California’s Delta-Groundwater Nexus: Delta Effects of Ending Central Valley Overdraft?

By Timothy Nelson, Heidi Chou, Prudentia Zikalala, Jay Lund, Rui Hui, and Josué Medellín–Azuara.

Surface water and groundwater management are often tightly linked, even when linkage is not intended or expected. This link has special importance in drier regions, such as California. A recent paper examines the economic and water management effects of ending long-term overdraft in California’s Central Valley, the state’s largest aquifer system.  These effects include changes in regional and statewide surface water diversions, groundwater pumping, groundwater recharge, water scarcity, and resulting operating and water scarcity costs.

The analysis used a hydro-economic optimization model for California’s water resource system (CALVIN) that suggests operational changes to minimize net system costs for a given set of conditions, such as ending long-term overdraft. Based on model results, ending overdraft could induce some major statewide operational changes, including significantly greater demand for Delta exports, more intensive conjunctive-use operations to increase artificial and in-lieu groundwater recharge, and greater water scarcity for Central Valley agriculture. Figure 1 summarizes these changes.

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Figure 1 Average annual changes to accommodate ending groundwater overdraft of 1.2 million acre-ft in California’s Central Valley

Ending overdraft in the Central Valley increases economic demands for additional Delta exports, additional groundwater recharge, and additional water market sales, but these are not enough to prevent increased water scarcity to agriculture.

The statewide costs of ending roughly 1.2 maf/yr of groundwater overdraft in the Central Valley are probably at least $50 million per year from additional direct water shortage and additional operating costs. The costs of ending Central Valley overdraft could be much higher, perhaps comparable to the recent economic effects of drought ($1.5 billion/year) (Medellín-Azuara et al. 2015; Howitt et al. 2014).  There is, of course, some uncertainty on both the quantity of Central Valley overdraft and how agencies will manage without it.

Driven by recent state legislation to improve groundwater sustainability, ending groundwater overdraft will have statewide implications for water use and management.  In particular, these implications extend to the Sacramento–San Joaquin Delta, where ending Central Valley overdraft amplifies economic pressure to increase Delta water exports rather than reduce water exports.  California’s largest water management problems are often tied together.

Delta exports and groundwater overdraft in the southern Central Valley have a long intertwined history.  Both the federal Central Valley Project and the State Water Project were developed in part to alleviate groundwater overdraft in the southern Central Valley and improve the sustainability of the region’s agriculture.  The fundamentals of California’s geography, hydrology, and economy of water uses continue to challenge and bind the state and individual regions to balance limited water supplies.

Greater demands for Delta water exports from ending overdraft will probably further complicate potential solutions to Delta problems.  Conversely, the great and perhaps insurmountable difficulties to increasing Delta exports are likely to hinder ending groundwater overdraft in the Central Valley (and increase its costs).  While solving local and regional problems, connections to the statewide system will remain important.  Integrated modeling studies can provide useful insights for these problems, and sometimes insights for solutions.

Originally posted at the California Water Blog.

The authors were or are affiliated with the Department of Civil and Environmental Engineering at the University of California – Davis for this work.  Many have moved on, but some have stayed behind.

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