Sustainable Groundwater Management: What We Can Learn from California’s Central Valley Streams
The Nature Conservancy
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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 conse¬quence, decisions about groundwater extractions have generally failed to address the resulting impacts to sur¬face flows and 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. With the passage of the Sustainable Groundwater Management Act (SGMA), groundwater sustainability agencies across the state will soon be required to manage groundwater resources to avoid causing unde¬sirable results to groundwater levels and interconnected groundwater and surface water. These groundwater levels and areas of interconnection support ground¬water-dependent ecosystems (GDEs). Therefore, an important first step in sustainable groundwater man¬agement is to understand how groundwater pumping impacts surface water, including streams, and GDEs. To build the case for ecosystem protections now found in SGMA, The Nature Conservancy completed a study to illustrate how groundwater pumping is affecting streams and rivers in California’s Central Valley. The report uses an inte¬grated hydrologic model to reconstruct the historical impacts of groundwater use on groundwater levels and stream flow conditions. Our study focused on the state’s Central Valley because of its importance in California’s overall water supply. We used a model developed by the Department of Water Resources (DWR) to simulate the Central Valley’s hydrologic conditions during the years from 1922 to 2009, a period during which groundwater production grew threefold. The results reveal profound impacts to the stream flow conditions due to increased groundwater pumping. Across the Tulare Basin, San Joaquin Basin and Sacramento Valley these changes have differed in mag¬nitude, but share a similar trend. In areas with hydraulic connection between groundwater and surface water, increases in groundwater extraction continue to cause declines in groundwater levels that reduce stream flow. In many of these historically interconnected areas, declines in groundwater levels have caused segments of streams to change from gaining to losing reaches. In other words, portions of streams that once gained sur¬face flows from groundwater were converted to losing reaches, where surface flows are being lost to ground¬water. This reversal from gaining streams to losing streams has been most dramatic in the Tulare Basin, which converted largely by the 1920’s, with the San Joaquin Basin largely converting in the 1960’s. Our results indicate that the Sacramento Valley may be at a tipping point in this transformation, with some areas likely begin¬ning to lose stream flows to groundwater. The results of our study pre-date the extended drought that began in 2011 and it is likely that the drought has exacerbated stream depletions. In addition, our study illustrates that the effects of groundwater pumping can take years—even decades—to recover. This means that the full extent of the impacts of groundwater pumping during the drought will continue to plague California for many years.