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).

水龄与径流路径之间理应存在关联，但目前学界仍普遍未能明确完整流域的产流机制如何决定出口断面的径流年龄分布。Lapides等人（2021）结合美国加利福尼亚北部海岸山脉地区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，探究了河流水龄与径流路径之间的关系。Dry Creek流域是一处监测密度极高的研究流域，属地中海气候，地下关键带较薄。由于蓄水容量有限，流域产流响应迅速（约1~2小时），结合饱和区域野外填图与径流阈值分析可知，其年总径流量以饱和地表径流为主。尽管SAS模型模拟结果显示，在更高湿润状态下河流水龄更年轻，但径流总体仍普遍大于1天。由于流域径流以地表径流为主，这意味着相当一部分地表径流并非本次降雨产生的“新水”，而是源自回流至地表的较老地下水，这与野外观测到的渗出水头梯度、大孔隙回流以及暴雨事件后大范围区域持续饱和的现象一致。本研究认为，即便在普遍发育地表径流的流域中，径流路径仍可能比预期更长，这一结论对污染物输运与生物地球化学反应研究均具有启示意义。我们的研究结果也对经典流量过程线分割的假设提出了挑战，即地表径流是否全部由本次降雨的“新水”组成。
本研究将Benettin与Bertuzzo（2018）开发的Matlab版SAS模型代码移植至Python语言，并在此提供Python版SAS模型代码集，以及由Lapides等人（2021）在其SAS模型研究中使用的加利福尼亚州Dry Creek流域示例数据集。