Climate change is intensifying the hydrological cycle, leading to more frequent and severe droughts and floods across the globe. We learned this the hard way… As natural water reservoirs, groundwater aquifers can provide a crucial buffer against surface water variability. However, many aquifers are in a state of chronic overdraft, jeopardizing water supply reliability. At the same time, increases in heavy rain and earlier snowmelt exacerbate the risk of flooding, threatening existing infrastructure.
Now, this might seem counterintuitive…
Managed aquifer recharge (MAR) offers an integrated solution to simultaneously manage droughts, floods, and depleted groundwater. MAR involves improving connections between surface water infrastructure (e.g., reservoirs, conveyances) and underground aquifers to capture floodwater for storage and later use. Compared to traditional surface storage, this “Flood-MAR” approach can be more cost-effective, sustainable, and adaptive to a changing climate.
Despite the potential benefits, deployment of MAR is fundamentally constrained by how much water is available for recharge (WAFR) at present and under future climate change. This article outlines a novel, climate-informed and policy-relevant framework to quantify WAFR, its uncertainty, and associated policy actions. We apply this framework in California, where groundwater overdraft and flood risks coexist, to guide effective investment decisions on diversion infrastructure and inform sustainable water management practices.
Assessing Water Available for Recharge under Climate Change
Our analysis reveals that future maximum WAFR potential in California will increase overall, but to what extent this potential can be captured is largely constrained by the physical capacity of diversion infrastructure. Moreover, a warming climate will likely exacerbate existing regional disparities in WAFR, elevating it in Northern California while decreasing it in the drier Southern regions.
Specifically, we find widespread and robust increases (over 56-80% of subbasins) in WAFR potential during 2070-2099 under both moderate (RCP4.5) and high (RCP8.5) emissions scenarios. The largest increases are concentrated in Northern California, including the Sacramento River and North Coast regions. In contrast, there is almost no increase (and in some cases, even a decrease) in the drier Southern California.
This emerging “wet-gets-wetter, dry-gets-drier” pattern under climate change will pose additional challenges to California’s existing water infrastructure and ecosystems. Existing aqueducts and reservoirs may not be able to effectively move and store the intensified high-magnitude-flows from Northern to Southern regions, especially during wet seasons when the system is already at full capacity.
Moreover, future WAFR will be concentrated over a narrower wet season compared to historical periods, as evidenced by the projected increase in WAFR “sharpness” (a measure of seasonality). This implies heightened winter flooding risk that may further stress aging infrastructure.
Capturing the Maximum Recharge Potential
While the projected increases in WAFR potential present great opportunities for state-wide recharge practices, the extent to which this maximum potential can be actually captured (WAFRcap) is largely constrained by the physical capacity of diversion infrastructure.
Our analysis finds that with a referenced diversion capacity equal to the design capacity of the Friant-Kern Canal (150 m³/s), only about 25% of the projected WAFR potential over the San Joaquin Valley can be captured in the far future (2070-2099). This is likely not enough to fully offset the historical groundwater overdraft in the region.
Even more concerning, we find that the share of captured WAFR is likely to further decrease in the future, especially under the high-emissions scenario (RCP8.5). This is due to the increasing seasonality of extreme rainfall, which translates to more “flashy” streamflow conditions that are harder to divert.
To tap the elevated WAFR potential under deep uncertainties, expanding diversion capacity will be crucial. However, the marginal gain in WAFRcap decreases rapidly per unit expansion in diversion capacity, and is associated with large uncertainties from both climate models and greenhouse gas scenarios.
Prioritizing Infrastructure Investments
Given the limitations of existing infrastructure and the large uncertainties surrounding future WAFR, we outline a novel, robustness-based policy typology to guide infrastructure investment decisions. This typology explicitly incorporates and classifies key uncertainties, focusing on whether they are reducible in the near-term (e.g., policy decisions) or subject to deep, long-term climate changes.
We find that most subbasins in California (67 out of 135) have limited near-term opportunities to invest in diversion infrastructure, due to the low potential of initial robust expansion. These regions, mostly located in the drier Southeast, should consider alternative water sources (e.g., treated wastewater, desalinated brackish groundwater) and non-physical interventions (e.g., demand management, water trading) to facilitate recharge.
In contrast, nine subbasins are identified as high investment priorities, as they exhibit both high initial robust expansion potential and low sensitivity to remaining uncertainties. Traditional cost-benefit analysis combined with multi-objective optimization can be used to determine the size, sequence, and location of these proposed diversion infrastructure projects.
For the remaining subbasins, investment decisions need to carefully weigh the relative importance of climate versus policy uncertainties. Subbasins suffering from deep climate uncertainties should consider building flexible, modular infrastructure that can be adapted as climate behavior is realized. In contrast, subbasins predominantly impacted by policy uncertainties should prioritize resolving these uncertainties before major infrastructure investments.
Towards an Integrated Approach for Flood and Drought Resilience
The spatial mismatch between high-magnitude-flow availability and groundwater overdraft revealed in our analysis poses a profound challenge for recharge practices. Northern California, where large volumes of WAFR are projected, is likely to face an increased risk of flooding. Meanwhile, the drier Southern regions with greater groundwater depletion will have less WAFR available.
To address these challenges, a holistic, integrated approach is needed that simultaneously mitigates flood and drought risks, while enhancing groundwater sustainability. This includes not only expanding diversion infrastructure, but also pairing it with other strategies such as reservoir re-operations, demand management, and ecosystem-based solutions.
Moreover, the fragmented governance of groundwater in California, with multiple management entities within a single basin, creates specific challenges for coordinating cross-basin infrastructure investments. Policy-makers should consider the variations in local governance context when translating the proposed typology into practical guidance.
Looking beyond California, the untapped MAR potential demonstrated in this study could serve as a proof of concept to help translate MAR from local aspiration to global practice, especially in other water-stressed regions facing the dual threats of droughts and floods. However, regional differences in hydroclimatic conditions, water management policies, and data availability need to be carefully considered when applying this framework elsewhere.
By quantifying the changing WAFR characteristics under climate change and developing a policy-relevant investment typology, this work provides a critical first step towards realizing the full potential of MAR as an integrated, climate-adaptive water management strategy. Continued research and collaborative efforts across disciplines and jurisdictions will be essential to build a more resilient and sustainable water future.
Tip: Implement real-time monitoring to swiftly respond to flood risks
