There is a need to quantify the relationships between the extent and distribution of urban land cover features, stream networks, and hydrologic flows. Hortonian stream network properties and other geomorphologic characteristics have been consistently observed across several stream orders and correlate well with discharge in a diversity of undeveloped basins. Yet there is a lack of understanding about drainage network structure in urban settings. The objective of this research was to assess surface stream patterns within urbanized catchments in two cities with long-term social-ecological research programs, and determine if network relationships persist across stream orders in association with the proportion of urban development. Twelve subbasins were evaluated, six each in Baltimore, MD, and San Juan, PR spanning urban-rural gradients. Land cover was classified and quantified using automated object-based image analysis and visual interpretation of high resolution aerial imagery. Stream network structure was generated and quantified based on slope-dependent threshold analysis of 5 m and 1 m (Baltimore only) digital elevation models (DEMs) and field-verified locations of headwater streams, using the TauDEM toolbox. Bifurcation, length, and drainage area ratios were calculated per subbasin, compared against values reported in the literature, and analyzed in relation to percent impervious cover.
Analysis of surface network properties from the 5 m DEM revealed some distorted stream reaches and branching patterns following urbanization features (e.g., roads), but average network structural ratios of sub-basins in both cities fell within typical ranges observed in rural basins located in each region. Nor was percent impervious cover related to the network ratios in either city. Therefore, at the coarser topographic resolution modeled, underlying terrain features remain as dominant drivers of hydrologic networks, resulting in similar properties between urban and closely related natural watersheds. In contrast, preliminary findings from the 1 m DEM analysis indicate a stronger urban signal on drainage network structure in both cities. This suggests that to distinguish the effects of urbanization on structural patterns (e.g., hydrologic features), and by extension functional processes (e.g., stormwater runoff and the production of flood hazards) in social-ecological systems, there is a need for high resolution elevation data, knowledge of surface-subsurface infrastructure linkages, and algorithms that capture the complexity of natural and built landscape components. At present, only some US cities have this detailed information, and most cities in the tropics and global South do not, hindering the development of resilient management strategies in response to global environmental change.