Abstract
A novel, non-invasive imaging technique is proposed that determines 2D maps of water content in unsaturated porous media. This method directly relates digitally measured intensities to the water content of the porous medium. This method requires the classical image analysis steps, i.e., normalization, filtering, background subtraction, scaling and calibration. The main advantages of this approach are that no calibration experiment is needed, because calibration curve relating water content and reflected light intensities is established during the main monitoring phase of each experiment and that no tracer or dye is injected into the flow tank. The procedure enables effective processing of a large number of photographs and thus produces 2D water content maps at high temporal resolution. A drainage/imbibition experiment in a 2D flow tank with inner dimensions of 40cm×14cm×6cm (L×W×D) is carried out to validate the methodology. The accuracy of the proposed approach is assessed using a statistical framework to perform an error analysis and numerical simulations with a state-of-the-art computational code that solves the Richards’ equation. Comparison of the cumulative mass leaving and entering the flow tank and water content maps produced by the photographic measurement technique and the numerical simulations demonstrate the efficiency and high accuracy of the proposed method for investigating vadose zone flow processes. Finally, the photometric procedure has been developed expressly for its extension to heterogeneous media. Other processes may be investigated through different laboratory experiments which will serve as benchmark for numerical codes validation.
Highlights
Due to its location at the interface between the atmosphere, vegetation and the land surface, the vadose zone plays a crucial role in the hydrological cycle; it supports many processes controlling the complex relationships between precipitation, infiltration, surface runoff, evapotranspiration and groundwater recharge (Corwin et al, 2006; Harter and Hopmans, 2004; Selker et al, 1999)
Laboratory or small-scale field experiments are well suited to investigate the interconnected processes that occur in the vadose zone and lead to improved understanding through measurements
Outside the scope of this paper, which focuses on the vadose zone, previous laboratory experiments investigating variable density groundwater flow (Konz et al, 2008 - 2009a – 2009b; Schincariol and Schwartz, 1990; Simmons et al, 2002) or contaminant hydrogeological processes that involve, for instance, nonaqueous phase liquid (NAPL) flow behaviour in porous media (Chevalier and Petersen, 1999; Kamaruddin et al, 2011; Kechavarzi et al, 2000; Kechavarzi et al, 2005; McNeil et al, 2006; Oostrom et al, 2003; Oostrom et al, 2007; van Geel and Sykes, 1994) are worth mentioning, as they can provide inspiration in term of experimental design and monitoring solutions
Summary
Due to its location at the interface between the atmosphere, vegetation and the land surface, the vadose zone plays a crucial role in the hydrological cycle; it supports many processes controlling the complex relationships between precipitation, infiltration, surface runoff, evapotranspiration and groundwater recharge (Corwin et al, 2006; Harter and Hopmans, 2004; Selker et al, 1999). Outside the scope of this paper, which focuses on the vadose zone, previous laboratory experiments investigating variable density groundwater flow (Konz et al, 2008 - 2009a – 2009b; Schincariol and Schwartz, 1990; Simmons et al, 2002) or contaminant hydrogeological processes that involve, for instance, nonaqueous phase liquid (NAPL) flow behaviour in porous media (Chevalier and Petersen, 1999; Kamaruddin et al, 2011; Kechavarzi et al, 2000; Kechavarzi et al, 2005; McNeil et al, 2006; Oostrom et al, 2003; Oostrom et al, 2007; van Geel and Sykes, 1994) are worth mentioning, as they can provide inspiration in term of experimental design and monitoring solutions. All these experiments allow for direct investigation of the physical processes of concern and produce datasets that are valuable for calibrating and validating numerical models
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