Simulation of water and solute transport with MACRO model in Cecil loamy sand soil

Abstract

Modelling preferential flow increases understanding of flow and transport processes in the unsaturated (vadose) zone and, hence, helps prevention of groundwater contamination. A dual-porosity model, MACRO, was evaluated for long-term drainage flow and short-term chloride-tagged water flow simulations in well-structured Cecil loamy sand soil. Water flow in micropores is calculated by the Richards’ equation, and simple gravity flow is assumed in the macropores. Solute transport in the micropores is calculated by the convection–dispersion equation (CDE), and the dispersion and diffusion in the CDE is neglected for the solute transport in the macropores. Based on the statistical criteria, the model accurately simulated drainage flow with depth and time. The average values of 3 statistical parameters (coefficient of residual mass, model efficiency, correlation coefficient) for drainage flow of different plots and times were 0.0057, 0.972, and 0.987, respectively. Similarly, the average values of the 3 statistical parameters in the same order for water flow and chloride transport of 4 plots were 0.097, 0.628, and 0.915, and 0.167, 0.938, and 0.982, respectively. The model simulated long-term drainage flow better than the short-term applied chloride-tagged water. The percentage recovery of measured water and chloride 2 h after the application ceased was 83 and 63 in the 1.05-m-deep profile of plot 1. This was a strong indication of preferential flow in this soil. Two-domain flow concept was required for acceptable simulation of this type of flow. In the 2-domain flow, boundary tension, boundary hydraulic conductivity, and effective diffusion path-length were the 3 most important parameters controlling flow and transport between the 2 domains. The effective diffusion path-length represented the structural development with depth in Cecil loamy sand soil. The relationships between the variability in flow and transport characteristics and fundamental soil properties and, hence, the associated key model parameters suggest that pedotransfer functions can be developed for the estimation of dual-porosity model parameters that control preferential flow.