Glossary - D - drawdown
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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.
Permeability of aquifer materials and relation to drawdown cone
Pumping rates are important for efficient use of aquifers, and knowledge of aquifer material is important. This is the case for the Lockyer valley alluvial aquifer, and it is important to know what material the bore is screened in.
This is shown in the figure comparing three aquifer materials at the same pumping rate (after Manning, 1992).
Coarse sand and gravels: highly transmissive and water is released easily - little drawdown, and cone will spread out; could sustain pumping at a higher rate.
Fine sand: moderately transmissive, steeper gradient and drawdown.
Sandy silt: quite low permeability, low transmissivity, needs steep gradient to release water, so long drawdown, but less spread of cone. Can easily reach maximum production rate.
example of different materials with constant pumping rate:
- slope of cone: by permeability and quantity of water removed
- depth of cone: by pumping rate
- radius of cone: by time since pumping began