The downward movement of water through soil and rock under the force of gravity, beyond the root zone toward the water table. It is distinct from infiltration, which occurs at the surface.
Percolation is the gravity-driven movement of water through the unsaturated zone (vadose zone) of soil and rock below the root zone, ultimately contributing to groundwater recharge when it reaches the water table. While infiltration refers to water entering the soil at the surface, percolation describes the continued downward migration of that water through deeper soil horizons and geological formations. The rate of percolation depends on the hydraulic conductivity of the subsurface materials, the moisture content gradient, and the presence of impeding layers such as clay lenses or hardpans. Percolation tests (perc tests) are commonly used in site evaluations for septic system design, where the rate at which water drains from a test hole indicates the suitability of the soil for wastewater disposal. In the context of the water budget, deep percolation represents water that moves beyond the reach of plant roots and is no longer available for evapotranspiration. Understanding percolation rates is important for groundwater recharge estimation, irrigation efficiency assessment, and predicting the movement of contaminants through the subsurface.