Groundwater storage and safe/sustainable/optimal yield
In addition to functioning as the plumbing system for groundwater flow, the saturated geologic units that compose the WBRB groundwater basins also function as reservoirs that store enormous volumes of groundwater. Understanding how groundwater is stored and how to utilize the resource without depleting it (safe/sustainable yield) is of concern in most development projects. An exception is coalbed natural gas (CBNG) development, where the lowering of hydraulic pressure at depth is the purpose of groundwater extraction.
To assess groundwater resources on either a local or a regional basis, both the total volume of groundwater present and the fraction of groundwater that can be accessed, developed at acceptable cost, and put to beneficial use should be determined. Technical, financial, and cultural factors determine what part of a total stored volume of groundwater can be considered an available resource; for example:
- Only part of the groundwater contained within an aquifer will be producible, and part will be retained within the aquifer.
- The depth to the resource and other physical, cultural, legal, and institutional constraints may limit accessibility and preclude groundwater development due to cost or technical infeasibility.
- Groundwater must be of suitable quality to satisfy the requirements for its intended use.
The amount of water that an aquifer will yield to natural drainage or to pumping is determined by its hydraulic properties. The basic hydraulic property with respect to sustainable yield is effective porosity, expressed as specific yield in unconfined aquifers and as storage coefficient in confined aquifers.
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