A Mechanistic Assessment of a Near-stream Saturated Area Dynamics in a Headwater Catchment

Abstract:

Saturation excess overland flow generated in near-stream saturated areas is a mixture of rainfall and exfiltrating sub-surface water that can be quickly transported to the stream network during rainfall events. While many isotope hydrograph separation studies have demonstrated generally the dominance of pre-event water in the channel hydrograph, the mixing processes within the saturated area itself are poorly understood. Here, we isolated and measured discharge generated within a 100-m² saturated area of a headwater reach of the 45-ha Weierbach catchment (Luxembourg). We quantified surface saturation using ground-based thermal infrared imagery and in-site piezometers throughout a series of rainfall events. This, combined with isotope and geochemical tracing enabled us to assess the role of surface saturation dynamics on mixing and storm hydrograph response. Surprisingly, our detailed analysis showed that surface saturation dynamics were weakly correlated to discharge and precipitation; cumulative rainfall on near-stream saturated areas alone was unable to explain the flow generated within this reach. Streamflow isotopic response was essentially unchanging within the 5-week monitoring period (-55 to -58 δ²D and -8.5 to -9 for δ¹⁸O ‰) suggesting that well-mixed streamflow sources that were little affected by rainfall sources. While Na⁺, Mg²⁺, and Ca²⁺ show slight flushing and dilution responses, riparian and streamflow sources responded differently across the event. Overall, groundwater exfiltration within the saturated area appeared to generate streamflow throughout the reach, effectively erasing any rainfall onto saturated area signal.