In
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
-related science and
engineering
Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...
, there are two similar but distinct definitions in use for the word drawdown:
* In subsurface
hydrogeology
Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquif ...
, drawdown is the reduction in
hydraulic head
Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22.
It is usually measured as a liquid surface elevation, expressed in units of length, ...
observed at a
well
A well is an excavation or structure created in the ground by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...
in an
aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
, typically due to pumping a well as part of an
aquifer test An aquifer test (or a pumping test) is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" ( drawdown) in observation wells. Aquifer testing is a common tool that hydroge ...
or
well test
A well test is conducted to evaluate the amount of water that can be pumped from a particular water well. More specifically, a well test will allow prediction of the maximum rate at which water can be pumped from a well, and the distance that the ...
.
* In
surface water
Surface water is water located on top of land forming terrestrial (inland) waterbodies, and may also be referred to as ''blue water'', opposed to the seawater and waterbodies like the ocean.
The vast majority of surface water is produced by prec ...
hydrology and
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 public works such as roads, bridges, canals, dams, airports, sewage ...
, drawdown refers to the lowering of the surface elevation of a body of water, the water table, the piezometric surface, or the water surface of a well, as a result of the withdrawal of water.
In either case, drawdown is the change in hydraulic head or water level relative to the initial spatial and temporal conditions of the system. Drawdown is often represented in cross-sectional diagrams of aquifers. A record of hydraulic head, or rate of flow (
discharge), versus time is more generally called a
hydrograph
A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).
It can als ...
(in both groundwater and surface water). The main contributor to groundwater drawdown since the 1960s is over-exploitation of groundwater resources.
Drawdown occurs in response to:
# pumping from the bore
# interference from a neighbouring pumping bore
# in response to local, intensive groundwater pumping
# regional seasonal decline due to discharge in excess of recharge
Terminology
*
Aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
is an underground layer of permeable rock or sand, that hold or transmit groundwater below the water table that yield a significant supply of water to a well.
*
Aquifer test An aquifer test (or a pumping test) is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" ( drawdown) in observation wells. Aquifer testing is a common tool that hydroge ...
(or a pumping test) is a field experiment in which a well is pumped at a controlled rate and the aquifer's response (drawdown) is measured in one or more observation wells.
*
Cone of depression
A cone of depression occurs in an aquifer when groundwater is pumped from a well. In an unconfined aquifer (water table), this is an actual depression of the water levels. In confined aquifers ( artesian), the cone of depression is a reduction in ...
is a conically-shaped depression that is produced in a water table as a result of pumping water from a well at a given rate.
*
Groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
is water located beneath the earth's surface in pores and fractures of soil and rocks.
*
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22.
It is usually measured as a liquid surface elevation, expressed in units of length, ...
(or piezometric head) is a specific measurement of the potential of water above a vertical datum. It is the height of the free surface of water above a given point beneath the surface.
* Pumping level is the level of water in the well during pumping.
* Specific capacity is the well yield per unit of drawdown.
* Static level is the level of water in the well when no water is being removed from the well by pumping.
*
Water table
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
T ...
is the upper level of the zone of saturation, an underground surface in which the soil or rock is permanently saturated with water.
* Well yield is the volume of water per unit time that is produced by the well from pumping.
Methods for measuring drawdown
*
Transducer
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and contr ...
s are used to measure water levels in groundwater wells, rivers, streams, tanks, open channels and lift stations.
*Acoustic well sounders or echometers are a simple, cost effective, and minimally intrusive tool used to measure subsurface pressures and levels.
*Electric sounders are a practical land cost-effective method used to measure well water levels. This method uses a weight attached to a stranded insulated wire and an
ammeter
An ammeter (abbreviation of ''Ampere meter'') is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. For direct measurement, the ammeter is connected in series with the circuit ...
to indicate a closed circuit. Current supplied from a small battery flows through the circuit when the tip of the wire is in contact with the surface of the water.
*Air line method is a convenient and nonintrusive method used to measure water levels that is often used for the repeated testing of wells over 300 feet deep.
This method obtains water table depth using a pressure gauge and water displacement.
*Wetted tape method is a commonly-used method for measuring water levels up to roughly 90 feet deep. This method uses a lead weight attached to a steel measuring tape.
Ecological impacts of groundwater drawdown
Groundwater drawdown due to excessive water extraction can have adverse ecological impacts. Groundwater environments often have high
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
, however, drawdown alters the amount and types of nutrients released to surrounding organisms. In addition, nearby wetlands, fisheries, terrestrial and aquatic habitats may be altered with a reduction in the water available to these ecosystems, sometimes altering species
ecophysiology
Ecophysiology (from Greek , ''oikos'', "house(hold)"; , ''physis'', "nature, origin"; and , '' -logia''), environmental physiology or physiological ecology is a biological discipline that studies the response of an organism's physiology to enviro ...
.
[Antunes C, Chozas S, West J, et al. Groundwater drawdown drives ecophysiological adjustments of woody vegetation in a semi‐arid coastal ecosystem. GlobChange Biol. 2018;24:4894–4908. https://doi.org/10.1111/gcb.14403]
Extracting groundwater at a rate that is faster than it can be naturally replenished is often referred to as
overdrafting
Overdrafting is the process of extracting groundwater beyond the equilibrium yield of an aquifer. Groundwater is one of the largest sources of fresh water and is found underground. Groundwater depletion is comparable to a bank account in which mor ...
. Overdrafting may decrease the amount of groundwater that naturally feeds surrounding water bodies, including wetlands, lakes, rivers and streams.
[“Environmental Impacts of Water Withdrawals and Discharges in Six Great Lakes Communities: A Role for Green Infrastructure”, J. W. Ridgway, R. Higuchi, L. Hoffman, and R. Pettit, Environmental Consulting & Technology Inc. Report, 45 pp, May 2016] Additionally, when a
cone of depression
A cone of depression occurs in an aquifer when groundwater is pumped from a well. In an unconfined aquifer (water table), this is an actual depression of the water levels. In confined aquifers ( artesian), the cone of depression is a reduction in ...
is formed around a pumping well due to groundwater extraction, nearby groundwater sources may flow toward the well to replenish the cone, taking water from local streams and lakes. This may result in poor water quality in these local water bodies as baseflow water contribution is reduced, which could result in
perennial streams becoming more intermittent, and
intermittent streams
A stream is a continuous body of surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long large streams ar ...
becoming more
ephemeral
Ephemerality (from the Greek word , meaning 'lasting only one day') is the concept of things being transitory, existing only briefly. Academically, the term ephemeral constitutionally describes a diverse assortment of things and experiences, fr ...
. Finally, drawdown from groundwater extraction may lead to an increased sensitivity of the ecosystem to
climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
and may be a contributing factor to
sea-level rise
Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cryo ...
and
land subsidence
Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
.
Related
*
Subsidence
Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
References
{{DEFAULTSORT:Drawdown (Hydrology)
Hydrology
Aquifers
Water
Water wells
Hydraulic engineering