Groundwater 101


Image adapted from California Department of Water Resources "Groundwater: An Essential Part of California’s Water Budget"

Water moves throughout our world in a process known as the water cycle.
Rain Lands
Cities and Towns
Lakes, Rivers & Streams



During that cycle, precipitation and snowmelt interact with various land uses, such as:

Cites & Towns

Lakes, Rivers & Streams



During that cycle, precipitation and snowmelt interact with various land uses, such as:
As part of the water cycle, some water seeps through layers of rock and soil underground. This process is known as percolation. The water that is stored in the many layers of rock, sands, gravels, and clay underneath the earth's surface is called groundwater.
Aquifer Diagram

Image Credit: Adapted from USGS Sacramento Valley Hydrology Diagram

Groundwater is stored in an aquifer, which is an underground layer of rock and sediments, like sand and clay. Aquifers fill up slowly over time. If groundwater is used faster than aquifers can be refilled, aquifers can be depleted.
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Groundwater and Us

How do we use groundwater?

Hands pumping water from a well.

“Beneficial User and Uses”

Communities, economies, and ecosystems all depend on groundwater for different uses. Water law in California requires that water is used in ways that are both beneficial and reasonable. This is why water uses and users are referenced as “beneficial uses and users”. Examples include households using water for drinking, farms using water for irrigation, restaurants using water for washing dishes, and habitats such as wetlands and streams. This term refers to all of the legal applications of how groundwater can be put to “beneficial use”.


Water quality affects how groundwater can be used.

Over time, groundwater can become contaminated. Groundwater contaminants can occur naturally or as a result of human-caused pollution (e.g. pesticides, dumping). In some cases, even if groundwater is available to pump from our wells, it may not be safe to use without additional treatment.

Groundwater Dependent Ecosystems

The many plants, animals, and ecosystems which rely on groundwater are known as “Groundwater-Dependent Ecosystems” or GDEs. Some trees’ roots rely on shallow groundwater, such as our local oaks, willows, and cottonwood trees. Wetlands and streams also rely on sustained groundwater levels, especially in late summer or fall or during droughts, to provide habitat for migratory birds, such as sandhill cranes, and fish, such as salmon.

Without a healthy groundwater system, these habitats risk going dry, threatening our local plants and wildlife. It’s up to us to protect these important ecosystems in groundwater planning and decision-making.

Under the Sustainable Groundwater Management Act, beneficial users and uses must be engaged and considered in the development of local Groundwater Sustainability Plans.

How do we access groundwater?

Types of Wells

Groundwater is pumped to the surface through wells. These wells vary in size and depth. There are a few factors which help determine the type of well used to pump groundwater:

  • The purpose or “beneficial use” of how the groundwater will be used (e.g. drinking water, crop irrigation, public water supply);and,
  • The underlying geology underneath the well.

Groundwater in California

California is celebrated for its beautiful waterways, productive agriculture, and diverse communities and landscapes. Groundwater, one of California’s rich resources, is at risk .
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California depends heavily on groundwater to meet its water supply needs. Groundwater, which largely comes from rain and snow that seeps through soil and rock, supplies approximately 40-60 % of the state water supply. Meanwhile some communites rely on groundwater for 100% of their water needs.
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Image Credit: Department of Water Resources

Over the last century, California has experienced groundwater overdraft. Overdraft occurs when more groundwater is pumped out from underground than replaced - caused by demand from agricultural, urban, residential, and industrial development and drought.
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Participation Toolkit

Find more resources & learn more!

Find your Groundwater Sustainability Agency

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Provide information about your water interests and needs

Attend a local meeting in your community