Two-Layer Vadose Zone Model for Surface and Groundwater Interactions

Differentiation of upper and lower regions of the unsaturated (vadose) zone is important for modeling evapotranspiration, infiltration, and water table recharge. This study developed a methodology to describe relative moisture conditions in both zones for modeling soil hydrologic response. A comprehensive distributed parameter model was developed based on deterministic and physically based soil and vegetative properties. The Integrated Hydrologic Model (IHM) uses a unique relative soil moisture approach for land segment integration and is intended to simulate the complex interaction between surface-water and groundwater systems. No prior rigorous investigation or validation of the upper and lower soil response has been made. In this study, field soil moisture observations and soil characterization data were used to formulate a new basis for the upper and lower zones in the IHM. Several tests were performed to illustrate how the model reduces field-scale variability in soil moisture behavior and enhances representation of antecedent conditions. Results show that the two zones commonly exhibit different relative moisture conditions. Evidence is presented to document the existence of prolonged (many days) air entrapment and excess pore pressure, which affect soil water storage and observed water table levels.