underground runoff (or seepage)

Water flowing toward stream channels after infiltration into the ground.

undifferentiated soil map unit

Soil mapping unit in which two or more soil units occur but not in a regular geographic association. For example the steep phases of two or more soils might be shown as a unit on a map because slope is the prime characteristic. See also association / soil and complex / soil.

unsaturated flow

The movement of water in a soil that is not filled to capacity with water.

upper plastic limit

See liquid limit.

urban land

Areas so altered or obstructed by urban works or structures that identification of soils is not feasible. A miscellaneous land type.