Specific Yield Definition

Definitions of Groundwater Sustainable Yield (SY) used in this site:

The most important component of all groundwater resource studies is the estimation of sustainable yield (SY). Unfortunately many hydrogeologists assume that SY is a fixed estimate which has to be used every year. As a matter of fact SY is a dynamic estimate and differs from one year to another depending on the amount of additional replenishment to the system, and the reduction in discharge component which can be regained, with the emphasis that in SY there is a critical limit which must not be surpassed due to socio-economic and bio-ecological constraints. Also the definition of SY differs from each aquifer type to another.

• SY for regional aquifers: is defined as the additional groundwater output (computed annually) from the system which will cause minimal and acceptable levels of stress to the ecosystem with maximum benefits to the society and to the economy

• SY for alluvial aquifers: is defined as the annual volume of water available for abstraction which is the recharge component (computed annually) to the system which fills the alluvial aquifer to exploitable levels and will cause minimal and acceptable levels of stress to the ecosystem with maximum benefits to the society and to the economy

• SY for fractured rock aquifers: is defined as the additional groundwater output (computed annually) from the system which will not cause excessive groundwater levels fall that will disrupt inter-connectivity between the productive zones of the aquifer and will cause minimal and acceptable levels of stress to the ecosystem with maximum benefits to the society and to the economy

• SY for unconfined aquifer in agricultural areas: is defined as the additional groundwater output (computed annually) from the system which will not cause excessive recharge from the irrigation system that breach the surface water allocations and will cause minimal and acceptable levels of stress to the ecosystem with maximum benefits to the society and to the economy

• SY for bio-ecologically sensitive areas: is defined as the additional groundwater output (computed annually) from the system which will cause minimal and acceptable levels of stress to the ecosystem and conserve the habitat value of the environment.

In all the above cases the Initial (predevelopment) recharge is of no importance whatsoever in the estimation of the sustainable yield. Additional recharge and reduced discharge which does not stress the ecosystem are the only groundwater components which enter in the sustainable yield calculations

On the other hand in case of areas which are suffering from dryland salinity, the sustainable yield will be converted to unsustainable:

• Unsustainable Yield for aquifers at risk of salinity: is defined as the additional groundwater input (computed annually) to the system which will cause rise in water levels and increased saline groundwater discharge above the acceptable levels of stress to the ecosystem and cause suffering to the society and to the economy.