Tracking California's water supplies

Stay informed on critical water issues during drought years and wet seasons.

California has endured deluge after deluge this winter, including several atmospheric rivers that dumped tons of rain all at once. But one good winter after years of drought does not mean California no longer faces water shortages.

Droughts are becoming more common and more extreme as the climate crisis intensifies, and communities across the state dependent on depleted underground aquifers and parched Colorado River supplies do not have enough water to meet the demands of their farmers and cities. Groundwater in the San Joaquin Valley has been drained more quickly than it can be recharged, so thousands of wells have gone dry. And reservoirs storing Delta water have experienced record lows in recent years.

California's statewide snowpack on April 3 tied with 1952 for the highest on record for that date. Meanwhile, snow water content in the central and southern Sierras broke records, while in the northern Sierras snow water content levels are second to recorded historical levels from 1983.

This dashboard provides a current and historical perspective on water issues facing the state using a variety of publicly available datasets.

Key Numbers









Would you like to know more?

Surface water

How long have we been in a drought?

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How full are our reservoirs?

  • The State Water Project manages -- California reservoirs.
  • Reservoir levels are currently -- acre-feet of water.
  • The system can store -- acre-feet of water, putting us at --% of capacity.

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Major Reservoir Levels

  • This graphic, updated daily, compares the water level of each of 12 major state-managed reservoirs to that day’s average since 1990.

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Moving Water Around the State


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The fundamental challenge with water in California, at least the above-ground supply, is that most of it isn't where most of the people are. Each dots represents 10 people living in the state.

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Much of California’s water falls as snow in the Sierra Nevada and other northern mountain ranges. To satisfy human demands, we have transformed the state’s natural water systems to transport water hundreds of miles from its source.

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The federal government built the Central Valley Project, which primarily conveys water from Northern California to irrigate crops in the San Joaquin Valley.

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The state built the California Aqueduct, which snakes down the Central Valley for more than 400 miles. Much of the water in the state aqueduct moves downhill by gravity.

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But it's not all downhill from there. To get over the Tehachapi Mountains near Los Angeles, the water is pumped nearly 2,000 feet high by the most powerful water lifting system in the world.

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In addition, Los Angeles has built its own system, the Los Angeles Aqueduct, to carry water from the Owens Valley into the city.

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But none of that is enough. The Colorado River, which winds through four other states before reaching the southeastern tip of California, is another major source. The state is entitled to 4.4 million acre-feet of water a year under a multi-state contract. But it's difficult to know how much will be available in the coming decades. The US Geological Survey estimates that the river's flow, once mighty enough to carve the Grand Canyon, will decline by nearly a third over the next 30 years because of climate change.

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The Metropolitan Water District, which provides drinking water to more than 19 million people in Southern California, operates the Colorado River Aqueduct to carry water from the eastern edge of the state across the desert and into the Los Angeles basin. This aqueduct supplies about a quarter of the district’s water.

How much water came from the sky?



  • Rain helps increase reservoir levels, and it seeps deep into the ground to help replenish underground aquifers.
  • Too much water over a short period of time can be bad, leading to flooding and mudslides which damage property and put people at risk.
  • Prolonged drought conditions make it more difficult for rain to seep into the ground, as the ground tends to be harder and not as porous as it would be with more moisture.

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  • When rain falls as snow, it freezes and creates a water reserve for warmer times of the year. This water, when it melts, washes back into waterways and into the ground, supporting our water needs when rain isn't in the forecast.
  • The California Department of Water Resources uses a metric called "snow water content" to measure how much water is frozen.
  • Historically most snow has fallen by April 1, so scientists compare the snow water content from that day to the same day in previous years to access how likely the state will be to satisfy water requests.
  • California's statewide snowpack on April 3 tied with 1952 for the highest on record for that date. Meanwhile, snow water content in the central and southern Sierras broke records, while in the northern Sierras snow water content levels are second to recorded historical levels from 1983.

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How much water ends up in streams?

  • The US Geological Survey has gauges in streams across the state and continuously measures their flow.
  • Like all water data in California, these measurements are very spiky and hard to understand. But we can compare each stream to its past measurements.

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Who will get that water?

  • The State Water Project doles it out based on a system dating back to the Gold Rush.
  • Those who take the water deliveries are called contractors — mainly agencies managing water for residential, industrial, recreational and agricultural use.
  • Several times a year, the project announces what percentage of a contractor's request will be allocated for the water year. In times of extreme drought, allocations tend to drop moving into the summer if snowpack levels disappoint expectations.

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Water Usage

What's going on with urban water usage?

  • In mid-July 2021, Governor Newsom urged Californians to cut their water use by 15%. The ask is voluntary, for the moment, and there are no statewide emergency water mandates in effect.
  • As such, the state is leaving it up to individual water suppliers to bear responsibility for the reductions and deciding on how to make it happen.
  • Water suppliers are required to track residential water use. Using that analysis, the state is tracking both water savings from July 2021, and monthly water use compared to 2020.

