Abstract
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.