Ephemeral streamflow generation and its influence on downstream perennial watershed observations

Abstract:

Our understanding of the processes leading to stream network expansion and contraction is limited. To address this, we utilize an ephemeral-to-perennial drainage network in the Piedmont region of the United States to gain new understanding about the drivers of runoff generation, streamflow activation thresholds, and the hydrologic and chemical connectivity between upstream and downstream systems across wetness states. We use hydrometric (precipitation, discharge, and distributed groundwater levels) and chemical (dissolved organic carbon, electrical conductivity, and major ions) data from nested wells and piezometers to the outlets of zero through second order catchments to characterize overland, shallow soil, and deep subsurface flow across landscape positions. These measurements across a wide range of antecedent moisture conditions and storm sizes and intensities allow elucidation of the relationships between the spatiotemporal dynamics of runoff generation processes and observed biogeochemical behavior in this highly weathered, low relief landscape. Perched shallow throughflow and deeper groundwater contributions, both prominent in headwater hillslopes, can produce varying discharge-solute hysteresis directions at the watershed outlet as a function of antecedent moisture conditions. When active, ephemeral headwater systems provide disproportionately more runoff than their downstream counterparts. Together, this suggests ephemeral headwater systems play an active role in dictating biogeochemical fluxes at the watershed scale.