Biological and Environmental Engineering Soil & Water Research Group Hydrological pathways in a glaciated watershed in the Catskill Mountains Adrian 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. 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 km 2

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 events Saturated 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.

Biological and Environmental Engineering Soil & Water Research Group How do we measure saturation areas in space and time? Single well-scale: watershed size Well field:scale: watershed size Foot-surveys: watershed size Common features across scale: 1. Fast response of water table to rainfall 2. Depends on antecedent conditions 3. Water table