loading page

Controls on streamwater age in a saturation overland flow-dominated catchment
  • +2
  • Dana A Lapides,
  • W. Jesse Hahm,
  • Daniella Rempe,
  • William E Dietrich,
  • David N Dralle
Dana A Lapides
Pacific Southwest Research Station, United States Forest Service | Simon Fraser University

Corresponding Author:[email protected]

Author Profile
W. Jesse Hahm
Simon Fraser University
Author Profile
Daniella Rempe
University of Texas at Austinn
Author Profile
William E Dietrich
Universith of California Berkeley
Author Profile
David N Dralle
Pacific Southwest Research Station, United States Forest Service
Author Profile


Water age and flow pathways should be related; however, it is still generally unclear how integrated catchment runoff generation mechanisms result in streamflow age distributions at the outlet. Here, we combine field observations of runoff generation at the Dry Creek catchment with StorAge Selection (SAS) age models to explore the relationship between streamwater age and runoff pathways. Dry Creek is a 3.5 km2 catchment in the Northern California Coast Ranges with a Mediterranean climate, and, despite an average rainfall of ~1,800 mm/yr, is an oak savannah due to the limited water storage capacity. Runoff lag to peak—after initial seasonal wet-up—is rapid (~1-2 hours), and total annual streamflow consists predominantly of saturation overland flow, based on field mapping of saturated extents and an inferred runoff threshold for the expansion of saturation extent beyond the geomorphic channel. SAS modeling based on daily isotope sampling reveals that streamflow is typically older than one day. Because streamflow is mostly overland flow, this means that a significant portion of overland flow must not be event-rain but instead derive from older, non-event groundwater returning to the surface, consistent with field observations of exfiltrating head gradients, return flow through macropores, and extensive saturation days after storm events. We conclude that even in a landscape with widespread overland flow, runoff pathways may be longer and slower than anticipated. Our findings have implications for the assumptions built into widely used hydrograph separation inferences, namely, the assumption that overland flow consists of new (event) water.
Apr 2022Published in Water Resources Research volume 58 issue 4. 10.1029/2021WR031665