Calculating streamwater age using StorAge Selection functions at Dry Creek, CA

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. Lapides et al. (2021) combined 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 an intensively monitored catchment in the northern California Coast Ranges with a Mediterranean climate and thin subsurface critical zone. Due to limited storage capacity, runoff response is rapid (~1-2 hours), and total annual streamflow consists predominantly of saturation overland flow, based on field mapping of saturated extents and runoff thresholds. Even though SAS modeling reveals that streamflow is younger at higher wetness states, flow is still 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 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 than anticipated, with implications for contaminant delivery and biogeochemical reactions. Our findings have implications for the assumptions built into classic hydrograph separation inferences, namely, whether overland flow consists of new water. For this work, we translated SAS modeling code in Matlab from Benettin and Bertuzzo (2018) to Python and provide here a set of code for SAS modeling in Python and example data for Dry Creek, CA produced for the SAS modeling publication by Lapides et al. (2021).径流年龄与水流路径本应存在关联，但目前学界仍普遍不清楚流域整体产流机制如何决定出口断面的径流年龄分布。Lapides等人（2021）将干燥溪（Dry Creek）流域的产流野外观测数据与储水年龄选择（StorAge Selection, SAS）模型相结合，以探究河流水年龄与径流路径之间的关联。干燥溪（Dry Creek）流域位于加利福尼亚北部海岸山脉，属地中海气候，地下关键带（subsurface critical zone）较薄，是一处受高密度监测的典型流域。由于储水能力有限，流域产流响应迅速（约1~2小时）；结合饱和区域测绘与产流阈值的野外观测结果可知，年总径流以饱和坡面流（saturation overland flow）为主。尽管SAS模型结果显示，流域湿度越高，河流水年龄越年轻，但实际径流年龄通常仍超过1天。鉴于径流以坡面流为主，这意味着相当一部分坡面流并非来自本次降雨事件，而是源自返出地表的老化地下水——这与野外观测到的渗出水头梯度、通过大孔隙（macropores）的回流以及暴雨后大范围持续饱和的现象一致。本研究得出结论：即便在坡面流广泛分布的流域中，径流路径可能也比此前预期更长，这一结论对污染物输运与生物地球化学反应研究具有参考价值。我们的研究结果也对经典水文过程线分割（hydrograph separation）推断中的核心假设提出了挑战，即坡面流是否仅由本次降雨的“新水”构成。 本研究将Benettin与Bertuzzo（2018）开发的Matlab版SAS模型代码移植至Python，并在此提供Python版SAS模型代码集，以及用于Lapides等人（2021）SAS模型研究的加利福尼亚州干燥溪（Dry Creek）流域示例数据集。