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| Titel |
Assessing temporal variations in connectivity through suspended sediment hysteresis analysis |
| VerfasserIn |
Sophie Sherriff, John Rowan, Owen Fenton, Phil Jordan, Alice Melland, Per-Erik Mellander, Daire Ó hUallacháin |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122063
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| Publikation (Nr.) |
 EGU/EGU2016-994.pdf |
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| Zusammenfassung |
| Connectivity provides a valuable concept for understanding catchment-scale sediment
dynamics. In intensive agricultural catchments, land management through tillage,
high livestock densities and extensive land drainage practices significantly change
hydromorphological behaviour and alter sediment supply and downstream delivery. Analysis
of suspended sediment-discharge hysteresis has offered insights into sediment dynamics but
typically on a limited selection of events. Greater availability of continuous high-resolution
discharge and turbidity data and qualitative hysteresis metrics enables assessment of sediment
dynamics during more events and over time. This paper assesses the utility of this approach to
explore seasonal variations in connectivity.
Data were collected from three small (c. 10 km2) intensive agricultural catchments in Ireland
with contrasting morphologies, soil types, land use patterns and management practices, and
are broadly defined as low-permeability supporting grassland, moderate-permeability
supporting arable and high-permeability supporting arable. Suspended sediment
concentration (using calibrated turbidity measurements) and discharge data were collected at
10-min resolution from each catchment outlet and precipitation data were collected
from a weather station within each catchment. Event databases (67-90 events per
catchment) collated information on sediment export metrics, hysteresis category (e.g.,
clockwise, anti-clockwise, no hysteresis), numeric hysteresis index, and potential
hydro-meteorological controls on sediment transport including precipitation amount,
duration, intensity, stream flow and antecedent soil moisture and rainfall. Statistical
analysis of potential controls on sediment export was undertaken using Pearson’s
correlation coefficient on separate hysteresis categories in each catchment. Sediment
hysteresis fluctuations through time were subsequently assessed using the hysteresis
index.
Results showed the numeric hysteresis index varied over time in all three catchments. The
exact response was catchment specific reflecting changing sediment availability and
connectivity through time as indicated by dominant controls. In the low-permeability
grassland catchment, proximal sources dominated which was consistent with observations of
active channel bank erosion. Seasonal increases in rainfall increased the erosion potential but
continuous grassland cover mitigated against hillslope sediment contributions despite high
hydrological connectivity and surface pathways. The moderate-permeability arable catchment
was dominated by events with a distal source component but those with both proximal and
distal sediment sources yielded the highest sediment quantities. These events were driven by
rainfall parameters suggesting sediment were surface derived and the hillslope was
hydrologically connected during most events. Through time, a sustained period of rainfall
increased the magnitude of negative hysteresis, likely demonstrating increasing
surface hydrological connectivity due to increased groundwater saturation. Where
increased hydrological connectivity coincided with low groundcover, the largest
sediment exports were recorded. Events in the high permeability catchment indicated
predominantly proximal sediments despite abundant distal sources from tilled fields. The
infiltration dominated high permeability soils hydrologically disconnected these
field sources and limited sediment supply. However, the greatest sediment export
occurred in this catchment suggesting thresholds existed, which when exceeded
during higher magnitude events, resulted in efficient conveyance of sediments.
Hysteresis analysis offers wider utility as a tool to understand sediment pathways
and connectivity issues with applications to catchment management strategies. |
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