Water-Use Efficiency is:

The ratio of volume of water retained to restore soil moisture in a crop’s rooting zone to field capacity, expressed as a percentage of the total volume of water applied.  This is an indicator of the relationship between the amount of water required for a particular purpose and the amount of water used or delivered.

Though the two terms water conservation and water efficiency are often used interchangeably, they are different in that water conservation aims to reduce wastage of water.