What is a Safe Yield?

Managing water withdrawals from aquifers so as to not exceed “safe yields” is an emerging water supply issue.  Safe yield is the amount of water that can be withdrawn from an aquifer without significant ecological impacts.  Significant ecological impacts could result from reductions in streamflow where groundwater discharge to the stream provides baseflow.  Water withdrawals can be balanced with return flows to the aquifer, which can include wastewater returns, after appropriate treatment, and collecting and infiltrating treated stormwater, artificial recharge.

Determining safe yield limits is an evolving discipline.  It requires a clear understanding of climate, hydrogeology, and ecological thresholds, especially where streams and wetlands are hydrologically connected to the aquifer.  This requires careful integration between groundwater science and land-use policy. 

Baseflow to a river is affected by pumping wells located within the watershed.  Pumping a well intercepts groundwater that would otherwise flow into a river as baseflow, thus reducing the river’s source of water to keep it flowing.  In addition, wells located near a river or its tributaries can cause induced infiltration.  The effect of pumping a well is to lower the groundwater table surrounding the well into a “cone of depression.”  This cone changes the slope of the groundwater table, potentially creating a gradient from the nearby river toward the well, that results in either: 

a) Intercepting groundwater baseflow that would otherwise discharge into the river, supplying flow, resulting in reduced surface flow; or

b) Inducing infiltration, thus causing surface water from the river or stream to flow from there through the ground and into the well, resulting in reduced surface flow, dry riverbeds in the vicinity of the wellfield, and reverse flow.

These effects of groundwater pumping are sometimes exacerbated when municipalities relying on this source must pump the most water in summer-early fall to meet high seasonal demand.  This pumping occurs exactly when the natural flow system it is at its most critical stage: low seasonal flows coupled with the growing season. 

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