In water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

-related science and engineering

Engineering

Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

 there are two similar but distinct definitions in use for drawdown.

• In subsurface hydrogeology
  Hydrogeology
  Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, . The term geohydrology is often used interchangeably...
  
  , drawdown is the change in hydraulic head
  Hydraulic head
  Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
  
   observed at a well
  Water well
  A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
  
   in an aquifer
  Aquifer
  An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
  
  , typically due to pumping a well as part of an aquifer test
  Aquifer test
  An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
  
   or well test
  Well test
  This article discusses water well testing; the testing of other wells, eg. petroleum wells, is a separate field.A well test is conducted to evaluate the amount of water that can be pumped from a particular water well...
  
  .

• In surface water
  Surface water
  Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
  
   hydrology and civil engineering
  Civil engineering
  Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings...
  
  , 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

Hydrograph

A hydrograph is a graph showing the rate of flow versus time past a specific point in a river, or other channel or conduit carrying flow...

 (in both groundwater and surface water).

Effects

Aquifer drawdown

Drawdown

Drawdown has four distinct meanings:* Drawdown , a lowering of a reservoir or a change in hydraulic head in an aquifer, typically due to pumping a well.* Drawdown , decline in the value of an investment, below its all-time high....

 or overdrafting

Overdrafting

Overdrafting is the process of extracting groundwater beyond the safe yield or equilibrium yield of the aquifer.Since every groundwater basin recharges at a different rate depending upon precipitation, vegetative cover and soil conservation practises, the quantity of groundwater that can be safely...

 and the pumping of fossil water

Fossil water

Fossil water or paleowater is groundwater that has remained sealed in an aquifer for a long period of time. Water can rest underground in "fossil aquifers" for thousands or even millions of years...

 increases the total amount of water in the hydrosphere

Hydrosphere

A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

 that is subject to transpiration

Transpiration

Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

 and evaporation

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 thereby causing accretion

Accretion (atmosphere)

Accretion is an atmospheric science term for when an ice crystal or snowflake hits a supercooled liquid droplet, which then freeze together. This increases the size of the water particle. A common example of this that is visible to people is graupel....

 in water vapour and cloud cover

Cloud cover

Cloud cover refers to the fraction of the sky obscured by clouds when observed from a particular location...

 which are the primary absorbers of infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 radiation in the earth's atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. Adding water to the system has a forcing effect

Global warming

Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 on the whole earth system

Gaia hypothesis

The Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life on the planet.The scientific investigation of the...

, an accurate estimate of which hydrogeological fact is yet to be quantified.