The Green and Ampt Infiltration Model Accounting for Air Compression and Air Counterflow in the Shallow Water Table Environment: Laboratory Experiments

Water infiltration into the unsaturated zone especially in a shallow water table environment is affected by air compression ahead of the wetting front and air counterflow. Neglecting air compression in infiltration modeling can overestimate infiltration and infiltration rates, whereas not accounting for air counterflow can underestimate infiltration and infiltration rates due to unrealistic buildup of air pressure resistance ahead of the wetting front. A method, derived on the basis of the Green and Ampt (1911) infiltration model, is introduced to simulate air compression and air counterflow during infiltration into a shallow water table. The method retains the simplicity of the Green and Ampt (1911) model but adds the air pressure resistance term ahead of the wetting front. Infiltration equations are derived on the basis of the Green and Ampt (1911) and Sabeh’s (2004) infiltration model which accounts for air compression and air counterflow. The difference between this method and Sabeh’s (2004) model is that air counterflow, air compression, and infiltration are decoupled and updated with each wetting front increment whereas Sabeh’s (2004) method uses time step as a decoupling mechanism. Air compression ahead of the wetting front is predicted using the perfect gas law. Laboratory experiments showed that the introduced method is reasonably accurate when modeling cumulative infiltration values. Results of laboratory experiments were compared to results of the modeled infiltration methods: original Green and Ampt (1911) model and Green and Ampt with air compression and counterflow. The advantage of this new method is its simplicity. The new method uses parameters that are generally needed for modeling infiltration with the Green and Ampt (1911) approach. Disadvantages of the model are assumptions of the uniform water content and the uniform pressure. Another shortcoming of the model is that it does not account for air compression and air counterflow prior to ponding. Laboratory experiments described in this work and a proposed model can be further used for modeling and studying infiltration with air effects. In addition, this work can be of use to someone studying irrigation techniques of rice or other crops