It may be defined as
The portion of precipitation on a water shed that can be collected for use is called yield.
It includes direct run off and that water which passes underground before appearing as stream flow.
Safe yield :
It is the amount of water that can be drawn throughout the year.
It is the minimum yield that can be recorded for a given past period.
Draft (Demand) :
It is the actual or intended quantity of water drawn for use.
In the water supply projects, the minimum daily flow of stream should be well above the maximum daily draft.