Quantification of Dynamic Effects in Capillary Pressure for Measuring Water Retention Curve

Abstract:

Understanding soil moisture processes in the subsurface is a requirement to solve a variety of hydrologic problems. Traditionally, the water retention curve is measured in the laboratory under equilibrium conditions, and it is generally assumed that the estimated hydraulic properties can be used to model transient water flow under field conditions. However, recent theories have indicated that the hydraulic characteristics are flow rate-dependent and the dynamic effects should be considered. In this study, a series of laboratory experiments were performed in a homogeneous column to investigate the significance of dynamic effects. The experimental set-up consists of a cylinder of 30 cm in diameter and 70 cm in height filled with the field sand and quartz sand, and equips with the time domain reflectometry (TDR) and tensiometer to measure the moisture content and capillary pressure, respectively. Through a continuous cycle of drainage and imbibition, the dynamic retention curves are established and the magnitude of dynamic coefficient (τ) is quantified. Experimental results show that the water saturation and capillary pressure are affected by the flow rate and produce different water retention curves. At a given pressure, more water is retained in the sand for the transient flow case than for the static equilibrium case during the drainage, but the result is opposite during the imbibition. The measured τ varies with saturation, generally increased with decreasing saturation during the drainage. The measured τ is also dependent on porous media mean grain size, and its value increased in the finer sand. Although the mechanisms of dynamic effects are not clear, their effects on the water retention curve cannot be ignored when quantifying the hydraulic properties.

Keywords: unsaturated zone, water retention curve, dynamic effect, capillary pressure, experiment