Quantifying groundwater dependency of riparian surface hydrologic features using the exit gradient

Numerical groundwater flow models necessarily are limited to subsurface flow evaluation. It is of interest, however, to examine the possibility that, for unconfined aquifer systems, they could be used to proportionately measure the magnitude of seepage they estimate when these aquifers intersect the landscape surface. Our goal in this study was to determine the degree to which an unconfined groundwater model can estimate run-off or seepage at the land surface during winter time wet season conditions, as well as in the dry season, when evapotranspiration is a major part of the water balance, using a lowland basin-fill example study area in the Pacific Northwest. The exit gradient is a metric describing the potential for vertical seepage at the landscape surface. We investigated the spatial relationship of mapped surface features, such as wetlands, streams and ponds, to the model-predicted mapped exit gradient. We found that areas mapped as wetlands had positive exit gradients. During the wet season, modelled exit gradients predicted seepage throughout extensive areas of the groundwater shed, extending far beyond mapped wetland areas (355% increase), associated with previously observed increases in nitrate-nitrogen in streams in wet season. During the dry season, exit gradients spatially corresponded with wetland areas. The increase in in-stream nitrogen corresponds with shorter residence times in carbon-rich wetland zones because of the onset of saturation overland flow. We present results that suggest that the exit gradient could be a useful concept in examining the groundwater-surface water linkage that is often under represented physically in watershed flow models. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.