- Stream discharge is a key water balance component and important factor in global change evaluations. Nevertheless, the mechanisms for streamflow generation are poorly understood. Near- stream surface saturation during precipitation events is one of the most iconic, visible indicators of rapid runoff production in upland humid catchments around the world. Despite years of study, we lack understanding of what occurs within the near-stream saturated area, its mixing dynamics and how this affects catchment geochemical- and flow-response dynamics during events. This thesis explores the mechanisms that control near-stream saturated area behavior in a headwater catchment.
First, I explore the relation between catchment geochemical response and the flow duration curve (FDC) for the 46-ha Weierbach catchment in Luxembourg across 10 years of runoff monitoring. The shape of the Weierbach FDC suggested a two-phase system, a high-flow, precipitation-driven period and a dry, evapotranspiration-driven, low-flow period. I hypothesized that the two phases were correlated with activation of shallow hillslope and subsurface streamflow sources and that the activation of these sources would be reflected in stream chemistry and surface saturation dynamics. During high-flow periods of the FDC, stream geochemistry was largely unchanging, lacking a dilution effect and appeared a mix of the highly variable soil and groundwater. Thermal infrared (TIR) imagery suggested large surface saturation dynamics at high flows. The geochemical signature of streamflow and soil riparian water during low-flow periods most closely resembled groundwater chemistry and led to increasing base cation concentrations and electrical conductivity.
Secondly, to better understand the effect of rain falling on saturated areas and the contribution of rainfall to saturation excess overland flow, I quantified surface saturation dynamics in a near- stream area during rainfall events using high-frequency TIR imagery. During 10 rainfall events across a 34-day period starting December 2013, a total of 161 mm of rainfall elicited 133 mm of runoff at the 6-ha outlet. Surface saturation within a 25-m² thermal infrared imaged area increased from 2 to 20% but was highly variable and weakly correlated to discharge and precipitation. Rainfall onto mapped, near-stream saturated areas accounted for little of the flow generated within a headwater reach. Streamflow isotopic composition at the 6-ha, headwater outlet deflected little throughout the 30-day rainfall period, 0.7 and 1.2 ‰ for δ¹⁸O and δ²H, respectively. Groundwater exfiltration within the saturated area generated nearly all of the streamflow as well the persistent saturation throughout the event.
Thirdly, I examined the underlying controls on streamwater chemostasis in a forested, headwater catchment. Thermal infrared imagery was simultaneously used to quantify saturation expansion and groundwater exfiltration hotspots within the headwater reach. Streamflow during a series of rainfall events responded chemostatically, most measured geochemical species (Ca²⁺, Mg²⁺, Na⁺, SiO₂, Cl⁻, SO₄²⁻ and NO₃⁻) varied little (< 0.5 mg/L), despite discharge increases from 0.2 to 4 L/s. Groundwater levels within the saturated zone increased after an initial 24 mm of event rainfall and remained within 0.05 m of the soil surface throughout the runoff period. TIR imagery identified consistent groundwater exfiltration zones from temperature differences across the event-period in the saturated zone. This suggested that unlike many headwater systems, the alluvial aquifer was well connected to groundwater outside the riparian zone and the mapped seepage area was a focused discharge point for the catchment-scale groundwater flow system.
Overall this work suggests that for this catchment, groundwater exfiltration in the near-stream zone strongly controls stream geochemical response as well as the timing, duration and quantity of streamflow generation.