Cohesion is the molecular attraction between like substances, most importantly the force that causes water molecules to stick to one another due to hydrogen bonding. In stormwater management and soil physics, cohesion is a fundamental property of water that influences how it behaves within soil pores, on surfaces, and during runoff.
Water molecules are polar, meaning they have a slight positive charge on one side and a slight negative charge on the other. This polarity allows adjacent water molecules to form hydrogen bonds, creating a cohesive force that holds them together. As a result, water tends to form continuous films, droplets, and connected pathways rather than dispersing into isolated molecules.
Cohesion works in tandem with adhesion, which is the attraction between water and other materials, to produce capillary forces. These combined forces enable water to move through fine soil pores, resist gravity to some extent, and maintain continuity in unsaturated flow conditions. Cohesion is also responsible for surface tension, which allows water to form curved surfaces and influences how it interacts with soil and vegetation.
In the context of stormwater management, cohesion plays a key role in infiltration, soil moisture retention, and the movement of water through the vadose zone. It helps maintain continuous flow paths in soils, allowing water to be transmitted through small pores rather than breaking apart. Cohesion also contributes to the formation of raindrops and affects how water accumulates and flows across impervious surfaces.
Although cohesion primarily refers to interactions between water molecules, the concept also applies more broadly in soil mechanics, where cohesive forces between fine soil particles, especially clays, contribute to soil strength and resistance to erosion. Overall, cohesion is a fundamental force governing the physical behavior of water in stormwater systems and natural hydrologic processes.