Drawdown (hydrology)

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).

Effects

Aquifer drawdown or overdrafting and the pumping of fossil water increases the total amount of water in the hydrosphere that is subject to transpiration and evaporation thereby causing accretion in water vapour and cloud cover which are the primary absorbers of infrared radiation in the earth's atmosphere. Adding water to the system has a forcing effect on the whole earth system, an accurate estimate of which hydrogeological fact is yet to be quantified.