Impacts of Urbanization on U.S. Watersheds
Urbanization is inevitable with a growing population, but what consequences does this have for the water we rely on?
Cheng Li, a former visiting scholar at North Carolina State University from the Guangdong Academy of Sciences, along with USDA Forest Service scientists Ge Sun, Peter Caldwell, and Erika Mack modeled the effects of urbanization on surface water across the contiguous U.S. The results were published in Water Resources Research.
“Forests serve as powerful biological pumps and can return more than half of precipitation back to the air, and thus can greatly reduce urban runoff,” says Sun.
The team used a Forest Service model called WaSSI to update and resolve conflicting results from other studies on urbanization and plant water use. The team specifically addressed vegetation factors in their model, which they show to be a cause of error in previous models.
“The power of vegetation transferring water to the air through evapotranspiration processes was greatly underestimated in some popular literature and publications. They were not based on solid forest hydrology science, especially when compared to our model, which better handles the forest evapotranspiration processes,” says Sun.
Using WaSSI, the team tested their hypotheses that water runoff will increase due to greater urbanization and that local climate and vegetation conditions have a strong influence on this relationship.
The model simulated water budget dynamics over 81,000 different watersheds, between the years 2000 and 2100. Watersheds that were predicted to increase in urban acreage were of particular importance for the study, so analysis focused on these areas. This study is the first to examine water-urbanization dynamics in such detail across the entire U.S.
Total urban acreage in watersheds across the U.S. is expected to increase approximately four percent from 2010 to 2050, and again from 2050 to 2100. Greater urban area — and thus more roads, parking lots, and other impermeable surfaces — leads to less water retention from the decreased magnitude of soil and plants. With this reduced ability to store water, these environments contribute to more significant runoff.
The study shows that impermeable surfaces, local precipitation, and vegetation are the most significant players in determining change in water retention in these watersheds.
Why worry about changes in water runoff? Water retention plays many roles in watershed ecosystems. “Changes in water runoff can lead to flooding,” says Sun. “This could lead to stress on reservoirs and worsening water quality in different communities.” Increasing urban areas and water runoff through the year 2100 will thus strain water resources across the U.S.
Although the team clarified the importance of vegetation in watershed models, the study did not consider climate change. The researchers caution that bigger storms from climate change will exacerbate changes in water dynamics across the U.S. and the world.
The study suggests that retaining and increasing appropriate vegetation cover can combat increased runoff in urbanizing watersheds. Urban planning can do this with green infrastructure, by retaining patches of native vegetation and restoring and protecting wetlands.
Learn more about urbanization impacts on watersheds.
For more information, email Ge Sun at ge.sun@usda.gov.