Changes in urban land use throughout the Edwards Aquifer: A comparative analysis of Austin, San Antonio, and the Interstate–35 corridor
Introduction
Urbanization has historically occurred near water to ensure a reliable supply for domestic, industrial, and municipal uses (Kummu et al., 2011; McCool et al., 2008). However, a lack of access to freshwater is a growing threat to the future sustainability of urban centers, as illustrated by the water crises in Cape Town, South Africa (Wrottesley, 2018) and Chennai, India (BBC, 2019) during 2018 and 2019, respectively. The growing strain placed on water resources is partly due to the continued concentration of individuals in urban areas, with the proportion of global urban inhabitants increasing from approximately 30% of the total population in 1950 to over 55% by 2018 (United Nations, 2019).
The state of Texas provides no exception to these global population trends. In 1910, 24% of the Texas population resided within urban areas, but this proportion increased to 85% by 2010 (White et al., 2017). Cities in South–Central Texas have undergone particularly rapid urban expansion that threatens water resources. San Antonio was the fastest growing city in the United States between 2016 and 2017, as it added an average of 66 people per day, and nearby Austin ranked 12th (U.S. Census Bureau, 2018). Additionally, Comal County, which is located along the Interstate–35 corridor connecting Austin and San Antonio, exhibited the second largest percentage growth between 2017 and 2018 (U.S. Census Bureau, 2019).
This pronounced urban expansion has resulted in drastic land use changes throughout San Antonio, Austin, and the Interstate–35 corridor via the addition of new housing units, transit construction projects, and widespread commercial development (Corridor Advisory Committee, 2012; Marcus & Millichap, 2018; O'Hare, 2018). Importantly, the urban population of Texas is expected to double in 40 years (White et al., 2017) with much of the future growth in South–Central Texas occurring atop or proximate to the Edwards Aquifer, which is the region's primary water source. Continued urbanization not only enhances the demand for water withdrawals from the Edwards Aquifer but also threatens the quality and quantity of water recharging the aquifer due to the transformation of the natural landscape (Veni, 1985). Overall, urban development degrades the numerous ecosystem services provided by the Edwards Aquifer, such as water filtration, water storage, and the provision of unique habitats that support biodiversity (Kreuter et al., 2001; Yi et al., 2018; Yi et al., 2019).
Quantifying the pace of urban development across the Edwards Aquifer and understanding the degree to which it has been influenced by protective measures is critical to successfully conserving the water resource. Unfortunately, few recent studies have explored these linkages. One of the most comprehensive assessments of urbanization throughout the Edwards Aquifer analyzed land use trends between 1986 and 2000 (Peschel, 2004), which predated the implementation of more substantive aquifer protection policies. This research aims to expand upon the work of Peschel (2004) by analyzing the spatiotemporal characteristics of urban development across the aquifer after 2000. Specifically, the paper addresses the following research questions:
- 1.
How has the rate of urbanization upon the Edwards Aquifer in San Antonio, Austin, and the Interstate–35 corridor changed between 2001 and 2016?
- 2.
What specific portions of the aquifer have undergone the most rapid development during the study period?
- 3.
Are any differences in the rate of aquifer urbanization between Austin and San Antonio related to the varying protective measures implemented in the two regions?
Exploring these research questions will highlight the particular areas of the aquifer that are urbanizing most rapidly and potentially warrant additional protection. Comparing the degree of aquifer urbanization between Austin and San Antonio will also help evaluate the effectiveness of the various measures implemented to protect the Edwards Aquifer, which could inform future policy efforts. The following background section describes in greater detail the adverse impacts of urbanization on the Edwards Aquifer and also chronicles the protective policy measures established to conserve the resource. Section 3 outlines the methods and data used to quantify and analyze the rate of urban development throughout the aquifer. The results of the analysis and a discussion are provided in Section 4. Finally, Section 5 summarizes the major findings of the research and highlights the broader policy implications.
Section snippets
Impacts of urbanization on the Edwards Aquifer
The Edwards Aquifer is a karst aquifer where carbonate limestone rock has dissolved preferentially to form conduits that enable the rapid recharge of water (Loáiciga et al., 2000). Although the high levels of permeability and porosity are largely responsible for the aquifer's notable storage capacity, transmission capability, and highly productive wells, they also enhance the vulnerability of the aquifer to urbanization since there are limited opportunities for the subsurface filtration of
Edwards Aquifer study region
The Edwards Aquifer underlies an expansive multi–county region of South–Central Texas characterized by brush and grasslands in the southeast that transition gradually to rolling, wooded rangelands northwest of the Balcones Fault Zone (Fig. 1). Climatically, the region is sub–humid to semi–arid with hot summers and mild winters (Musgrove et al., 2016). Precipitation varies spatially and temporally throughout the study area, as conditions tend to oscillate between pronounced dry and wet periods (
Urbanization across the Edwards Aquifer and Edwards Plateau
When analyzing the Edwards Aquifer overall, 23,482 ha or 1.2% of the total aquifer area was urbanized between 2001 and 2016 (Table 1). The combination of urbanization during the focus period with development prior to 2001 indicated that 6.3% of the Edwards Aquifer was developed by 2016. Importantly, the temporal trends highlighted that the rate of urban development across the aquifer overall continuously declined. Between 2001 and 2006, the rate of aquifer urbanization was 1,774 ha/year, but it
Conclusion
By analyzing LULC data across the Edwards Aquifer, this study quantified the rate of aquifer urbanization, evaluated the effectiveness of aquifer protection policies, and identified the specific portions of the aquifer urbanizing most rapidly in South–Central Texas. Overall, the rate of urbanization upon the Edwards Aquifer recharge zone declined during each time period while the pace of development within the contributing zone increased during 2006–2011. These findings were consistent when
CRediT authors contribution statement
Justin Fabio Guerra: Conceptualization, Methodology, Writing, Visualization. Neil Debbage: Conceptualization, Methodology, Writing, Visualization, Supervision.
Declarations of competing interest
None.
Acknowledgements
This research was supported by a grant from the Office of Sustainability at the University of Texas at San Antonio.
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