Streamflow life cycles spanning the USA

Abstract:

Rivers are replenished by precipitation that works its way through watersheds and into stream networks. The time that precipitation requires to travel into a stream regulates contaminant transports, nutrient mobility and bedrock weathering, but has not yet been evaluated at a continental scale. Here we synthesize a pan-U.S.A. dataset of rain, snow and streamflow ¹⁸O/¹⁶O and ²H/¹H ratios and analyze the data to show that the lion’s share of USA streamflow is generated by precipitation that takes ~2 months to ~2.5 years to flush through watersheds and into networks of streams (i.e., rivers replenished by “infant-to-toddler aged” precipitation). These streamflow ages are considerably shorter than the average amount of time that water spends within streams themselves (~1 month, globally), and much shorter than the global groundwater residence time of more than ~1000 years. We also estimate the depth of “dynamic” groundwater storage that actively generates the majority of streamflow and discover that less than ~1% of watershed flowpaths generate the bulk of continental runoff. Our finding showcases that the most hydrologically-active zone within Earth’s hydrosphere is located nearest to the surface where atmosphere-biosphere-lithosphere interactions are at a maximum. This research emphasizes the importance of critical zone research for developing accurate forecasts of how human modifications to the land and climate will impact downstream water, nutrient and contaminant fluxes.