ABSTRACT
The term ‘‘percolation’’ refers to the downward flow or movement of water through the soil profile. More precisely, percolation is defined as the downward flow of water in saturated or nearly saturated soil at hydraulic gradients of 1.0 or less.[1] Although the terms ‘‘infiltration’’ and ‘‘percolation’’ are often used interchangeably, infiltration refers to the entry of water into soil,[1]
which typically occurs after rainfall or irrigation. In contrast, percolation refers to water movement that occurs following an infiltration event, once the soil profile has become saturated or nearly saturated with water. Such post-infiltration water movement is commonly referred to as internal drainage. Hillel[2] employed the term ‘‘deep percolation’’ to specify internal drainage of water occurring below the root zone, which is not influenced by water losses due to evaporation or transpiration (evapotranspiration) via plant roots.
