Here, we studied the spatio-temporal relationship between human populations and water resources in the conterminous US for the decades 1790–2010 (excluding 1960, for which population data are lacking) 24. However, a recent reconstruction of historical human population distribution in the conterminous US 24 based on decennial census data has provided an opportunity to conduct a quantitative spatio-temporal analysis of the coevolution of human settlement locations and water. Spatially explicit population maps, including the Gridded Population of the World (GPW) 19, the Global Rural Urban Mapping Project (GRUMP) 20, LandScan 21, and WorldPOP 22, are of limited utility for long-term (e.g., multi-decadal) dynamic analyses since they only became available after the 1990s, while historical census data for the conterminous US, although available from 1790 to 2010, lack detailed information within counties 23. Therefore, the changing role of water resources in regulating where people live over longer timescales (e.g., centuries) remains to be examined, and an integrated approach in space and time is required to answer this question quantitatively.Ī significant obstacle is the availability of spatially explicit population data over long time periods. Also, the spatial units of analysis at continental or country scale 18 may not provide sufficient perspective for smaller areas, such as catchments. However, these studies were conducted as a temporal snapshot (for 2007) 5, or with a relatively narrow temporal window (between 19) 18. The relationship between human settlement density and distance to rivers has been analyzed at high spatial resolution at a global scale 5, 18, finding denser human population with increased proximity to rivers, but with regional variability based on climate, degree of urbanization, and economic development history. The dynamic and complex human–water interactions embedded within heterogeneous landscapes present significant questions about where humans live in relation to water resources. Since the 1940s and 1950s, groundwater extraction in the US has increased remarkably as groundwater pumping became economical and efficient due to the development of technology and widespread access to electricity 15, 17. Furthermore, groundwater has gradually emerged as a critical water source, largely due to its convenience, availability, and quality 15, 16. The importance of locating close to rivers has been increasingly reflected by the indirect environmental amenity value of water and the agglomeration economic effects of historical established settlements in proximity to rivers 12, 13, 14. Meanwhile, humans are more vulnerable to flood risk when settling adjacent to rivers and thus require development of flood protection measures to reduce flooding risks 11. However, humans have been able to decrease their reliance on direct proximity to rivers by developing advanced measures to transport adequate water from other sources (for example, canals or pipelines, groundwater pumping, and desalination) 6, 7, 8, 9, and shifting from waterborne transport modes to land (railways and roads) and air transport 10. Historically, humans have chosen to live close to rivers for domestic and agricultural water supply as well as for navigation purposes, which has led humans to follow the courses of rivers during migrations 1, 2, 3 and locate in proximity to rivers when establishing settlements 4, 5. Water is an attractive factor when people choose where to live. Our results reveal a historical coevolution of human-water systems, which could inform water management and contribute to societal adaptation to future climate change. Regional heterogeneity resulted in diverse trajectories of settlement proximity to major rivers, with the attractiveness of rivers increasing in arid regions and decreasing in humid areas. We show that humans were preferentially attracted to areas overlying major aquifers since industrialization due to the emergent accessibility of groundwater in the 20 th century. Here we show that humans moved closer to major rivers in pre-industrial periods but have moved farther from major rivers after 1870, demonstrating the dynamics of human reliance on rivers for trade and transport. Human societies evolved alongside rivers, but how has the relationship between human settlement locations and water resources evolved over time? We conducted a dynamic analysis in the conterminous US to assess the coevolution of humans and water resources from 1790 to 2010.
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