biological and environmental engineering soil & water research group hydrological pathways in a...
Post on 21-Dec-2015
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Biological and Environmental Engineering
Soil & Water Research Group
Hydrological pathways in a
glaciated watershed in the Catskill
MountainsAdrian Harpold
Biological and Environmental Engineering
Soil & Water Research Group
Improving our understanding of runoff processes in the Catskills
• Stream chemistry is a function of 1) sources, 2) flowpaths 3) age of water.
• Variable saturated areas (VSA) are a quick source of storm runoff.
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aSTWI
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Where from?
How long?
How did it get there?
• Spatial predictions of VSA are often based on topography and soil properties.
• Are we neglecting the effects of small-scale variability on saturated areas?
Biological and Environmental Engineering
Soil & Water Research Group
Learning from large-scale saturation maps
Biological and Environmental Engineering
Soil & Water Research Group
Identifying and monitoring controls on saturation areas
• Soil piping creates near-stream saturated areas
• Groundwater springs generate hillslope saturated areas
• Overland flow, water table heights, and water chemistry are collected across the hilllsope
36.8 km2
Biological and Environmental Engineering
Soil & Water Research Group
Response to rainfall events
Prior to rainfall
During rainfall
New saturated connections to the stream channel
Biological and Environmental Engineering
Soil & Water Research Group
Connection between stream discharge and hillslope features
• Water table shows hysteresis versus streamflow (delayed)
• Near-stream transient GW shows least delay
• Overland flow measurements are much more correlated to streamflow(no hysteresis)
Biological and Environmental Engineering
Soil & Water Research Group
Response of stream chemistry to rainfall
Single Event Nine Events
Biological and Environmental Engineering
Soil & Water Research Group
Identifying source areas to the stream via end-member mixing analysis
Groundwater contributes 53% to 95% of runoff volume during 9 eventsSaturated areas contribute 2% to 24% Throughfall contributes 4% to 25%
Groundwater dominates baseflow
Saturated areas respond quickly to
large rainfall
Rain response is delayed and
damped
Biological and Environmental Engineering
Soil & Water Research Group
• Contributions from VSA are dependent on antecedent conditions
Effects of hillslope processes on stream response for nine rainfall events
• Large storm runoff volumes have similar VSA contributions• Contributions from rainfall control the peak runoff for each event.
Near-stream saturation areas
active
Hillslope saturation areas connected (maximum
contributing area achieved)
Hillslopes begin to throughfall to stream (maximum
contributing area achieved)
Biological and Environmental Engineering
Soil & Water Research Group
Conceptualizing source areas and flowpaths in a glaciated watershed
• Previous conceptualization: saturated areas grow in extent from the toe of the hillslope up.
Prior to rainfall
During rainfall
• Alternative conceptualization: saturated areas near the stream are connected to discrete hillslope saturated areas.
• Maximum saturation contribtuions are limited by the ‘quick’ draining preferential flowpaths.
Biological and Environmental Engineering
Soil & Water Research Group
Conclusions• Subsurface variabilities (e.g. groundwater springs
and soil piping) can cause persistent spatial patterns of surface saturation, even during very dry periods.
• Expansion of near-stream saturated areas are a source of runoff in smaller storms (<7 mm), but hillslope saturation areas contribute regularly in larger events.
• After the maximum saturation extent has been reached, additional rainfall contributes throughfall to the stream.