The area under study consists of four major basins in the central part of the Sierra Nevada in California: the Stanislaus, Tuolumne, Merced and Upper San Joaquin river basins, which contribute the most to the San Joaquin River flow. They host various hydropower projects, including the expansive Big Creek hydropower system which is ranked among the world’s largest complexes of its kind. Collectively these projects generate approximately 25% of California’s hydroelectric energy, with a total generation capacity of over 2700MW.
In addition to the largest climate variability in the US, the Mediterranean climate regime found in California gives the state a more extreme mismatch in water availability and demands. Electricity and especially agricultural water needs are greater during California’s hot, dry summers, while most of the precipitation arrives primarily as a few atmospheric river events in the northern part of the state, in particular in the Coast and Sierra Nevada mountain ranges during the winter.
Climate change – warming in particular – further complicates that dynamic with a shift in hydrology towards less snow accumulation, earlier snowmelt and more precipitation contributing to streamflow as rain. Meanwhile, climate change is also causing increased climate “whiplash,” which refers to extreme annual shifts from one climate extreme to another, such as from severely dry to wet or wet to dry conditions.
Climate whiplash is primarily driven by rapid climate change, which intensifies the hydrological cycle and therefore climate variability. That rapid change impacts specific components of the climate system, such as causing disruptions in the polar vortex, which in turn affects the jet stream (fast-flowing air currents flowing west to east). These effects are especially noticeable in the US West Coast, but climate whiplash has also been recognised and is projected to intensify in parts of Europe and Asia.
The research on climate whiplash impacts on hydropower production underscores the challenges in managing extremes, revealing system vulnerabilities due to limited surface water storage capacity, and emphasising the need for adaptive strategies to enhance climate resilience in hydropower operations.
To simulate the impacts of whiplash events, we used a daily water system simulation model (CenSierraPywr) to stress test the water and power projects in the Central Sierra Nevada, and also constructed 200 synthetic hydrologic sequences by sampling the hydrological data derived from future climate projections for the years 2030 through 2060 (Figure 1).
We sampled the wettest (>80th percentiles) and driest (<40th and 20th percentiles) water years across projections of ten Global Circulation Models. Then, we combined these water years into 2-to-5-year long dry spells interspaced by 1-2 wet years, to be used as climate-perturbed hydrological inputs to the CenSierraPywr model.
Our findings show that whiplash impacts on hydropower production are significant due to increased spillage. Even though the basins examined are highly regulated, they can be affected due to their smaller surface water storage capacity. Storage capacity constrains the ability of these systems to absorb hydrologic shocks. A very wet year can alleviate the stress caused by droughts by bringing storage or energy generation levels close to or even above average historical averages. However, the first dry year following wet years faces an immediate and notable decrease in hydropower generation, demonstrating the system’s vulnerability to drastic changes in water availability. Wet extremes can momentarily benefit power generation, but do not result in longer-term system resilience.
Varying degrees of resilience to climate whiplash
Different basins exhibit varying degrees of resilience and performance. For instance, the northern basins (Stanislaus and Tuolumne) have large storage capacities and show some buffer in the first dry year after wet years, with approximately half the generation losses compared to preceding and subsequent dry years (Figure 2). The Stanislaus still generally requires two wet years to reach above-average storage values (Figure 3).
During the wet rebounds, the basin shows increased hydropower generation especially in late winter and spring (Jan-May), while losses in multi-year droughts are year-round and more apparent in late spring (May-Jun). The Tuolumne basin also has a large storage capacity and shows a more stable response to drought with losses in storage averaging around -30% in most dry years, compared to historical averages. Major losses in generation occur between the winter and summer (especially in Feb-Jul), while benefits from wet years are mostly seen in the winter of the second wet year (Jan-Mar).
The Merced and Upper San Joaquin basins, on the other hand, have lower storage capacity. The Merced shows the widest range of variation in generation due to its dependency on the water stored in the terminal reservoir in the basin’s outlet.
Therefore, generation ranges on average from approximately +40% in wet years to below -60% during prolonged droughts, compared to historical averages, and major losses concentrate in the summer when water is needed downstream for agricultural demands. Notably, the Upper San Joaquin basin performs the worst during droughts even though it is the most regulated river basin, due to the low storage capacity in its multiple reservoirs, leading to significant generation losses. Gains in generation in the wet year(s) are already lost in the first subsequent dry year. Losses during droughts and gains during wet years are both concentrated in the winter and summer.
Impacts on hydropower production
To lessen or alleviate these impacts, planners can invest in advanced forecasting and monitoring systems, adopt adaptable operational and storage strategies, and explore compensatory sources of power generation. For instance, California is actively advancing Forecast Informed Reservoir Operations to enhance flood control strategies. This approach relies on short-term forecasts of atmospheric rivers to determine when to release water (or not) for flood control, rather than following outdated and inflexible protocols from the mid-1900s of always preemptively emptying reservoirs.
Additionally, there is growing interest in integrating alternative energy sources with hydropower, such as taking advantage of the complementarity of solar energy. This includes utilising floating solar photovoltaics (PV) and solar PVs over canals, where possible, which not only reduces evaporation but also boosts solar energy efficiency and ensures greater energy security.
The concept of managed aquifer recharge is also being extensively researched; this approach aims to mitigate flood risks, counteract groundwater depletion, and meet agricultural water demands simultaneously. Given the vulnerability of existing hydropower systems to increases in hydrologic whiplash, bolstering climate resilience with more adaptable water resource infrastructure and management is paramount.
Reference
