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A sprinkling experiment to quantify celerity-velocity differences at the hillslope scale
Citation:
van Verseveld, W., H. Barnard, C. Graham, J. McDonnell, J. Renee Brooks, AND M. Weiler. A sprinkling experiment to quantify celerity-velocity differences at the hillslope scale. HYDROLOGY AND EARTH SYSTEM SCIENCES. EGS, 21(11):5891-5910, (2017).
Impact/Purpose:
Understanding how water and chemicals are delivered to streams through the terrestrial ecosystem is critical for EPA to quantify ecosystems services related to water quality and quantity within watersheds. Researchers studying water transport have identified a double paradox where they observe a rapid movement of event and stored water to the stream during a storm, while chemical constituents show a highly variable response. Resolving the double paradox is essential to improve our understanding of flowpaths and predicting transport of contaminants and natural (bio)-geochemical solutes within watersheds. One important aspect of this paradox is the differences between the flow velocities in the system (that control the tracer response) and the celerities (or speed with which perturbations are transmitted which control the hydrograph) and how these differences influence water quality and quantity in storm events. Here, we mechanistically assess the differences between the celerity and velocity of the hillslope hydrograph using a combination of a hillslope-scale sprinkler experiment and a process-based spatially explicit hydrologic model. This study helps resolve the double paradox by highlighting the mixing of event water with stored water within the upper soil profile and the importance of mobile flowpaths through the hillslope to the stream. These results will enable EPA to better predict how landscapes function in providing clean water to streams. This paper contributes to SSWR 4.03C.
Description:
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. δ2H label was applied at the start of the sprinkler experiment. Maximum event water (δ2H 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-1) than average vertical velocities of δ2H (on the order of 6 – 17 mm h-1). 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 δ2H 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 δ2H input signal, based on calculations with our mixing model. Our results suggest that soil depth variability played a significant role in the velocity-celerity responses. Deeper upslope soils damped the δ2H input signal and played an important role in the generation of the δ2H breakthrough curve. A shallow soil (~ 0.30 m depth) near the trench controlled the δ2H 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.
