Drawdown (hydrology)
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In hydrology, there are two similar but distinct definitions in use for the word drawdown:
- In subsurface hydrogeology, drawdown is the reduction 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 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.[1]
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 (in both groundwater and surface water). The main contributor to groundwater drawdown since the 1960s is over-exploitation of groundwater resources.[2]
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[3]
- Aquifer 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.[4]
- Aquifer test (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.[5]
- Cone of depression 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.[4]
- Groundwater is water located beneath the earth's surface in pores and fractures of soil and rocks.[6]
- Hydraulic head (or piezometric head) is a specific measurement of the potential of water above a vertical datum.[7] It is the height of the free surface of water above a given point beneath the surface.[4]
- Pumping level is the level of water in the well during pumping.[8]
- Specific capacity is the well yield per unit of drawdown.[8]
- Static level is the level of water in the well when no water is being removed from the well by pumping.[8]
- Water table is the upper level of the zone of saturation, an underground surface in which the soil or rock is permanently saturated with water.[9]
- Well yield is the volume of water per unit time that is produced by the well from pumping.[8]
Methods for measuring drawdown
- Transducers are used to measure water levels in groundwater wells, rivers, streams, tanks, open channels and lift stations.[10]
- Acoustic well sounders or echometers are a simple, cost effective, and minimally intrusive tool used to measure subsurface pressures and levels.[11]
- Electric sounders are a practical land cost-effective method used to measure well water levels.[12] This method uses a weight attached to a stranded insulated wire and an ammeter 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.[13]
- 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.[13] This method obtains water table depth using a pressure gauge and water displacement.[14]
- 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.[13]