A sprinkling experiment to quantify celerity-velocity differences at the hillslope scale Journal Article uri icon



  • <p><strong>Abstract.</strong> The difference between celerity and velocity of hillslope water flow is poorly understood. We assessed these differences by combining a 24-day hillslope sprinkling experiment with a spatially explicit hydrologic model analysis. We focused our work at Watershed 10 at the H. J. Andrews Experimental Forest in western Oregon. <i>δ</i><sup>2</sup>H label was applied at the start of the sprinkler experiment. Maximum event water (<i>δ</i><sup>2</sup>H labeled water) contribution was 26 % of lateral subsurface flow at 20 h. Celerities estimated from wetting front arrival times were generally much faster (on the order of 10–377 mm h<sup>−1</sup>) than average vertical velocities of <i>δ</i><sup>2</sup>H (on the order of 6–17 mm h<sup>−1</sup>). 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 an attenuated <i>δ</i><sup>2</sup>H in lateral subsurface flow. Furthermore, exfiltrating bedrock groundwater that mixed with lateral subsurface flow captured at the experimental hillslope trench caused further reduction in the <i>δ</i><sup>2</sup>H input signal. Our results suggest that soil depth variability played a significant role in the velocity-celerity responses. Deeper upslope soils damped the <i>δ</i><sup>2</sup>H input signal and played an important role in the generation of the <i>δ</i><sup>2</sup>H breakthrough curve. A shallow soil (~ 0.30 m depth) near the trench controlled the <i>δ</i><sup>2</sup>H peak in lateral subsurface flow response. Simulated exit time and residence time distributions with the hillslope hydrologic model were consistent with our empirical analysis and provided additional insights into hydraulic behavior of the hillslope. In particular, it showed that water captured at the trench was not representative for the hydrological and mass transport behavior of the entire hillslope domain that generated total lateral subsurface flow, because of different exit time distributions for lateral subsurface flow captured at the trench and total lateral subsurface flow.</p>;

publication date

  • November 27, 2017

Full Author List

  • van Verseveld WJ; Barnard HR; Graham CB; McDonnell JJ; Brooks JR; Weiler M

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