Anthropogenic and Biophysical Controls on Low Flow Hydrology in the Southeastern United States

The original paradigm of urban hydrology identified links between urbanization and changes in hydrologic processes. One of the prominent relationships identified was increased impervious surface coverage decreased subsurface recharge and subsequently low flows. This does not consider watershed-specific biophysical or anthropogenic characteristics that could increase low flows, such as direct discharge from wastewater treatment plants and indirect recharge to groundwater from leaking pipes and infiltration-based green infrastructure. Here, in an effort to sharpen this original paradigm, we take a statistical approach to evaluate relationships between natural and human-impacted watershed characteristics and a suite of low flow metrics that describe mean annual low flow magnitudes and interannual variability in 54 watersheds over 15 years in the Piedmont physiographic region of the southeastern United States. We find that annual low flow magnitudes are most strongly correlated to biophysical watershed factors, including positive relationships with topography and precipitation and negative relationships with poorly drained soils. In contrast, interannual variability is positively correlated to land use characteristics, including forest, and negatively related to urbanization. Similarities in best fit models across long-term average, wet, and dry years indicate a robustness of hydrologic drivers across watershed wetness conditions. Our results also showcase that while the original paradigm of urban hydrology may hold across traditional low flow metrics, such as baseflow index, extreme low flow drivers are complex and counterintuitive. Understanding these first-order controls on baseflow magnitude and variability across an urbanization gradient is needed to conceptualize the impacts that continued, rapid urbanization will have on low flow dynamics.