Hydrologic and hydrographic convergence in urbanizing landscapes

Background/Question/Methods

Urbanization involves major re-shaping of hydrologic systems, both intentionally via the construction of water infrastructure, and unintentionally via the varied hydrologic consequences of land conversion.  The effects of urbanization on recipient stream ecosystems, including habitat loss, eutrophication, and alteration of flow regime, are well-documented, but fewer studies have addressed the consequences of urbanization for aquatic systems at the landscape scale (‘hydroscapes’).  Since urbanization can result in major changes in the abundance, distribution, and characteristics of surface water bodies (hydrography) of landscapes, we view this as a pressing knowledge gap in the study of urban systems.  Here we present a detailed description of the spatial structure and temporal trajectory of an entirely man-made hydroscape: the South Florida Urban Lake District; and test the broader hypothesis that hydrography of diverse landscapes is homogenized by the process of urbanization.  We use both existing hydrographic data and new observations from high-resolution landscape mapping to describe to aggregate area, size distributions, and connectivity of different surface water bodies (lakes, streams, and wetlands).  Our study sites include the metropolitan statistical areas (MSAs) of Baltimore, Boston, Los Angeles, Miami, Minneapolis, and Phoenix.  Historical aerial photographs were used to evaluate the history of landscape transformation and lake construction within our more intensive study of the South Florida Urban Lake District.  

Results/Conclusions

We found that the South Florida Urban Lake District includes ca. 10,000 man-made lakes, ranging in size from 100 m² to hundreds of hectares, with lakes densities (and areal cover) reaching higher levels in the western portions of Miami-Dade, Broward, and Palm Beach Counties, which were historically part of the Everglades.  Trajectories of lake construction match trends in population growth.  We found no relationship between lake age and size or shape (measured as shoreline development factor), but did observe that lake size follows a clear pareto distribution with an exponent very close to unity.  In our cross-site comparison, we find that urban landscapes do converge toward a characteristic hydroscape as measured by the total area of surface water, a pattern largely driven by the extreme dry and wet end-members of Miami and Phoenix. However, the specific characteristics of urban hydroscapes (such as size-frequency distributions, water body shape, and relative abundance of different water body types) appears to have strong legacy effects and idiosyncratic trajectories of change.