Groundwater and surface water are essentially one resource, physically connected by the hydrologic cycle. Although water law and water policy often consider groundwater and surface water as separate resources, groundwater and surface water are functionally inter-dependent. Groundwater and surface water interactions are controlled by their hydraulic connection.
Hydraulically Connected Systems
If the groundwater table is in physical contact with the stream bed, it is a hydraulically "connected" system. The exchange of water between the groundwater system and a stream is controlled by the difference in elevation between groundwater table and the water level in the stream.
Gaining Stream (reference: USGS Circular 1139, 1998)
In a gaining stream, groundwater levels are above the water level in the stream. Under these conditions, the groundwater system discharges water to the stream, increasing flow in the stream.
Under natural conditions, prior to the pumping of groundwater from wells, streams are the primary discharge outlet for groundwater: rainfall percolates into the ground and recharges the groundwater system, and then water flows very slowly through the groundwater system, eventually discharging to streams. Groundwater discharge to streams provides the baseflow of streams and is often a primary component of the total streamflow.
When groundwater wells are developed, pumping captures water that would otherwise discharge to streams, which decreases the baseflow to the streams. This decrease in groundwater discharge to streams, caused by pumping, is called stream depletion.
Groundwater can also discharge to lakes, wetlands and other similar habitat in low-lying areas when groundwater is near the surface. Like streams, groundwater discharge to these surface water features is reduced by groundwater pumping.
Losing Stream (reference: USGS Circular 1139, 1998)
In losing streams, groundwater levels are lower than water levels in the stream, and water from the stream recharges the groundwater system. Several different conditions can cause a stream to be a losing stream.
For example, if groundwater pumping is sufficient to lower regional groundwater levels or if a well is located very near a stream, groundwater levels can drop below stream levels, inducing groundwater recharge from streams. These conditions can persist for months or years. This loss of water from stream flow, caused by pumping, is also called stream depletion.
Natural conditions, such as flood events, can also create losing-stream conditions. During flood events, stream levels can temporarily rise above groundwater levels, causing streams to recharge the groundwater system adjacent to the stream. However, when water levels in the stream return to normal, this water will drain back into the stream. This rapid exchange of water between the stream and the groundwater system during flood events is not called stream depletion.
Hydraulically Disconnected Systems
Disconnected Stream (reference: USGS Circular 1139, 1998)
If a stream is separated from the groundwater table by an unsaturated zone, it is a hydraulically "disconnected" system. In disconnected systems, although groundwater pumping does not affect streams, streams do affect groundwater through streambed seepage that recharges the groundwater system.
Groundwater systems are often disconnected from the streams in arid regions and in regions where groundwater pumping has significantly lowered groundwater levels.
Source for figures: Winter, T.C., Harvey. J.W., Franke, O.L. and Alley, W.M., 1998, Ground water and surface water–A single resource: U.S. Geological Survey Circular 1139, 79 p.