Numerical modelling of vadose zone flow for a shallow groundwater wetland using HYDRUS-1D

Abstract This study employs HYDRUS-1D for modelling the vadose zone processes in a shallow groundwater wetland. HYDRUS-1D is based on Richards equation whose numerical solution heavily relies on reliable data derived from soil water retention curves. This modelling was supported by extensive field and laboratory measurements. Both disturbed and undisturbed soil samples were collected at the four depths for determination of particle size distribution, bulk density, saturated hydraulic conductivity, and saturated water content. The van Genuchten retention curves were used to describe the relationship between pressure head and water content; model parameters were optimized through inverse modelling with initial estimates of residual water content, shape parameters (α and n) obtained from ROSETTA model 3 (provided in HYDRUS-1D) and saturated water content and saturated hydraulic conductivity, determined from laboratory tests. Independent measurements of soil moisture and soil tension measured both in the field and laboratory fit well with the water retention curve obtained from the optimized ROSETTA parameters. Model performance in simulating measured water contents was good (average RMSE = 0.02 m 3 /m 3 ) during calibration and validation (0.01 m 3 /m 3 and 0.03 m 3 /m 3 for two independent periods). Water content in upper soil layers was strongly influenced by atmospheric conditions, while groundwater level has the most influence in the bottom layers. Evapotranspiration was found out to be the dominant process in summer with total flux higher than that of infiltration while in winter evapotranspiration was relatively lower, even though still higher than infiltration.