Irrigation and drainage engineers must design irrigation and drainage systems such that there is proper maintenance of moisture, air, and salt balance for favorable plant growth. A major design consideration is getting water into or out of the soil. In either case, the flow phenomenon involved is flow through partially saturated porous media. When water enters a soil, air must be replaced, and when water is removed, air must enter. The flow, therefore, involves two immiscible fluid phases-air and water. Frequently, these problems are divided into two regimes of flow: (1) above the water table and (2) below the water table. Water table is defined as the locus of points at which the pressure of the water is zero (gage) and is therefore a convenient reference datum. Two distinguishing characteristics of the two regimes of flow are as follows: (1) Above the water table there exist functional relationships among saturation, pressure difference between air and water, and the permeabilities to air and water; and (2) below the water table, saturation and permeability are constants.