ABSTRACT
Water flow through soil in which both water and air exist is called unsaturated flow. This chapter discusses unsaturated flow near the region of saturated flow, where water exists in small pores in the soil while air occupies the large pores. It discusses infiltration based on the Green and Ampt model. The release of air bubbles is easier at low pressure than at high pressure. In a layered soil, air bubbles will be released where water pressure is negative. A negative water pressure would therefore be useful in preventing seepage loss. Flowing water usually contains dissolved air, which may be released within pores in the soil. Air bubbles will decrease hydraulic conductivity, but when flowing water dissolves air bubbles within the pores, the hydraulic conductivity increases. Flow with a free water surface may occur from seepage through earth dams or dikes or percolation from canals or paddy fields.
