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.


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.

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.

Further reading

Nelson, T., H. Chou, P. Zikalala, J. Lund, R. Hui, and J. Medellín–Azuara (2016), Economic and Water Supply Effects of Ending Groundwater Overdraft in California’s Central Valley, San Francisco Estuary and Watershed Science, Volume 14, Issue 1, Article 7,

Hanak, E., J. Lund, A. Dinar, B. Gray, R. Howitt, J. Mount, P. Moyle, and B. Thompson, Managing California’s Water:  From Conflict to Reconciliation, Public Policy Institute of California, San Francisco, CA, 500 pp., February 2011.

Medellín-Azuara, J., D. MacEwan, R.E. Howitt, G. Koruakos, E.C. Dogrul, C.F. Brush, T.N. Kadir, T. Harter, F. Melton, J.R. Lund,” Hydro-economic analysis of groundwater pumping for California’s Central Valley irrigated agriculture,” Hydrogeology Journal, Vol. 23, Issue 6, pp 1205-1216, 2015.

Howitt, R.E., Medellin-Azuara, J., MacEwan, D., Lund, J.R. and Sumner, D.A. Economic Analysis of the 2014 Drought for California Agriculture. Center for Watershed Sciences, University of California, Davis, CA, 28pp., July 28, 2014.

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

  1. Rich Persoff says:

    Let’s realize that the amount of water available for distribution is not as much as is wanted or needed to avoid financial losses to present consumers. It’s like musical chairs: more and more water consumers will inescapably be left out as supplies diminish.
    Mandatory permanent fallowing, with some compensation, for those crops which use the most water and have the least employment or other social benefit, could balance a vigorous agriculture and a limited water supply.
    One-time cotton and hay fields might well have greater economic benefits as solar energy farms, using present technology. And it would seem that EIR’s for cropped (e.g., not wild) land would require much less mitigation.

  2. Larry Farwell says:

    It seems odd not to mention that the reason the CVP and SWP did not end groundwater overdraft is because agric. acreage irrigated with groundwater increased as the surface water projects removed agric. acreage from groundwater use. As long as new lands can be brought into production using groundwater CA will never end the overdraft. Any agric. acreage brought into production since 1950 without a surface water supply should be retired.

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  5. The study was disappointing, and short on good science. Sadly, the Lund team’s results are often more “truthy” than complete.

  6. Boris Seymour says:

    A critique of this paper by Tim Stroshane:

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