When the California Legislature created the “modern” water rights regulatory system more than a century ago, it focused exclusively on surface water, exempting groundwater from the permitting system. Yet in most watersheds, surface water and groundwater are closely linked. Actions that change one often have an impact on the other. The arbitrary legal divide has made it harder to manage the state’s water. But a recent law and a new court decision have done a better job of connecting surface water and groundwater.

When rain falls or snow melts in the foothills and mountains of California, water follows several pathways downhill and into rivers and streams. Some water moves across the land or through deep soils and weathered bedrock, arriving in rivers hours to weeks after rain or snowmelt. And some percolates deep into the ground, becoming groundwater.

As groundwater tables rise in winter and spring, they discharge water into rivers and streams, either directly through river beds and banks or indirectly through springs. This is why many of California’s rivers and streams flow long after the rain stops.

The exchange between surface water and groundwater continues when rivers emerge from mountains and flow across the state’s large valleys. Winter flooding, along with rainfall in the valleys, recharges groundwater during the winter. Some of that groundwater moves back into rivers and helps maintain flow in the dry season. Shallow groundwater also supports wetlands and riparian vegetation.

Groundwater pumping alters the exchange between surface water and groundwater. In many of the state’s large valleys, such as the Central Valley and valleys in the Central Coast and Southern California, streams and rivers that historically flowed all year are now intermittent due to lowered groundwater tables. In many cases, groundwater tables are so far below the surface that water only flows one way: from the river bed to the aquifer.

The arbitrary legal divide between surface water and groundwater has begun to change. A big step was the 2014 enactment of the Sustainable Groundwater Management Act (SGMA). SGMA requires water users in overdrafted groundwater basins to develop and implement groundwater sustainability plans to bring groundwater use and recharge into balance by 2040. SGMA mandates that these plans include measures to avoid undesirable effects from pumping, such as reduced stream flows and harm to groundwater-dependent ecosystems like wetlands. This is one of the few explicit legislative acknowledgments of the links between groundwater pumping and surface water flows.

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