Automated hanging water column for characterizing water retention and hysteresis of coarse-textured porous media

Modeling and characterizing hysteretic water retention is critical for predicting hydrodynamic behavior in porous media. This is especially true in coarse-textured media used in geotechnical engineering, greenhouse, and landscape industries, where subtle changes in water status may lead to plant stress. However, based on the traditional hanging water column method, water retention measurements are laborious and time consuming because of the stepwise manual water potential adjustments and wait-time requirements for equilibrium conditions to develop. Therefore, we designed and fabricated an automated system to collect wetting- and drying-water retention data from coarse porous media. The basic system consisted of (1) a compound pressure transducer (± 70-cm range) providing both the porous medium's volumetric water content (θ) and matric potential (h) determinations, (2) a 70-cm linear actuator to vertically position a 50-mL burette, and (3) a diffuse laser distance sensor positioned by a 10-cm linear actuator to monitor the burette's vertical position relative to the sample position. This automated system determined the initial drying process beginning with a fully saturated sample (h = 0 cm) and determined subsequent wetting- and drying-water retention curves. Our automated water retention measurements in quartz sand (ASTM C778-21) exhibited maximum- and minimum-standard deviation in θ of 0.013 and 0.00044 cm³ cm⁻³, respectively. Parameters of the hysteretic water retention model of quartz sand were characterized using repeated measurements. Results of this research included the creation of an automated water retention system and the well-characterized hydraulic parameters for the original well-graded and narrowly sieved particle sizes of quartz sand.