Variably saturated porous media is a field of study within hydrogeology and geotechnical engineering that focuses on the behavior of subsurface materials, such as soils and rocks, that contain a mix of both water and air within their pore spaces. These materials can be in a partially saturated state, with some pores filled with water and others filled with air. Research in variably saturated porous media typically examines how water moves through these materials, how it interacts with other substances present in the pore spaces, and how these processes affect the overall behavior and properties of the porous media. This research is important for understanding groundwater flow, contaminant transport, and the stability of slopes and foundations in various engineering and environmental applications.