LAND USE CONTROLS ON GROUNDWATER INPUTS TO STREAMS

High levels of impervious surface area (ISA) in urban watersheds alter stream flood response, often leading to higher peak flows, shorter lag times, and lower baseflow contributions relative to their undisturbed, rural counterparts. However, few studies assess changes in event water (recent rainfall) and pre-event water (baseflow from groundwater) as a function of land use, and those which do use variable methodologies and occur across a wide range of climate and basin characteristics (e.g., lithology, soil cover, and vegetation), thus limiting the comparability of these studies. Some studies also note that “urban karst” enhances preferential flowpaths in developed watersheds thereby contributing greater quantities of baseflow to streams than might be predicted by ISA alone. This study assesses land use controls on baseflow inputs during floods for multiple streams (n = 11) with a range of ISA (1.5-35.3%), but similar climate (humid subtropical), geology (carbonates), soil (silt loams), and vegetation (temperate deciduous forests). We use high frequency monitoring of specific conductivity and stable isotopes (δ²H and δ¹⁸O) for two component hydrograph separations. Our results show that, on average, streams across a large range of ISA have high baseflow inputs during floods (60% of the total flow) that are not significantly different (p = 0.45; R² = 0.07). However, lag times are reduced substantially (66% decrease; p < 0.01; R² = 0.67) as a function of land use. Our results imply a decoupling of sourcing and timing of flow fraction delivery across a land use gradient. The faster delivery of groundwater in more urbanized streams implies greater water transport through preferential flowpaths. Our study streams overlie carbonates with known conduit flow that could enhance groundwater delivery rates. Urban karst may also accelerate groundwater transport to streams, but studies of proximal urban watersheds with ISA similar to our most developed sites, but overlying shales, show low baseflow inputs during floods (~25% of the total flow). This suggests that lithology is a more important control on flow fraction delivery than enhanced permeability from urban karst. Understanding water delivery during floods in populated areas improves flood control efforts and ecosystem management in an increasingly urbanized world.