Adapting the Green and Ampt Model to Account for Air Compression and Counterflow

• Main Author: Sabeh, Darwiche
• Format: Others
• Published: Scholar Commons 2004
• Subjects: rainfall; infiltration; sharp wetting front; water table depth; American Studies; Arts and Humanities
• Online Access: https://scholarcommons.usf.edu/etd/1230; https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=2229&context=etd

One of the earliest functions to express infiltration as a function of time was introduced by Green and Ampt. In this study their formula was modified to account for air compression and counterflow. Physically,infiltration, air compression, and counterflow occur simultaneously, while in this model they are decoupled within a time step. Counterflow is calculated as a mass flux and pressure is found using the perfect gas law. First, a comparison of three infiltration methods, the original Green and Ampt formulation, a modified version incorporating air compression only, and the third version including air compression and counterflow, was conducted. Then sensitivity of the model accounting for both air compression and counterflow was explored. Results showed that accounting for both air compression and counterflow improves the predicted infiltration rate. Air effect on infiltration can be significant even for environments with an impervious layer as deep as 10m; while for very deep water table environments (100m) the three models give similar results. In shallow water table environments (0.5m), air effect on infiltration rate, cumulative infiltration, ponding time, and saturation time is substantial. The model accounting for air compression and counterflow was then tested for different parameters. It provided reasonable results compared to the Green and Ampt model and the modified version accounting for air compression only. The advantages of this model are that no additional data is required other than what's needed for the original Green and Ampt formulation, and it can be applied for any environment. The assumption of uniform soil moisture content is a limitation for the model, especially for shallow water table environments where the variations in the soil moisture profile within the wetting front depth is substantial.