Urban Growth Trajectories as a Window into Understanding Hydrologic Changes

Abstract:

Understanding the causes of declining water quality and aquatic communities in urban streams requires a clear picture of the spatiotemporal dynamics of human infrastructure systems as a watershed undergoes urbanization. Urban watershed studies typically employ one of two approaches, 1) long-term watershed studies that examine changes over time in one watershed or 2) an urbanization gradient that examines changes during a snapshot in time among watersheds spanning rural to urban land use. We used both approaches to investigate hydrologic changes associated with urbanization in nine U.S. cities. We asked do development trajectories and hydrologic responses due to urbanization vary among cities and how does the approach used influence the interpretation of results. Daily mean streamflow records from USGS flow gages were used to calculate a range of ecologically relevant hydrologic metrics from 2000 to 2012 for the gradient approach and since the 1930’s and 1940’s in six long-term watersheds. Results from the urbanization gradient approach indicated similar, linear development trajectories among the nine cities but heterogeneity in the magnitude of hydrologic changes. In contrast, the long-term approach indicated development trajectories could be characterized by three stages that varied in development intensity, timing, and length. Results from the long-term watersheds indicated linear and rapid, non-linear increases in flow metrics across the time series. Regression analysis indicated that the intensity of urbanization during the peak growth period was the strongest driver of the magnitude of observed stream flow changes. Overall, the long-term approach provided a much clearer window into not only the type of hydrologic changes during watershed urbanization, but also the timing of and potential triggers driving flow changes.