Articles | Volume 21, issue 11
Hydrol. Earth Syst. Sci., 21, 5891–5910, 2017
Hydrol. Earth Syst. Sci., 21, 5891–5910, 2017

Research article 27 Nov 2017

Research article | 27 Nov 2017

A sprinkling experiment to quantify celerity–velocity differences at the hillslope scale

Willem J. van Verseveld1, Holly R. Barnard2, Chris B. Graham3, Jeffrey J. McDonnell4,5, J. Renée Brooks6, and Markus Weiler7 Willem J. van Verseveld et al.
  • 1Deltares – Catchment and Urban Hydrology Department, Delft, the Netherlands
  • 2Institute of Arctic and Alpine Research, Department of Geography, University of Colorado, Boulder, CO, USA
  • 3Hetchy Hetchy Water and Power, Moccasin, CA, USA
  • 4Global Institute for Water Security and School of Environment and Sustainability, University of Saskatchewan, Saskatchewan, Canada
  • 5School of Geoscience, University of Aberdeen, Aberdeen, Scotland
  • 6Western Ecology Division, US EPA/NHEERL, Corvallis, OR, USA
  • 7Chair of Hydrology, Faculty of Environment and Natural Resources, University of Freiburg, Freiburg, Germany

Abstract. Few studies have quantified the differences between celerity and velocity of hillslope water flow and explained the processes that control these differences. Here, we asses these differences by combining a 24-day hillslope sprinkling experiment with a spatially explicit hydrologic model analysis. We focused our work on Watershed 10 at the H. J. Andrews Experimental Forest in western Oregon. Celerities estimated from wetting front arrival times were generally much faster than average vertical velocities of δ2H. In the model analysis, this was consistent with an identifiable effective porosity (fraction of total porosity available for mass transfer) parameter, indicating that subsurface mixing was controlled by an immobile soil fraction, resulting in the attenuation of the δ2H input signal in lateral subsurface flow. In addition to the immobile soil fraction, exfiltrating deep groundwater that mixed with lateral subsurface flow captured at the experimental hillslope trench caused further reduction in the δ2H input signal. Finally, our results suggest that soil depth variability played a significant role in the celerity–velocity responses. Deeper upslope soils damped the δ2H input signal, while a shallow soil near the trench controlled the δ2H peak in lateral subsurface flow response. Simulated exit time and residence time distributions with our hillslope hydrologic model showed that water captured at the trench did not represent the entire modeled hillslope domain; the exit time distribution for lateral subsurface flow captured at the trench showed more early time weighting.

Short summary
How stream water responds immediately to a rainfall or snow event, while the average time it takes water to travel through the hillslope can be years or decades and is poorly understood. We assessed this difference by combining a 24-day sprinkler experiment (a tracer was applied at the start) with a process-based hydrologic model. Immobile soil water, deep groundwater contribution and soil depth variability explained this difference at our hillslope site.