Convergent Surface Water Distributions in U.S. Cities

Based on patterns in US cities, we conclude that urbanization causes the convergence of surface water abundance at broad scales. Surface water abundance in the undeveloped land of this study followed previously observed continental scale patterns in hydrography (Downing and others 2006); however, the surface water abundance of the urban landscapes did not conform to these hydrographic patterns. Continental scale variation among urban hydroscapes was far less than variation between undeveloped hydroscapes. In addition, the extent and direction of differences between urban and surrounding undeveloped hydrography appear to be related to the broad geographic patterns in climate and topography. Urban land in dry regions had greater abundances of surface water than surrounding undeveloped land, whereas urban land in very wet regions had lower abundances of surface water than their surroundings. Only by examining patterns at broad spatial scales are we able to understand the role of climatic, physiographic, and anthropogenic drivers of surface water distributions (Heffernan and others 2014; Thorp 2014).

The convergence of surface water abundance may also cause the convergence of surface water characteristics. Previous studies on urban stream loss document a preferential loss of smaller water features (Elmore and Kaushal 2008; Roy and others 2009; Larson and Grimm 2012). Urban water bodies tend to be more moderately sized and less connected to surface channels than their counterparts in undeveloped lands (Steele and Heffernan 2014). These differences in physical form may reflect the relative change in removal costs and value with size, certain functional needs, or aesthetic preferences. As a whole, the convergence of water body sizes suggests a reshaping of urban hydroscapes that is more pervasive and nuanced than simply adding or removing surface water from the landscape.

This study adds to the growing evidence that urbanization is a homogenizing process (Groffman and others 2014). Although cities are highly heterogeneous at local scales (Cadenasso and others 2007), the standardization of design, construction, and land use creates urban ecosystems that are broadly similar across large regions (Pouyat and others 2003, 2007; Pickett and others 2011; Seto and others 2012). Such similarities lead to urban growth and development patterns that converge toward a specific form (Seto and Fragkias 2005; Batty 2008). Ecological communities in cities are homogenized by the intentional and accidental shuffling of flora and fauna by commerce and through planting, which introduces a common pool of cosmopolitan species to similarly structured urban environments (McKinney 2006; Lososová and others 2012). Our data suggest that homogenization of urban environments extends beyond changes in land cover and species composition to include transformation of the land’s shape, and indeed, whether it is even dry land at all.

As predicted, increasing development intensity led to greater convergence of surface water abundance. We primarily attribute this pattern to the increased value of dry land for building, where land is scarce and demand for development is high. Alternatively, this pattern may also reflect a legacy of land use. Most urban cores in the US were established as centers of business and industrial production, activities for which water bodies and streams are of little use or value. As manufacturing industries relocate and city centers develop service oriented economies with larger residential populations and mixed land uses, we may observe a reincorporation of surface water into high density development. Surface water, such as park ponds and streams, serve as recreational amenities for urban residents and tourists and therefore are likely more highly valued in this type of urban center (Abbott and Klaiber 2013). Regardless of the drivers, it is important to note that the difference between urban and undeveloped land was far greater than the differences between the intensity classes for both water body coverage and channel density. Given that most urban development is low intensity, the majority of surface water in a city may still be substantially different than its original condition.

This study provides a unique examination of urbanization across a large number of cities and range of population sizes. Although many aspects of cities vary predictably with population (Bettencourt and others 2007), we find the population of cities only indirectly affects hydrographic characteristics. Cities with smaller populations tended to lack the high intensity urban development, where observed changes were most pronounced and covered smaller areas; however, the hydrography of other land cover types (open, low, and medium intensity) was indistinguishable between large and small cities. This finding is important given that the number of small towns far exceeds the number of large cities. Thus any positive or negative effects resulting from urban hydrographic convergence will affect a much larger, extensive area.

In both absolute terms and relative to lands surrounding cities, urban hydroscapes are both structurally and spatially complex, and exhibit strong geographic patterns. When the differences in both water body number and areal coverage for each MSA were observed in tandem, cities with minimal surface water abundance tended to have both a larger areal coverage and numeric density, although a few exceptions did not follow this pattern. Across the rest of the US, a large number of cities fell into the fragmented or consolidated categories. Cities in the fragmented category (ΔA_WB < 0, ΔN_WB > 0) were mostly located along the southern coastline, where expansive areas of wetlands are prominent in undeveloped lands. In comparison, the urban water bodies covered less area, but were greater in number. This fragmented pattern may reflect the drainage and breakup of these wetland tracts, as observed in case studies (Du and others 2010), or may reflect selection of sites around or between those features. The compensatory mitigation of wetland losses predominantly leads to wetland reconstruction outside of the affected watersheds (Kettlewell and others 2008). Similarly, large reservoirs that serve as municipal water supplies and flood control are often located outside of developed areas. In effect, relocation accumulates and consolidates water lost within the urban core elsewhere, but at this time its contribution to the observed patterns is unknown.

