Percolation

• The downward flow of water in unsaturated, saturated or nearly saturated porous medium at hydraulic gradients of the order of 1.0 or less.

• dh/dl = hydraulic gradient

• Pressure vs. length water flows from higher elevation to lower elevation

• Water held in soil or rocks by its surface charge. This water is returned to the atmosphere either through plants or evaporated from the soil. (evapotranspiration)