Controls on Stream Water Age in a Saturation Overland Flow‐Dominated Catchment

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 stream water 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 hillslope water storage capacity. Runoff lag to peak—after initial seasonal wet‐up—is rapid (∼1–2 hr), 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 1 day. Since streamflow primarily consists of overland flow, a significant portion of overland flow must not be event‐rain but instead derive from older, nonevent 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 watershed fed primarily by overland flow, runoff is primarily not composed of event water. Our findings have implications for the interpretation of stream chemistry and the assumptions built into widely used hydrograph separation inferences, namely, the assumption that overland flow consists of new (event) water.

Harnessing hyperspectral imagery to map surface water presence and hyporheic flow properties of headwater stream networks

“…Streamflow data near the outlet of Dry Creek (near gaging station; Figure 1 High flow survey We identified one cloud-free image that coincides with a very highflow rate of 34 mm/day. Field visits indicate that the channel network considered here is fully wetted at flow rates exceeding 10 mm/day (Lapides et al, 2022).…”

“…In the early winter months, infiltrating rainfall rapidly replenishes root-zone water storage deficits, leading to recharge of groundwater tables that rise to the ground surface, typically within 200 mm of seasonal total rainfall (Dralle et al, 2018). Once water tables intersect the ground surface, saturation overland flow is widespread and channel networks rapidly expand, with flows that can exceed 50 mm per day (Lapides et al, 2022). Rapid flow increases are followed by comparably fast flow recessions with attendant contraction of wetted channel extent.…”

“…Rapid flow increases are followed by comparably fast flow recessions with attendant contraction of wetted channel extent. Runoff responds rapidly to precipitation (typical lag-to-peaks in Dry and Hank Creeks of only 2-3 hrs (Lapides et al, 2022)). At runoff rates > 10 mm/day, most of the geomorphic channel network has flowing water and saturation overland flow extends up adjacent hillslopes (Lapides et al, 2022).…”

“…Runoff responds rapidly to precipitation (typical lag-to-peaks in Dry and Hank Creeks of only 2-3 hrs (Lapides et al, 2022)). At runoff rates > 10 mm/day, most of the geomorphic channel network has flowing water and saturation overland flow extends up adjacent hillslopes (Lapides et al, 2022). Thin weathering profiles with small water storage capacity also impact the site's plant community -a relatively sparse oak savanna, comprised of non-native annual grasses and Oregon white oak (Quercus garryanna) (Hahm et al, 2017(Hahm et al, , 2018(Hahm et al, , 2019.…”

“…Wondzell & Ward, 2022), water transit times (e.g. Lapides et al, 2022), and water-borne disease transmission (e.g. Perez-Saez et al, 2017).…”