To simulate perfectly the flow of water through the ground, one would need to know the details of every pore, passage, and air pocket in the soil. To circumvent such daunting requirements, researchers simulate soil moisture transport as if the water were flowing through a series of cylindrical tubes, with the groundwater transport influenced by both gravity and pressure. This simplified representation holds up well when the soil is wet, but the analogy begins to break down when moisture levels drop. At very low moisture levels a dynamic nearly negligible at high flow rates begins to dominate: the adsorption of water onto soil particles. Water creates a thin film around individual grains of soil through molecular, ionicelectrostatic, and other attraction forces, but these interactions, while well known, are generally overlooked in models of soil moisture transport.