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.
Highlights
Residence time distributions, velocities and celerities at the catchment and hillslope scales are poorly understood (McDonnell and Beven, 2014)
A detectable increase in lateral subsurface flow was observed within an hour of the start of irrigation on day of year (DOY) 208 (Fig. 2)
Celerities estimated from wetting front arrival times were generally much faster than average vertical velocities of δ2H. This was consistent with our modeling results that showed that the transport of δ2H through the hillslope was controlled by effective porosity, indicating a subsurface system where mixing is controlled by an immobile soil fraction
Summary
Velocities and celerities at the catchment and hillslope scales are poorly understood (McDonnell and Beven, 2014). Countless studies have shown that the hydrologic response of headwater catchments can be much faster (orders of magnitude) than the mean transit time of water (e.g., Martinec, 1975; Seeger and Weiler, 2014). Different mechanisms control celerities (technically, the celerity of the hydraulic potentials, hereafter called “celerities” and described as the hydrograph response to precipitation inputs) and flow velocities (i.e., the time it takes a tracer to travel through the system). While differences between the two are to be expected (McDonnell and Beven, 2014), few studies have quantified these differences and have explained the processes responsible for them. Studies have found different controls on how stored water travels rapidly to the stream: transmissivity feedback
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
