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In water-related science and engineering there are two similar but distinct definitions in use for 'drawdown'.

★ In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.

★ In surface water hydrology and civil engineering, drawdown refers to the lowering of the water level in a man-made reservoir or tank.
In either case, drawdown is the change in head or water level relative to background condition, indicating the difference in head which has occurred at a given location relative an initial time at the same location.
A record of hydraulic head through time is more generally called a hydrograph (in both groundwater and surface water).