High-Resolution Pore-Scale Water Content Measurement in a Translucent Soil Profile from Light Transmission

Abstract

HighlightsResearch methods are needed to study preferential flow processes at pore scale and high temporal resolution.Novel verification of the light transmission method shows high efficiency to measure rapid transient soil water flow.Recast of a previous physical model allows reliable pore-scale water content quantification in translucent soil profiles.Insights from the light transmission method can inform preferential flow modeling efforts.Abstract. Understanding rapid transient flows in the soil unsaturated zone continues to be a major challenge in hydrology and water quality engineering. For example, surface runoff mitigation by riparian buffers can be limited by rapid transient flows due to the natural propensity of these areas for preferential flow pathways (i.e., caused by roots, wormholes, or wetting/drying cycles). However, current monitoring technologies are limited in their ability to capture rapid soil preferential flows at high spatial and temporal resolutions. Among the state-of-the-art technologies to monitor preferential flow, the light transmission method (LTM) has become a promising tool to quantify pore-scale water contents at a laboratory scale, but its reliability and consistency need further study. The objectives of this study are to recast a previously developed LTM physical model, propose a novel verification method to assess LTM reliability to measure pore-scale water dynamics in laboratory translucent soil profiles, and identify the representative pore radius of translucent soil profiles based on their average number of pores. This study found a high measuring efficiency with LTM for soil moisture and drainage estimations (NSE > 0.98, RMSE < 5.4%), showing its potential for use in laboratory analysis of pore-scale rapid transient water dynamics typically found in preferential flow through the vadose zone. This study also shows that the parameter traditionally associated with the number of pores in a translucent soil profile is a fitting parameter with no direct physical meaning. Keywords: Beer-Lambert law, Fresnel law, Light transmission method, Preferential flow, Riparian buffer, Vadose zone.