Lab Scale Model Simulations of Unsaturated Flow Using Hydraulic Property Clusters.

Unsaturated fluid flow within a natural soil system is heterogeneous by nature. Quantification of this flow is difficult, but can be attributed to differences in hydraulic properties within the soil. We propose that model simulations of three-dimensional outflow with spatially varied hydraulic properties arranged in clusters will be able to quantify unsaturated heterogeneous flow. A multi-step outflow experiment with simulated rainfall infiltration was used on an undisturbed soil column over a 41 day period. Outflow capture was designed to account for spatial heterogeneity. Thirty samples of equal volume with known spatial variability were taken from the column after the outflow experiment. Hydraulic properties of the samples were determined using both an automated simplified evaporation method and through numerical inversion of tension data through the Hydrus-1D program. Hydraulic model parameter variation was analyzed to compare the relative effectiveness of both methods. Simulated homogeneous outflows were obtained using a homogeneous forward model in three dimensions and the obtained hydraulic model parameters in the Hydrus-3D program. Simulated homogeneous outflow values were used for grouping purposes, with two main groups being defined: high flow and low flow. Hydraulic properties of the representative samples in each group were averaged together to make an "effective" hydraulic property cluster. Initial spatial distribution of clusters was based on previous work done with the column involving Electrical Resistance Tomography (ERT) measurements. The outflow values from the use of these high and low flow clusters was compared to the measured values from the outflow for validation of the model. Spatial variation of heterogeneous flow was further studied using other configurations of hydraulic property clusters and observing the simulated outflows.