Exploring Linkages Between Transit Time and Water Quality in Streams Across the Conterminous USA

Many rivers are at risk of water quality deterioration induced by anthropogenic activities. Addressing this challenge requires a better understanding of the dominant controls on water quality across landscapes. Water transit time, defined as the time interval between the introduction and discharge of a parcel of water in a catchment, has been proposed as a fundamental characteristic linking hydrologic transport and the biogeochemical processes that determine water quality at the catchment scale. In this study, we assess the associations between the transit time of streamflow, which integrates water and solutes from different sources and pathways across a catchment, and water quality in a selection of different catchments distributed across the conterminous USA. We employ a continental scale, integrated hydrologic model coupled with Lagrangian particle tracking to simulate flow paths and calculate streamflow transit times. Water parcels in river cells are tracked backwards, along subsurface and overland flow pathways, towards their surficial entry points. The physically based modeling framework explicitly accounts for the effects of climate, topography, geology, and land cover on the simulated hydrologic response. Mean transit times and fractions of streamflow younger than specified age thresholds are evaluated. Various functional relationships between the stream transit time metrics and a suite of water quality parameters describing nutrients, dissolved salts, and suspended solids are tested. Inter-basin comparison of such relationships can help elucidate differences in catchment sensitivity to perturbations, improve predictions of surface water vulnerability to contamination, and provide critical information for developing appropriate management approaches.