Runoff generation across ephemeral to perennial Piedmont catchments
Ephemeral and intermittent streams comprise the majority of stream channel length worldwide. These non-perennial streams are important landscape features in that they transport materials and solutes from the terrestrial landscape to downstream aquatic systems, provide unique ecological habitats, and transmit, transform, and retain chemical species. Although these streams serve a myriad of hydrological and biogeochemical functions, the mechanisms that drive runoff generation as well as the hydrological and biogeochemical contributions of these non-perennial streams to downstream perennial stream systems are poorly understood. To address this knowledge gap, this dissertation focused on investigating dominant runoff sources, flowpaths, and stream-groundwater interactions across an ephemeral-to-perennial drainage network located in the humid Piedmont landscape of the Duke Forest, North Carolina, USA. Not only does this research work to fill an important knowledge gap, but it was conducted in a low relief landscape, which is a highly understudied landscape type.
A 48.4 hectare research watershed was designed, fully instrumented, and continuously managed to address the objectives of this dissertation. Through this, the timing and magnitude of precipitation, runoff, and the dominant runoff generating flowpaths were monitored across watershed scales at 5 minute intervals for two years. In the first chapter of this dissertation, observations of hydraulic gradients between groundwater and the stream were used to show event- and seasonal-driven bidirectional flow between these two systems. The results of this work indicated that non-perennial headwater streams can both lose to and gain water from the deep groundwater system. Calculations from an annual water balance in the headwater catchment confirmed this temporally shifting flow system that resulted in significant annual stream water loss to the groundwater system. In the second chapter, a data-driven visualization of the bidirectional stream-groundwater interactions across a characteristic hillslope that built on Chapter 1 results was presented. This animation and supporting text provided visualization of a shallow, perched, transient water table that drove runoff generation during periods of losing stream gradients. Chapter 3 built on these results through introduction of a conceptual model of the dominant runoff generating processes in this headwater catchment during ephemeral and intermittent runoff regimes. This chapter focused on two intensively sampled precipitation events to demonstrate that distinct runoff generating mechanisms dominate runoff regimes across different catchment storage states. Finally, in the fourth chapter, observations of dissolved organic carbon fluxes, runoff contributions, active surface drainage network length, and groundwater across different landscape elements (headwaters and lowlands), were used to investigate the balance of longitudinal, lateral, and vertical runoff processes on runoff and carbon export dynamics across watershed scales. Overall, these findings shed light on dominant runoff generation mechanisms in low relief, highly weathered landscapes, which are a globally understudied landscape type. These findings also fill a knowledge gap about the mechanisms that drive runoff generation and stream-groundwater interactions surrounding non-perennial streams and the resulting influences on carbon and water fluxes in downstream, perennial waterways.
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Rights for Collection: Duke Dissertations