Groundwater and Stream Interaction in California’s Central Valley (February 2016)

Groundwater and Stream Interaction in California’s Central Valley: Insights for Sustainable Groundwater Management

Groundwater is intimately connected to surface water, which has profound implications for sustainable water resource management. California has historically overlooked this important interaction and as a consequence, decisions about groundwater extractions have generally failed to address the resulting impacts to aquatic ecosystems such as rivers, wetlands and springs. This has contributed to a loss of approximately 95 percent of the historical wetlands and river habitat in California’s Central Valley.

In February 2016, the Conservancy published a study that uses an integrated hydrologic model to reconstruct the historical impacts of groundwater use on groundwater levels and stream flow conditions in California’s Central Valley. The results illustrate how rivers and streams that historically gained surface flows from groundwater now lose surface flows to groundwater. As a consequence, Central Valley rivers are now losing almost 1.5 billion gallons of water each day – that is enough water to supply 2.5 times the water needs for Los Angeles—than they did in the 1920s.  In addition, groundwater aquifers contain 6.5 trillion gallons less water now than they did at the start of the study period.

Groundwater sustainability agencies across the state will soon be required to manage groundwater resources to avoid causing undesirable results to groundwater levels and interconnected groundwater and surface water. These groundwater levels and areas of interconnection support groundwater-dependent ecosystems (GDEs). Therefore, an important first step in sustainable groundwater management is to understand how groundwater pumping impacts surface water and GDEs, both of which include streams.

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