Pumping from wells shifts ground-water divides by redirecting water that would ordinarily discharge at the land surface away from the surface toward the open interval of a well at somedepth within the ground-water system.

For this reason pumping can move ground water under surface-water divides.

Pumping from wells moves shallow local ground-water divides that under natural conditions approximately coincided with the topographic ridges that define surface-water divides:

Schematic sections of ground-water flow (A) under natural conditions and (B) affected by pumping (source: USGS Water-Resources Investigations Report 00-4008)

Note that the surface-water divides are not displaced by pumping, but that the ground-water divides move in response to the new flow field. What occurs at the scale of local ground-water flow can also occur at the regional scale. Consider the location of a regional divide that bounds ground water that is tributary to a Great Lake. Pumping can move the natural divide even if it is was originally many miles from the Lake:

Thumbnail schematic block diagrams of regional ground-water divides:

A) in the absence of pumping	B) with one pumping center	C) with two pumping centers
Note: The major shallow divide for ground water is approximately coincident with the topographic divide that defines the watershed boundary of the Great Lakes Basin. (source: D.T. Feinstein, U.S. Geological Survey)

Diagram A shows that under natural conditions long regional flow paths move under major and minor surface water divides and discharge toward the Lake. Pumping can entirely reverse this pattern and draw regional flow from beneath the Lake.

Diagram B contains a single pumping center consisting of municipal and industrial wells. It is inside the Great Lakes Basin (that is, east of the surface-water divide) but draws water from outside the Basin through long regional flow lines emanating from the other side of the major surface-water divide that bounds the Basin. Pumping increases the downward leakage from shallow local systems and fortifies regional flow.

Diagram C contains a second pumping center outside the Great Lakes Basin that intercepts some of the regional flow that would otherwise discharge to the first pumping center. The competition between pumping centers, called well interference, has a big effect on the flow pattern.

Note how the second pumping center captures water that originates inside the Basin by inducing flow across the surface-water divide toward the west. Under these conditions, precipitation and recharge that would naturally cycle to the Lake is rerouted to a different watershed, implying a loss of water to the Great Lakes system.

These diagrams are relevant to the evolution of deep ground-water flow in southeastern Wisconsin.

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