The consolidated pattern was particularly strong in the Midwest and Plains cities. Here water bodies in the urban land covered a larger area, but were fewer in number compared to those in undeveloped landscapes. Although we tried to minimize the presence of other land covers within the undeveloped area, it is possible that the consolidation pattern of the mid-section of the US reflects a legacy of agricultural development in the undeveloped land cover. Artificial drainage and the presence of agricultural farm ponds could generate a pattern of small areal coverage and high numbers of water bodies (Skaggs and others 1994; Downing and others 2006). Though we cannot determine agriculture’s contribution to these patterns with certainty, we still observe convergence at the US scale even when agriculture is included in the surrounding land estimate. This result suggests that urban hydrography is not simply inherited from an agricultural legacy, and highlights the need for investigation of agricultural hydrography at macro-scales, as well as how and where urban development incorporates or alters those characteristics.

Urban hydrographic patterns likely reflect two main mechanisms: alteration and choice of location for building. Alteration includes construction, removal, or relocation of surface water features during urban development. Direct observation from previous, more detailed case studies of urban hydrographic change (Elmore and Kaushal 2008; Roy and others 2009; Du and others 2010; Larson and Grimm 2012) suggests that hydrographic alteration is a dominant mechanism of change in some cities. An alternative explanation for the hydrographic patterns we observed is that cities are selectively built in areas with particular hydrographic characteristics. Arid cities, for example Phoenix, are often built along rivers and other locally “wet” areas within the arid landscape that could account for the apparent increase in surface water abundance. Many cities are built on major rivers or the shorelines of major water bodies and the tendency to build in very similar locations may contribute to overall similarities in hydrographic pattern.

The relative role of the mechanisms of hydrographic change, like other land use and land cover changes, is likely spatially and temporally heterogeneous. Spatial heterogeneity in the mechanisms may be correlated with the figurative distance of a city from the convergence point. For example, cities built in regions at the extremes of surface water abundance may require more substantial alteration (for example, Phoenix, Miami, Houston). In addition, the role of these mechanisms is also likely temporally heterogeneous. The distribution of the types of water bodies in cities of different sizes suggests selection may be more important in the initial stages of cities and changes as cities grow outward from their original settlements (Steele and Heffernan 2014). A case study of water bodies in Phoenix, wherein the initial settlement was located along a locally wet riparian area, supports the idea of temporal variation. As the city has grown outward, the density of built water bodies has increased, and the structure of watered lands has changed from one linearly centered along rivers and riparian areas toward more expansive watered landscapes that serve urban and residential purposes (Larson and Grimm 2012). It is important to mention, however, that using a space for time substitution limits our ability to draw inferences regarding how the hydrography of cities changes.

A number of ecological and ecosystem processes are mediated by the abundance of surface water and its geomorphology. The abundance of lakes, ponds, wetlands, and streams, as well as the size of water bodies and the connectivity between these features, mediates watershed fluxes of carbon, nitrogen, and phosphorus (Cole and others 2007; Fraterrigo and Downing 2008; Downing 2010) in addition to species abundance and distribution (Dunham and Rieman 1999; Dahlgren and Ehrlen 2005). Further research will be needed to determine if the convergence of urban hydrography leads to a convergence of biogeochemical and ecosystem properties related to the surface water abundance and characteristics. Some urban watersheds, for example those in coastal areas, may function very differently than the original ecosystem. For others, the differences in the hydroscape structure may be of little consequence to local ecosystems relative to the effects of pollution and other changes associated with urbanization.

Though the urban hydroscape is converging, this does not indicate that the management of urban watersheds should be likewise homogeneous. Cities manage surface water to supply a host of services as well as mitigate the negative consequences of an overabundance of water (for example, flooding, mosquitoes) or aridity (for example, scarcity, heat). We can only speculate about the extent to which the convergent pattern represents an optimization of services provided by surface water; however, increasing these benefits is likely to remain a continuing goal of landscape design and development. Depending on the service, some cities may benefit from returning to a hydroscape more closely resembling the antecedent one, whereas others may benefit from a further departure. For example, small water bodies in the urban landscape more efficiently regulate microclimates (Sun and others 2012; Sun and Chen 2012) and incorporation or reestablishment of small water bodies in cities may provide cooling benefits within the urban heat island and energy savings. For some cities that may have removed a significant portion of the small water bodies, this re-establishment would return the city to a hydroscape more similar to the antecedent one; however, for cities such as Phoenix, such perennial features would present a further departure.

Hydrographic patterns reflect the interaction between geophysical constraints and the policies and practices of diverse decision makers whose decisions shape water management, stream and wetland conservation, and urban land use and land cover change at a variety of scales (Roy Chowdhury and others 2011). At broad scales, these multi-scaled decisions lead to a configuration of urban development in the United States that is less dense and sprawling relative to cities elsewhere (Huang and others 2007; Schneider and Woodcock 2008). Higher density development in this study increased the convergence of urban surface water on a “drier” hydroscape relative to lower densities. Whether similar patterns occur in more consolidated, densely built regions of the globe might indicate how different priorities and constraints shape relationships among people, water, and the built environment across a wide range of political, economic, and environmental conditions.