Capillary forces are the intermolecular forces that cause water to move through small pores in soil due to the combined effects of adhesion, cohesion, and surface tension. In stormwater management and soil physics, capillary forces are responsible for the movement and retention of water in unsaturated soils, particularly above the groundwater table.
These forces arise from two primary interactions. Adhesion is the attraction between water molecules and solid surfaces, such as soil particles. Cohesion is the attraction between water molecules themselves. Together, these forces create surface tension at the air-water interface, allowing water to be drawn into and held within small pore spaces, even against the force of gravity.
Capillary forces are strongest in fine-textured soils, such as clays and silts, because these soils contain many small pores that enhance the surface area available for adhesion. As a result, water can be pulled upward or retained more effectively, sometimes forming what is known as capillary rise above the water table. In contrast, coarse-textured soils like sand have larger pores, which reduces capillary action and allows water to drain more freely.
In the context of stormwater management, capillary forces play a key role in controlling soil moisture distribution, infiltration behavior, and the availability of water for vegetation. They influence how long water is retained in practices such as bioretention systems and infiltration basins, affecting both treatment performance and drainage rates. Capillary forces also contribute to the movement of dissolved pollutants, as water held in small pores can transport nutrients and contaminants through the soil profile over time.
Capillary forces are a fundamental mechanism governing the storage, movement, and retention of water in soils, especially under unsaturated conditions where gravity alone does not dominate flow.