Looking beyond MS4 communities: Assessing impervious cover and population dynamics in small towns.

Water quality and quantity are strongly influenced by land use and land cover, with impervious surfaces serving as a key indicator of watershed health. Increased imperviousness alters watershed hydrology, shifting it from infiltration-dominant to runoff-dominant, leading to reduced groundwater recharge, higher runoff, increased peak discharge, and channel enlargement. In the United States, policy regarding stormwater management is predominantly focused on towns with populations greater than 10,000. However, any increase in impervious cover can negatively affect hydrology and water quality, and communities with populations smaller than 10,000 often lack the financial and technical resources to mitigate issues from impervious cover. This study examines changes in impervious cover in small towns and HUC-12 watersheds in eastern South Dakota between 2001 and 2019, alongside population changes from 2000 to 2020 in towns with populations under 10,000. Imperviousness changes were evaluated using the NLCD, and population shift was assessed using U.S. census data. A t-test was performed to compare changes over time, followed by linear regression and generalized additive model to examine the effects of population density dynamics and proximity to larger cities on changes in imperviousness. Results reveal significant demographic changes, with more than half of the towns studied experiencing population declines, while those closer to larger cities saw growth. Despite these population declines, many towns saw significant increases in imperviousness, highlighting that population change alone does not fully explain shifts in impervious surfaces and that other factors, such as land-use policies and socio-economic factors, play a significant role. Although small towns contributed modestly to overall watershed imperviousness, more than half (52 %) of these towns had already exceeded the 10 % threshold commonly associated with ecological impairment by 2019. Additionally, the average imperviousness in these towns rose from 9.3 % in 2001 to 12.7 % in 2019, highlighting the need for expanding the focus of stormwater policy beyond larger communities. Interestingly, while imperviousness significantly increased in small towns, most watersheds experienced a decline, and only 28.2 % of watersheds saw increases primarily among those near larger urban centers, underscoring complex spatial patterns in land use. These findings highlight the importance of addressing these challenges with finer-scale watershed delineations, localized management support, and strategies to enhance community resilience and improve water resource management in small towns.