Using estimates of nutrient retention to inform riparian buffer metrics for water quality improvement

Restoration of riparian buffer zones has been a major focus of water-quality initiatives in an effort to reduce the negative effects of nonpoint source pollution.  Newly derived flow-path metrics have shown a potential to guide land management decisions because of their ability to efficiently quantify likely riparian function at the watershed scale, but were developed assuming that buffers filter nutrients solely as a function of width without accounting for local factors that may affect retention.  The purpose of this study was to develop weighted variations of flow-path metrics that incorporated retention estimates and test their ability to predict nitrate-nitrogen concentrations at watershed outlets for 1,613 Maryland watersheds.  We found that weighted flow-path metrics predicted a reduction in filtering potential.  In some watersheds, differences between weighted and unweighted metrics dramatically changed the outcome of how well watersheds were perceived to be buffered.  For each physiographic province filtering capacity was limited at similar proportions of watershed cropland, emphasizing the importance of hydrologic connectivity.  Weighted metrics showed promise in improving the ability to relate nitrate-nitrogen concentrations to measures of riparian function.  The metrics developed and tested in this study were an important step towards developing tools to aid restoration strategies.