by Mustafa Dogan, Ian Buck-Macleod, Josue Medellin-Azuara, and Jay Lund

Groundwater overdraft is a major problem globally and has been a persistent and growing problem in California for decades. This overdraft is predominantly driven by the economic value of water for agricultural production and cities. Spurred by the recent drought, California passed legislation requiring the elimination of groundwater overdraft by 2040. To explore potential water supply effects of ending long-term groundwater overdraft in California’s Central Valley, we compared several water policies with historical and warmer–drier climates, employing a statewide hydroeconomic optimization model, CALVIN, in our new paper. Hydro-economic optimization models like CALVIN allocate water to agricultural and urban users considering hydrologic conditions, infrastructure, and environmental restrictions among other factors, such that systemwide water scarcity and operation costs are minimized.

Groundwater overdraft is a common response to surface water scarcity when the economic value of water use in agriculture and cities exceeds pumping costs. In California, supply and demand disparity combines with a great seasonal and geographical imbalance of water supplies and demands. Water is much more available during winter in northern California, but water demands are mostly in central and southern California during spring and summer.

The Sacramento-San Joaquin Delta (Delta) is the major hub in California’s water system (Figure 1), with environmental and water allocation policies also affecting operation, planning, and management decisions. Changes in regulations and climatic conditions and increasing water demands make Delta water management complex.

Figure 1. California’s groundwater basins, aggregated wildlife refuges, and minimum in-stream flow requirements represented in CALVIN, and the Delta water balance (Dogan et al., 2019)

Management Policies

Policy cases examined here put different restrictions on Delta water operations and eliminate groundwater overdraft under 82-year (1921–2003) historical and warmer–drier climates with 2050 agricultural and urban water demands (Table 1). Policy 1 allows historical overdraft rates in CALVIN’s 82-year modeling horizon. A no-overdraft policy is applied to the Central Valley groundwater aquifer in Policies 2–5, requiring that groundwater storage at the end of each 82-year run cannot be less than groundwater storage at each run’s beginning. Policies 3–5 add different Delta export constraints to this no-overdraft policy. Policy 3 maintains historical Delta outflows, by month, in addition to the no-overdraft policy, so reductions in Delta outflows cannot substitute for lost historical supplies from groundwater overdraft. Policy 4 further restricts Delta export operations by constraining water exports to historical quantities, in addition to maintaining a no-overdraft policy. This prevents the optimization model from curtailing water use north of the Delta to supply water south of the Delta to accommodate supplies lost there from ending groundwater overdraft. Policy 5 reduces Delta exports by 95%, largely eliminating them, in addition to maintaining a no-overdraft policy.

Table 1. Policy cases evaluated under historical and perturbed (warmer–drier) climates (modified from Dogan et al., 2019)

Conclusions

All analyses have weaknesses and limitations, but the model’s results support several consistent and insightful conclusions, many of which merit further analysis and discussion.

• California’s two largest water problems, groundwater sustainability and the Sacramento-San Joaquin Delta, are closely tied.
• With “no overdraft” policy (Policy 2), the 82-year period has two large recovery durations, lasting 62 and 12 years, demonstrating the importance of long-term groundwater planning and the difficulty of assessing groundwater sustainability.
• If Delta outflow cannot be reduced for environmental and operational reasons (Policy 3), opportunities for Sacramento Valley users to sell some water to south-of-Delta users would be economically worthwhile under historical climatic conditions.
• If permitted environmentally, diversions from surplus Delta outflow (Policy 3) can reduce (but not eliminate) water scarcities.
• If Delta exports cannot be increased for environmental and management reasons (Policy 4), regional water trades help reduce water scarcities.
• Surface water availability and the reliability of Delta exports are significantly reduced with a warmer-drier climate.
• A warmer-drier climate combined with ending groundwater overdraft and some water management policies would further exacerbate water scarcities and increase environmental and economic costs.
• Delta water supplies and operations become more important with the end of overdraft and climate change.
• Economically useful adaptations include more diversions from surplus Delta outflow, increased water transfers involving Delta, water markets and trades, conjunctive use of groundwater and surface water, and recycled wastewater. These would come at a cost and often with additional controversy.

Further Readings

Dogan, M.S., Buck, I., Medellin-Azuara, J., and Lund, J.R. (2019), “Statewide Effects of Ending Long-term Groundwater Overdraft in California,” Journal of Water Resources Planning and Management, 145(9).

Lund, J.R. (2016), “California’s agricultural and urban water supply reliability and the Sacramento-San Joaquin Delta,” San Francisco Estuary Watershed Sci. 14(3).

Dogan, M. S., J. D. Herman, and M. A. Fefer. 2017. “CALVIN source code.” Accessed July 21, 2018. https://github.com/ucd-cws/calvin.

Buck, I. (2016), “Managing to end groundwater in California’s Central Valley with climate change.” M.S. thesis. Dept. of Civil and Env. Engineering, University of California, Davis.

Dogan, M. S. (2015), “Integrated water operations in California: Hydropower, overdraft, and climate change.” M.S. thesis. Dept. of Civil and Env. Engineering, University of California, Davis.

Mustafa Dogan recently completed his PhD in Civil and Environmental Engineering at the University of California, Davis and is now an Assistant Professor of Civil Engineering at Aksaray University in Turkey. Ian Buck-Macleod is an engineer with Stantec. Josue Medellin-Azuara is an acting Associate Professor of Environmental Engineering at the University of California, Merced.  Jay Lund is a Professor of Civil and Environmental Engineering at the University of California, Davis.