Low Flow Database

Groundwater Withdrawls

What is Groundwater?
Groundwater is the water that is stored in the interstitial space, or space between particles, underground. Some soil materials have the ability to store more water in their interstitial space than others. As a general rule, materials made of large particles like sand and gravel (below, left) have more interstitial space than soils made of smaller particles like silt and clay (below, right) because they cannot be packed together as tightly.

What Happens When Groundwater is Pumped?
Regardless of the type of aquifer in which a well is located, groundwater responds to pumping in two ways:

1. The level of groundwater, or water table, is lowered and a cone-shaped depression on the surface of the water table is formed near the well; and
2. Water from other areas rushes toward the well to replace the water that is removed. This phenomenon is called recharge and occurs where there is no barrier to water movement.

In most cases, both effects are observed. Initial pumping causes the water table near the well to drop and forms a cone-shaped depression on the surface of the water table (see figure above). As pumping continues and more water is removed, the cone expands, but at an ever-slower rate because the expansion of a large cone of depression requires more water to be withdrawn than a smaller one.

How do Groundwater Withdrawals Affect Streamflow?
In some cases, as the cone of depression expands, it hits a recharge feature, or another source of water such as a river or lake. Continued pumping of the well may induce infiltration, or cause water to flow from the recharge feature into the ground and slow or halt the expansion of the cone of depression. If the stream is small relative to the amount of water being withdrawn, it may dry completely, especially during periods of naturally low flow (see figure below). If this happens, the cone of depression will extend beyond the recharge feature and continue to expand. On the other hand, if there is an impermeable layer, such as clay, on the lake or river bottom, there may be little interaction between groundwater and surface waters.