Technical Note: Deciphering the Hydrologic Response of Riverbeds across Changes in Recharge with Electrical Resistivity Imaging

Abstract. The fluxes between groundwater and surface water play a significant role in quantifying water balance along stream reaches to continent scales. Changes in these dynamics are occurring due to aquifer depletion, where river flow from predevelopment baseflow conditions with groundwater to surface water have evolved to enhanced recharge through streambeds of ephemeral flows to groundwater. This problem is studied along the Arkansas River in Western Kansas across a stream reach that transitions from near equilibrium of fluxes to a losing river that contributes recharge to a depleting High Plains Aquifer. Existing hydrologic data illustrates the lack of understanding they provide related to the control of fluxes exerted by alluvial deposits. We employ electrical resistivity imaging (ERI) along this river transect to elucidate the intricate pathways of hydrologic connectivity existing between the Arkansas River and underlying Arkansas Alluvial and Ogallala Aquifers. Time-lapse ERI profiles quantify the temporal changes in resistivity across the riverbed, and these changes are associated with the distribution of soil physical properties and hydrologic conditions below the water-sediment interface. Results utilize a recently discovered vadose zone property whereby fine grained inclusions may become revealed by their different water holding capacity relative to that of a surrounding matrix of coarser grained soil across changes in recharge (caused by changes in stream discharge). These findings corroborate the role of large-scale geologic features in maintaining streamflow in regions of near-surface impermeable layers, and the localized recharge that occurs to the High Plains Aquifer through embedded assemblages of fine and coarse grained soils.