The Big Water Supply Shift in California

Groundwater Key to Water Security in California’s Changing Climate

Published Nov 13, 2015


California’s recent struggles with drought have brought state water supplies to unprecedented and dangerously low levels. At the same time, the growing consequences of climate change are becoming increasingly evident, with far-reaching implications for how the state manages its water supplies.

California is increasingly turning to groundwater to meet its water needs. To ensure enough water for generations to come, the focus must shift to make groundwater supplies more reliable and sustainable.

Lake Oroville California Low Reservoir Water Levels
Decreased snowpack has brought less water into reservoirs (such as Lake Oroville, pictured, in Northern California), while increased temperatures have led to greater evaporation of surface water.
Photo: California Department of Water Resources

Current impacts and future risks

Temperatures are rising in California as heat-trapping emissions continue to accumulate in the atmosphere.

The statewide average temperature during the winter of 2014–2015 was the warmest ever recorded (50.5°F)—more than 5°F warmer than the average for the twentieth century. Warming has been most evident in the Sierra Nevada, where much of the state’s water supply originates, with average minimum temperatures registering above freezing for the first time and the lowest-ever snowpack in 2015.

In addition, the timing of the snowmelt is shifting, becoming out of sync with communities’ needs. Snow is already melting as many as 30 days earlier than in the mid-twentieth century, reducing summer flows in many snow-fed rivers. This means less water is available during the hotter months, when water demand tends to be highest.

Higher temperatures are also expected to cause more snow to fall as rain, which paradoxically may exacerbate water shortages. California's water systems are built to hold snowmelt as it is slowly released from mountain snowpack. When too much rain falls during the winter, reservoirs accumulate too much water, too quickly, and must release water to ensure that the dams are not overtopped.

In addition to causing changes to the timing and quantity of surface water supply, global warming is also causing more severe impacts from drought. For example, hotter temperatures lead to greater evaporation from reservoirs, farms, and lawns.

Groundwater: current problem, future solution

Until recently, surface-water storage and conveyance have been the major focus of water management. Yet, as surface water becomes less reliable, Californians are turning increasingly to groundwater.

Groundwater supplies between 30 and 50 percent of California’s water supply, depending on precipitation, and represents a storage reservoir that is over three times greater than available surface water storage.

Today, groundwater supplies up to 50 percent of California’s water, but the state's prolonged drought has led to the overpumping of groundwater,  overdrafting the Central Valley’s aquifers.

Graph of groundwater depeltion in California Central Valley

In 2014, the ongoing drought helped spur the passage of the Sustainable Groundwater Management Act (SGMA), the first ever statewide requirement for groundwater management.

Sustainable groundwater management offers a new pathway, allowing the state both to mitigate and to adapt to climate change while increasing water reliability in the future.

The Union of Concerned Scientists, along with many others, has recommended that the regulations include a common framework for setting thresholds that quantitatively describe what is considered unsustainable, as well as protective triggers that require management actions to avoid crossing thresholds. This framework will provide greater regulatory certainty and pave the way for successful implementation of SGMA.

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