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WIREs Water
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From hillslope to stream: methods to investigate subsurface connectivity

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Hydrologic connectivity is the linkage of separate regions of a catchment via water flow. Knowledge of hillslope–stream connectivity (both at the surface and in the subsurface) is essential for understanding and predicting runoff responses and streamwater quality. Connectivity can be very dynamic: hillslopes may connect to the stream only during certain events or seasons. While surface connectivity is often discussed, particularly in the context of sediment transport, subsurface connectivity is more difficult to describe and assess. This difficulty has led to a wide variety in methodologies that are used in various contexts. Field approaches have focused on intensive monitoring of processes on the hillslope or the fingerprint of connectivity in the stream. Combining experimental studies with modeling allows for testing of hypotheses with respect to thresholds and controls on connectivity, and extrapolation from the hillslope scale to the catchment scale. However, as most modeling approaches are based on datasets from a few intensively studied hillslopes, this carries the inherent risk of oversimplification because it assumes that the observed hillslope responses are representative for the catchment or even the region. Focussed efforts on catchment scale assessment of hillslope–stream connectivity, as well as site intercomparisons and the search for similarity measures may allow us to capture the wider picture of the mechanisms and factors that control hillslope–stream connectivity, and its effects on flow and transport at the catchment scale. This overview focuses on how hillslope–stream connectivity has been studied and describes the advantages, disadvantages, and challenges of the different methods. WIREs Water 2015, 2:177–198. doi: 10.1002/wat2.1071 This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality
Diffuse hillslope–stream connectivity (a), focused hillslope–stream connectivity due to concentrated flow at the soil–bedrock interface (b), and concentrated flow in macropores (c).
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Focused hillslope–stream connectivity: water flowing out of preferential flow pathway in Luxembourg (Photo: Theresa Blume).
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Establishment of subsurface connectivity across a hillslope during events.
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Hillslope‐centered measurements: (a) subsurface flow measurements at the Panola trench showing the 2 m sections collecting diffuse subsurface flow and macropore flow collectors collecting concentrated subsurface flow, (b) tipping buckets collecting subsurface flow from the different trench sections, (c) groundwater wells for detailed measurements of saturation at the soil–bedrock interface at the Panola hillslope, showing the trench on the lower part of the hillslope, (d) dye staining in Maimai to study channelized subsurface flow at the soil–bedrock interface, (e) GPR measurements during a tracer experiment in Luxembourg (Photos: (a)–(c): Ilja van Meerveld, (d): Chris Graham, (e): Niklas Allroggen).
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Hillslope‐centered (e.g., piezometers, soil moisture, or ERT probes in green) and stream‐centered (sampler to collect streamwater samples in purple) approaches to study hillslope–stream connectivity.
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Science of Water > Water Quality
Science of Water > Hydrological Processes

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