Biological and Environmental Engineering Soil & Water Research Group Hydrological pathways in a...

Biological and Environmental Engineering

Soil & Water Research Group

Hydrological pathways in a

glaciated watershed in the Catskill

MountainsAdrian Harpold



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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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?



Learning from large-scale saturation maps



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



Response to rainfall events

Prior to rainfall

During rainfall

New saturated connections to the stream channel



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)



Response of stream chemistry to rainfall

Single Event Nine Events



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



• 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)



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.



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.

Biological and Environmental Engineering Soil & Water Research Group Hydrological pathways in a...

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