Correction factor for measuring mean overland flow velocities under rainfall using dye tracer

Authors: Polyakov, Viktor; LI, L. - University Of Arizona; Nearing, Mark

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2021
Publication Date: 5/15/2021
Citation: Polyakov, V.O., Li, L., Nearing, M.A. 2021. Correction factor for measuring mean overland flow velocities under rainfall using dye tracer. Geoderma. 390, Article 114975. https://doi.org/10.1016/j.geoderma.2021.114975.
DOI: https://doi.org/10.1016/j.geoderma.2021.114975

Interpretive Summary: Velocity of overland flow is a key variable for understanding and predicting soil erosion processes. One of the simplest and most common methods to measure the velocity is by introducing dye tracer into the flow. However, this method provides only surface (or maximum) velocity. A correction coefficient is then needed to convert the latter to the mean flow velocity. In this study we investigated factors that affect the correction coefficient using erodible soil plot under simulated rainfall. We developed a prediction equation to determine the correction coefficient based on flow rate, travel distance, and maximum velocity. In our prediction equation we emphasized the use of variables that are readily available in field experiments, which might aid in its wider adoption.

Technical Abstract: Dye tracing is a common and simple method for measuring surface velocity of overland flow. However, a correction factor is needed to convert the velocity of dye front to mean velocity. Selecting the appropriate correction factor for particular conditions is complex and its behavior has not been well studied in sheet flow under rainfall on actively eroding surfaces. A series of simulated rainfalls were conducted on a 2 m by 6 m plot with gravely sandy loam soil. The plot was progressively eroded to examine a wide range of hydraulic conditions. A total of 178 velocities (maximum and mean) were measured using electrolyte and dye tracers over three travel distances (1.65, 3.5, and 5.8 m), and three slope gradients (5%, 12% and 20%). Discharge, sediment yield, random roughness, rock cover, and a range of hydraulic variables were also determined. Mean flow velocity was derived from the centroid of electrolyte breakthrough curve. Our findings suggest that velocity correction factor is a dynamic, site specific property. A linear model was proposed to estimate the correction factor based on readily available predictor variables, those being travel distance, unit discharge, and maximum velocity. The results will benefit further investigations of overland flow hydraulics on semiarid rangelands by providing more comprehensive correction factors to determine mean velocity.