Yu Fang & James W. Jawitz, The evolution of human population distance to water in the USA from 1790 to 2010, Nature Communications | (2019) 10:430 | https://doi.org/10.1038/s41467-019-08366-z (open access)
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I really enjoyed this paper; it's a novel subject and one that has important ramifications for future population migrations in response to climate change.
The text below consists solely of the abstract and the discussion. To access the entire paper online so that you can read the entire text with hot links, click here.
Thanks to Tim Smith for alerting me to this fascinating paper.
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Abstract
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. 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. We show that humans were preferentially attracted to areas overlying major aquifers since industrialization due to the emergent accessibility of groundwater in the 20th century. 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. Our results reveal a historical coevolution of human-water systems, which could inform water management and contribute to societal adaptation to future climate change.
Discussion
Societal progress and technological development have greatly changed how people obtain water resources. Are humans also changing where they live in relation to water? This study provided a quantitative analysis of human water coevolution over a timescale of centuries. For the conterminous US, our analysis demonstrated a persistent preference of living close to major rivers, with consistently negative relationships between population density and DMR from 1790 to 2010 (Fig. 3a, Fig. 3b). We also quantified the dynamic desirability of living adjacent to rivers using the linear slopes of NPD and DMR, which echoed changing societal reliance on adjacency to major rivers over time.
In pre-industrial society, humans were highly reliant on proximity to rivers for agriculture, water supply, and navigation. With industrialization, human settlements transformed their transport modes from primarily water-borne to other means such as roads and railroads As human settlements have continued to grow, water demand in some cities has increased beyond the local supplies, as illustrated by cases such as Los Angeles, Phoenix, and Atlanta. Thus, water supply in post-industrial societies has evolved from people moving to the water to people moving the water. For example, in Los Angeles since 1900, as the population has grown, the water supply from the local Los Angeles River became limiting and the city began to import water from hundreds of kilometers away, including the Owens Valley in 1913, Mono Basin in 1940, the Colorado River in 1941, and the rivers of the Central Valley in 1971. Modern cities can bring adequate water from previously remote and inaccessible sources by building canals or by groundwater pumping, increasing water availability and greatly reducing the portion of the total population considered at risk for water scarcity26. Around 80% of large cities globally would have to travel less than 22 km to reach a potential unstressed water source adequate for several million people9. Thus, water transfers have contributed to the increase in the distance of cities from water sources. Consequently, the desirability of living close to rivers gradually declined and people are no longer constrained to live only in water- rich places, with burgeoning cities also existing in arid regions. Thus, the increasing human distance to water in recent decades reflects a process of telecoupling in human–water interaction.
We identified three types of human–water coevolution (Fig. 4 and Supplementary Fig. 3) that were associated with regional differences in climate, water resource endowments, and settlement history. Regions (here based on HUCs) such as New England, with long settlement history and early industrialization, had stable distances from rivers with early evolution from frontier to cities (Fig. 2a). Regions with temporally increasing desirability of proximity to rivers were located in the arid west, while regions with decreasing desirability were mostly found in humid regions with less vulnerability to water scarcity. Additionally, the correlation between fast growth in population density and high-recharge major aquifers suggested an increasing attractiveness of places overlying productive aquifers. We found consistently high mean population density in areas overlying rapidly recharged major aquifers, even after controlling for mean annual temperature (Supplementary Table 3).
Our analysis was conducted at a continental scale over a time period of hundreds of years and is thus subject to the limitation of currently available data. Our analysis did not evaluate water quantity (for example by differentiating by stream order) or quality, which could affect the attractiveness to human settlements. We compared published river course migration rates, and found overall migration rates to be much smaller than the 1-km resolution of our analyses (see Supplementary Note and Supplementary Tables 4 and 5 for details). Continental-scale information about aquifer structure and recharge rates have only relatively recently become available, however, these data are still at much lower spatial resolution than corresponding information about river networks. The timescales for large-scale changes in whole-river network structures and aquifer boundary migration are geological rather than human in scale. Thus, at this spatio-temporal scale the assumption of stationarity of river networks and groundwater seems reasonable. However, the importance of aquifer volume changes due to groundwater pumping has been growing in recent decades, and is expected to continue to grow in significance in coming decades. The potential changes of aquifer boundaries and storage volumes due to groundwater withdrawal should be given consideration when projecting human distance to water in the future.
Finally, we note that the dynamic human distance to water also implies changing anthropogenic impacts on water resources over time. Human settlements located adjacent to rivers exert direct pressure on local water quality, resulting in serious impacts to health and ecosystems. Water redistribution has relieved the previous strong dependence on adjacent rivers, but with a high dependence on technology and infrastructure. Meanwhile, the rapid development of some aquifers has contributed to serious groundwater depletion issues, e.g., in the California central valley and High Plains aquifers. The shifted human distance to water resources has thus been mainly achieved by co-evolution of infrastructure and water management institutions. For example, a majority of US cities have achieved artificial- or super-abundance of water supply through substantial complexity of hydraulic infrastructure institutions. Ensuring adequate infrastructure investment is therefore extraordinarily essential to sustain stable water supplies. However, further infrastructure development to extract water from farther away is becoming more expensive or even infeasible due to the physical, economic, and ecological limits. Infrastructure projects such as large dams or water transfer projects have also often resulted in forced relocation of people. Thus, as water resources become increasingly scarce in the future, whether people move closer to water, or water is moved to the people, feedbacks between these should be considered in future evaluations of the dynamic evolution of the human distance to water.
Enjoy!
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