Adhesion is the molecular attraction between water and the surfaces of other materials, particularly soil particles, caused by electrostatic and chemical interactions at the interface between different substances. In stormwater management and soil physics, adhesion is a key mechanism that allows water to cling to soil grains and move through pore spaces.
At the microscopic level, adhesion occurs because water molecules are polar, meaning they have a slight positive charge on one side and a slight negative charge on the other. Soil particles, especially clays and organic matter, often carry surface charges that attract water molecules. This attraction causes water to spread along and adhere to particle surfaces rather than simply draining away under gravity.
Adhesion works together with cohesion, which is the attraction between water molecules, to produce capillary forces. These combined forces enable water to move upward or laterally through small soil pores, retain moisture in unsaturated soils, and form thin films of water around particles.
In the context of stormwater management, adhesion is important because it contributes to water retention within soils, particularly in fine-textured soils with high surface area. It affects infiltration and drainage behavior by slowing the movement of water, helping soils hold moisture for plant uptake, and influencing how pollutants interact with soil surfaces. Adhesion also plays a role in the initial wetting of dry soils, as water must first overcome resistance and attach to particle surfaces before infiltration can proceed efficiently.
Simply put, adhesion is a fundamental force that governs how water interacts with soil, influencing moisture distribution, capillary movement, and the effectiveness of stormwater treatment processes.