biota, and destroy riparian wetland form and function. The following work quantifies peak flow and microclimate alteration due to urbanization in a large, multi-use watershed of the central U.S.


A long-term stream flow monitoring program was established in the Hinkson Creek Watershed located in Boone County, mid-Missouri during the fall of 2008. Hydroclimate stations were installed at five locations along Hinkson Creek within predominantly forest, cropland, and urban environments. Continuous stream stage and flow data from these sites were used to quantify peak flow in different environments of the watershed. Preliminary results indicate that forested environments tend to attenuate peak flows from small precipitation events (< 0.80 mm), whereas events of similar magnitude in urban settings cause spikes in flow and a flashier hydrograph. With larger precipitation events (> 1.0 mm), the time from peak rainfall to peak flow is decreased by as much as 13% in urban settings accompanied by at least 4 cm higher peak flows and 15-20% greater flow volume. Average air temperatures in the urban region exceed the forested region by at least 15% during winter months, holding important implications for snow melt and runoff, peak flow and ecosystem processes. Continued monitoring and analyses will more precisely quantify the effects of urbanization on these phenomena and will provide critical information for the sustainable management of urban natural resources.