REPORT.OF INVESTIGATIONS NO. 42


137


Time of concentration. The time .required for the water to flow from the
 farthest point on the watershed to a gaging station or to another specified
 point.
Transpiration. The process by which water vapor escapes from a living plant
 and enters'the atmosphere.
 Water loss. The difference between the average precipitation over a drainage
 basin and the runoff adjusted for changes in storage and for interbasin
 movement of ground water. The basic concept is that water loss is equal
 to evapotranspiration; that is, water that returns to the atmosphere and
 thus is no longer available for use in the area. However, as used in this
 report, the term applies to differences between measured inflow and outflow
 even where part of the difference may be seepage.
 Zone of saturation. The zone in which the permeable rocks are saturated with
 water under pressure equal to or greater than atmospheric.