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

Area of Science:

• Hydrology
• Environmental Science
• Soil Science

Background:

• Quantifying hillslope water flow celerity and velocity differences is crucial for understanding hydrological processes.
• Previous studies have limited data on the factors controlling these flow dynamics.

Purpose of the Study:

• To quantify the differences between hillslope water flow celerity and velocity.
• To explain the hydrological processes controlling these differences using a combined experimental and modeling approach.

Main Methods:

• Conducted a 24-day hillslope sprinkling experiment at Watershed 10, H. J. Andrews Experimental Forest.
• Employed a spatially explicit hydrologic model for analysis.
• Utilized wetting front arrival times for celerity estimation and stable isotopes (δ²H) for velocity measurements.

Main Results:

• Hillslope water flow celerity was generally faster than the average vertical velocity of δ²H.
• Model analysis revealed that an immobile soil fraction controlled subsurface mixing and attenuated the δ²H signal.
• Exfiltrating deep groundwater and variable soil depth also influenced δ²H signal attenuation and response.

Conclusions:

• Immobile soil fractions and deep groundwater mixing significantly reduce the δ²H input signal in lateral subsurface flow.
• Soil depth variability critically affects celerity-velocity responses and signal damping.
• Water captured at the trench represents only a portion of the hillslope, with an earlier time weighting in its exit distribution.