Climate Change Impact Analysis on Groundwater Availability and Managed Aquifer Recharge in California

Project ID: 23041
Principal Investigator: Kirk Nelson
Research Topic: Groundwater Storage
Funded Fiscal Years: 2023 and 2024
Keywords: None

Research Question

Climate change presents a challenge to Reclamation in meeting its multiple objectives in reservoir operations. It is likely that climate change impacts will make existing challenges more difficult by altering the timing and magnitude of snowmelt feeding the headwaters above our reservoirs, increasing drought risks, and increasing the frequency and magnitude of extreme events. The importance of conjunctively managing surface and groundwater supplies has become increasingly recognized in recent years. The research question being asked in this project is: How will climate change impact groundwater availability and the ability for groundwater to meet demands during drought periods, and how effective is MAR as a management strategy to increase our ability to balance water budgets?

We hypothesize that as radiative forcing increases, climate change will reduce the availability of both surface water and groundwater supplies resulting in ever increasing challenges in meeting demands. We further hypothesize that MAR is a strategy that can mitigate the potential water shortages. The results of the study will provide guidelines for how to optimize the implementation of MAR under assumed future climates. As such, MAR represents a strategy for Developing Water Supplies.

It is important to recognize that climate change will impact both the volume and seasonality of surface water supplies. Increased frequency and/or duration of droughts will decrease surface water availability. Rising temperatures will lead to more precipitation falling as rain rather than snow, causing increased winter flow and decreased spring/summer flows. Increased winter flow may lead to the dams/reservoirs reaching their capacity and correspondingly increasing flood risks. Reduced spring/summer flows will increase the necessity of attaining groundwater reliability. MAR can be part of the equation balancing the impact by capturing excess winter flow and delivering it to suitable areas to recharge aquifers.

Need and Benefit

Ongoing overdraft overprinted by climate change effects that have accentuated drought conditions have dramatically impacted Reclamation's ability to deliver water, both in terms of flows and infrastructure capacity reduction due to subsidence. Mounting evidence also suggests that these difficulties will only grow as a result of climate change which is causing reduced snowpack, spring snowmelt to be reduced and shifted to earlier in the year. The likelihood of extreme events is also expected to increase which poses a risk in flood operations. Simply put, the timing, phase and magnitude of precipitation is negatively impacting Reclamation's ability to store and deliver water. At the same time, groundwater aquifers are being depleted and there is an effort (e.g., SGMA in California) to bring sustainable management to the aquifers. Facing these challenges requires new approaches. This research will develop a methodology for planning Managed Aquifer Recharge (MAR) which will address the problems described herein by capturing excess water during flood events by directing the water to recharge aquifers. In this way, the lost storage from changing snowpack dynamics can be replaced with additional groundwater storage.

MAR is an approach that has gained increasing attention in recent years, but there is a lack of comprehensive and robust modeling tools to plan its implementation. It is a challenging problem especially within the context of climate change that requires the linkage of models including the GCMs projecting future climatic conditions, rainfall-runoff models simulating the inflows to reservoirs, reservoir operations models to simulate the routing of surface water and diversions/deliveries and water available for MAR, and integrated surface-subsurface hydrologic models to simulate the physical processes governing recharge, groundwater storage, and subsidence (note that MAR can also provide mitigation against subsidence; CVHM2 contains a rigorous subsidence module, so part of our analysis will include this factor). There is no existing capability internal to Reclamation that can model this entire suite of linked processes. Externally, relevant work has been done to estimate the impact of climate change on groundwater storage in California's Central Valley (Alam, 2019) and the potential for MAR to achieve groundwater sustainability (Alam, 2020). However, no work exists either internally or externally that has combined an analysis of climate change impacts on groundwater storage and subsidence with the implementation of MAR as a mitigation measure against climate change with the dual goals of increasing water supplies and achieving groundwater sustainability.

The research products from this project will have an immediate benefit to Reclamation's contractors in the Delta-Mendota subbasin by providing a tool that will help them implement MAR as a water management strategy to overcome the challenges of climate change and meet the dual goals of ensuring groundwater sustainability (as required by California's SGMA) and optimally performing conjunctive management to balance water budgets.

The longer lasting impact of this research will be to provide a methodology that can be applied in Reclamation's other regions to conjunctively manage their surface and subsurface water resources by planning and designing effective MAR projects and mitigating land subsidence. The methodology will also be easily adaptable to consider mitigation/adaptation approaches other than MAR. This directly contributes to Reclamation's mission of managing water resources in an environmentally and economically sound manner in the interest of the American public. The net impact that will result is increased water delivery, groundwater storage increases, and land subsidence decreases.

Contributing Partners

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Research Products

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Last Updated: 6/22/20