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What's going on with residential water use?

  • Since October 15, 2014, water suppliers are required to estimate or report the amount of water used for residential purposes. They are required to calculate per capita use monthly.
  • This is often reflected as Residential Gallons Per Capita Day, or R-GPCD, a calculation based on the total population served by a water supplier verses the total water produced and used for residential use only over however many days are in that month.

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For undefined, the average amount of water used for residential was undefined gallons per capita each day in undefined.

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Residential water use statewide in undefined null undefined gallons per capita used daily compared to the five-year average of null.

What's going on with agricultural water use?

  • Good question; we really don't know. Unlike residential water use, collecting data on commericial agricultural water use is not required by law, and water suppliers occasionally supply that information.
  • According to the California Department of Water Resources, agriculture accounts for about 80% of water use.
  • The state does track the amount of water flowing to fields as they pass through water gates, but the data is unreliable, as water could flow back through the system and be double counted. Groundwater wells marked for agricultural use could also be measured, but the frequency varies (usually twice a year).
  • CalMatters is currently working on ways to reliably, and accurately, track agricultural water use.



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Where is the groundwater?

This map shows the 515 groundwater subbasins — underground features that collect water.

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How much groundwater is there?

We can see the distribution of extremely low wells based on each groundwater subbasin.

  • 0%
  • 25%
  • 50%
  • 75%
  • 100%

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How does the groundwater get extracted?

This map shows the distribution of wells per county.

  • 0
  • <500
  • 500-999
  • 1000+

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How many wells are at their lowest levels?

This map shows the distribution of wells per county.

  • 0
  • <50
  • 50-99
  • 100+

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How many wells are running out of water?

This map shows how many households reported well water shortages in the past year per county.

  • 0
  • <50
  • 50-99
  • 100+

Groundwater Wells

house icon

Household Water Shortages

  • The California Department of Water Resources started a program in 2014 to more systematically track household water shortages, initially to help facilitate drought assistance.
  • Most reported shortages occur during the summer months (Q3), regardless of whether there is a drought.
  • The state provides county-level data on these reported shortages

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well icon

Groundwater Well Rankings

  • The California Department of Water Resources compares current measurements for wells with historical measures for the same month. At minimum, wells are measured twice a year in April (peak high) and October (peak low).
  • This offers a glimpse into how many wells are experiencing historically low water levels.
  • All wells that contain 10 years of historical data for the month in question and wells that were measured within the last 18 months are included in the analysis.
  • Wells are grouped into percentiles based on their most recent measurements. For example, wells that are within the 10th to 25th percentile of their historical measurements are in the "10-25%" bucket, and wells measuring their historic low are in the "Lowest" bucket.
  • You can read more about the statistics here.

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drill icon

New Groundwater Wells

  • The California Department of Water Resources maintains a database of applications for new wells and tracks their completion.
  • Included is information about location, depth and planned use. We reviewed well completion reports starting in 2019.
  • You can review a map showing where wells are being built here.

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subsidence icon

Land Subsidence

  • A major risk of over-pumping an aquifer: triggering “land subsidence,” when the ground sinks slowly or suddenly due to shifts underground.
  • The worst instances of land subsidence in California are occurring in the southern end of the San Joaquin Valley, where over-pumping for agriculture is common, even in non-drought years.
  • Included is information about location, depth and planned use. We reviewed well completion reports starting in 2019.
  • The map below shows total land subsidence between June 2015 and June 2018. Some areas in the San Joaquin Valley have sunk 100 feet below historic norms.
  • -0.25 – -1 ft
  • -1 – -1.75 ft
  • -1.75 – -2.5 ft
  • -2.5 – 3 ft

Get Involved

Interested in joining the conversation about how to protect your local water supplies from mismanagement or climate-induced disasters such as drought? Here are options:

Groundwater Sustainability Plans

Local water agencies must develop plans to avoid seawater intrusion, degrading water quality and land subsidence.

The Sustainable Groundwater Management Act gave local water providers in critically over-drafted basins 26 years — until 2040 — to stop the impacts of the overdraft from worsening, and until 2042 for those managing less-depleted water supplies.


Sustainable Groundwater Management Act signed into law.


Water agencies in critically over-drafted basins must begin to implement sustainability plans.


All other agencies must begin to implement sustainability plans.


Agencies in critically overdrafted basins must achieve sustainability goals.


All other agencies must achieve sustainability goals.


days since signed until now


days from now until fully implemented


  • Engineering: Jeremia Kimelman, John Osborn D'Agostino, and Erica Yee
  • Data: Jeremia Kimelman, John Osborn D'Agostino, and Erica Yee
  • Design: Jahm Alissa
  • Editors: Vicki Haddock, Marla Cone, and John Osborn D'Agostino
  • Product Manager: Sapna Satagopan

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