Groundwater-Surface Water Interactions
How does groundwater interact with surface water?
The interaction of groundwater with surface water is a key element of the hydrologic cycle. Groundwater moves below the earth’s surface and is an important water source for rivers, streams, lakes, and wetlands. The interaction between groundwater and surface water can be thought of as a piggy bank. Sometimes surface water enters the ground for storage (savings), and returns to the surface when surface water is low (withdrawal). Groundwater can stay underground for a few days, weeks, years, decades, centuries or millennia before returning to the surface.
Rivers and Groundwater
Rivers have complex interactions with groundwater. A river coursing down a mountainside loses water upstream where it seeps into the dry ground. This water becomes groundwater. The groundwater then flows to areas of low elevation, just like the river downstream. When the groundwater meets the river, the water stored underground will enter the river and continue flowing downstream as surface water. The groundwater that enters the river is called the baseflow. If there is a large amount of groundwater entering the river, groundwater flooding can occur.
Groundwater makes up the baseflow of a river. In the winter when there is not as much rainfall as there is in warmer months, groundwater contributes to streams and rivers, thus forming the baseflow (see Figure 1), which is the base amount of water in streams and rivers during low flow periods. During high flows on the surface, this interface may reverse and the river or stream will provide water to the groundwater system.
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Groundwater flooding occurs when a large amount of groundwater comes to the surface quickly and causes flooding. When groundwater levels rise but remain underground, flooding can also occur. For example, basements that are deep and closer to a groundwater source can flood when the water levels rise.
Groundwater flooding can happen on its own, or at the same time as surface water flooding. When a river has high flows, some of this water goes underground. If groundwater levels are low, then this water is stored, and will slowly be released back to the river while the river flooding is ending. If groundwater levels are high, this water cannot be stored underground, so there is more water flooding the land.
Lakes and Groundwater
There are complex interactions between lakes and groundwater. The lake bed might feed the groundwater source, or the groundwater might feed into the lake. In some lakes, the direction of water flowing from the lake to groundwater (or groundwater to lake) does not change, but in instances the exchange might change depending on how much water is in the lake, how much precipitation there has been or the air temperature. The interactions between lakes and groundwater can vary tremendously. It is possible for a lake to receive groundwater in one part of the lake and lose water at another section of the lake.
Wetlands and Groundwater
Groundwater also often provides water to wetlands. Wetlands require water to maintain their healthy ecosystems. This makes groundwater is a vital component of wetland health. Wetlands also receive water from precipitation and runoff. When a wetland is full, it becomes a recharge zone. This means that the wetland has excess water that flows into the ground and replenishes the groundwater source.
How does human use impact groundwater-surface water interactions?
Groundwater used by humans affects groundwater-surface water interactions. When water is withdrawn from an aquifer, the water that might have migrated toward a river or wetland is redirected for human use or consumption. As the aquifer is pumped, less water from the ground reaches the stream and can even reverse the direction of flow of groundwater entering the river to river water entering the groundwater system. It may seem that using groundwater rather than surface water conserves surface water resources but since these systems are often connected both sources are impacted through groundwater use.