Riparian ground-water flow patterns using flownet analysis: Evapotranspiration-induced upwelling and implications for N removal

Ground-water flow paths constrain the extent of nitrogen (N) sinks in deep, stratified soils of riparian wetlands. We examined ground-water flow paths at four forested riparian wetlands in deep, low gradient, stratified deposits subjected to Southern New England's temperate, humid climate. Mid-day piezometric heads were recorded during the high water table period in April/May and again in late November at one site. Coupling field data with a two-dimensional steady-state ground-water flow model, flow paths and fluxes were derived to 3 in depths. April/May evapotranspiration (ET) dominated total outflux (44-100%) while flux to the stream was < 10% of total outflux. ET exerted upward ground-water flux through shallow carbon-rich soils, increasing opportunities for N transformations and diverting flow from the stream. Dormant season results showed a marked increase in flux to the stream (27% of the total flux). Riparian sites with deep water tables (naturally or because of increased urbanization or other hydrologic modifications) or shallow root zones may not generate ground-water upwelling to meet evaporative demand, thereby increasing the risk of N movement to streams. As water managers balance issues of water quality with water quantity, they will be faced with decisions regarding riparian management. Further work towards refining our understanding of ET mediation of N and water flux at the catchment scale will serve to inform these decisions.