[A-GE03_30AM2] Subsurface Mass Transport and Environmental Assessment

★ [AGE03-05] Estimation of Water Flux in Andisol with a Penta-Needle Heat Pulse Probe

*Masaru SAKAI¹, Naho KONDO¹, Scott JONES² (1.Graduate school of Bioresources, Mie University, 2.Department of Plants, Soils, and Climate, Utah State University)

Keywords:soil water flux, heat pulse probe, Andisol, dispersivity

The potential for using heat pulse probes for estimating soil water flux as well as soil thermal properties has received more attention this past decade. Although many studies were carried out to validate water flux estimation using heat pulse probes in sandy soils, few studies were reported for other soils. The purpose of this study was to estimate water fluxes in an aggregated Andisol using a heat pulse probe, and investigate the applicability with hydrodynamic dispersion in a soil.The Penta-needle heat pulse probe, which has a central heater needle surrounded by two pairs of orthogonally arranged thermistors, was used to estimate two directional water flux. Steady-state saturated water flow and unit-gradient unsaturated water flow experiments were conducted in Mie Andisol. To achieve saturated conditions, the Andisol was packed in the column with a bulk density of 0.85 g/cm³ and afterward it was saturated by applying water from column bottom. A glass filter was located at the bottom of the column. CaCl₂ solutions were applied from the top of the column at fixed rates using a peristaltic pump, and outflows from the bottom were measured by a scale. The flow rates were decreased stepwise from fast (around 350 cm/day) to slow rates (around 5 cm/day). Using faster flow steps, steady state saturated water flows were developed. Steady state conditions for unit-gradient - unsaturated water flow were developed by controlling suction at the column bottom, in which water contents were uniform and water flowed by gravity. At each flow steps, heat pulse measurements were conducted, and the influent solution concentrations were changed to obtain breakthrough curves (BTCs) by measuring soil electrical conductivities with four-probe salinity sensors. Water fluxes were estimated by applying an analytical solution to temperature rise data. Dispersivities were determined by applying the convection-dispersion equation to BTCs. Each experiment, including packing soil and water flow testing, were repeated a few times.In saturated conditions, water fluxes estimated by the heat pulse probe agreed well with independently measured water fluxes in one experiment and underestimations were found in two cases. For unsaturated conditions, estimated water fluxes agreed well with actual fluxes even in the experiment with disagreement in saturated conditions. The flux estimation errors were compared with dispersivities which can be interpreted as the scale of water flow spreading from mean displacement position. Large estimation errors were found for experiments with large dispersivities (λ > 1.5 cm), while errors were relatively small for conditions with smaller dispersivities both in saturated and unsaturated water flows. Generally, dispersivity values in aggregated Andisol is larger in saturated condition than in unsaturated condition. The experimental results in this study indicates that the applicability of heat pulse probe to aggregated soils potentially results in better water flux estimation in unsaturated conditions.