Storing and managing water for the environment is more efficient than mimicking natural flows

Dams and reservoirs, while crucial for water supply, significantly degrade freshwater ecosystems. To mitigate these effects, environmental flows are often released from reservoirs, creating a paradox: the source of ecosystem decline also holds the key to its recovery. Current approaches often focus on the effectiveness of environmental water (the degree to which flows achieve desired results), neglecting efficiency (accomplishing objectives with minimal resources). This study investigates whether an 'environmental water budget,' which explicitly allocates and manages reservoir storage for environmental needs, can improve efficiency. This contrasts with the prevalent approach of passing through a fixed percentage of inflow, which can lead to inefficient use of water resources. While a few examples exist (like Nevada's Marble Bluff Dam and Australia's 2007 Commonwealth Water Act), the widespread adoption of environmental water budgets in the USA remains limited. The study focuses on the San Joaquin River in California as a case study, comparing the pass-through approach mandated in some areas with a potential alternative using an environmental water budget. The importance of this research lies in the potential to improve the health of freshwater ecosystems while balancing competing water demands, especially in the context of climate change and increasing water scarcity.

Existing literature highlights the effectiveness of environmental flows in improving ecosystem health but often overlooks efficiency. Studies have focused on achieving desired ecological outcomes with little attention paid to minimizing water use, cost, and effort. While some research explores methods for prescribing appropriate environmental flows (e.g., functional flows, designer flows), these often treat environmental water as a constraint rather than a priority objective within multipurpose water management. Previous work has also shown the importance of incorporating thermal regimes into environmental flow management for a more holistic approach. The authors discuss the existing precedent for environmental water storage in Australia and limited examples in the US, such as the Marble Bluff Dam in Nevada and the California Water Storage Investment Program, emphasizing the need for a more comprehensive and efficient approach to managing environmental water.

The research utilizes a simple, priority-based water balance operations model coupled with a one-dimensional reservoir temperature model. The model is loosely based on California's Shasta Reservoir (5.55 billion cubic meters capacity), using its inflow data for simulations. The model operates on a monthly timestep for water years 1996-2021, capturing historical hydrologic variability. It simulates water allocation for five prioritized demands: environmental objectives (baseflows, flow shaping, optimal temperatures), wildlife refuges, in-basin uses (urban and agricultural), system water (for salinity management), and out-of-basin exports. Environmental demands receive senior priority in the model. Two management approaches are compared: (1) pass-through of 10-40% of inflows and (2) allocation of 10-40% of inflow and reservoir storage capacity for environmental demands. Some runs constrain minimum reservoir storage to maintain cold water for temperature objectives. The model assesses trade-offs between environmental objectives and other water demands across various scenarios and water year types (dry, wet, critically dry). The study uses Chinook Salmon as a focal species to demonstrate the concept and evaluate the potential benefits and trade-offs of different management approaches. The water temperature dynamics are modeled using the Water Quality for Reservoir-River Systems (WQRRS) model, a one-dimensional model that accounts for vertical stratification and temperature control devices.

The findings reveal significant differences between the two management approaches. Pass-through flows, while improving flow-related objectives at higher percentages, fail to adequately meet environmental demands at lower allocations (10%). Even with 40% pass-through, considerable shortages persist, particularly in dry years. Furthermore, the pass-through approach negatively impacts water temperature objectives as reservoir storage declines and the cold-water pool is depleted. Allocating a portion of reservoir inflow and storage capacity proves significantly more efficient. This approach allows for seasonal and interannual water storage, enabling targeted releases to fulfill environmental objectives without significant trade-offs. At allocations of 30% or higher, this approach meets most flow objectives and improves water temperature management, especially when combined with minimum reservoir storage constraints. However, allocating significant portions (30% or more) of inflow and storage capacity to the environment in dry years results in severe cutbacks for junior water users (out-of-basin exports). A detailed analysis of the 2019-2021 drought sequence exemplifies the benefits of storing water for the environment, showcasing how an environmental water budget effectively manages water for ecosystem needs despite drought conditions. The study also highlights a breakpoint in trade-offs: beyond 30% allocation, the environmental benefits diminish while shortages for other demands increase significantly. This suggests a balance point in resource allocation.

The study's findings directly address the research question by demonstrating the superior efficiency of an environmental water budget over the pass-through approach for managing environmental water. The results challenge the conventional wisdom of mimicking natural flows, highlighting the limitations of this approach, especially concerning water temperature maintenance. The significance of the findings lies in their potential to inform policy and management practices for large, multipurpose reservoirs. The identified breakpoint in trade-offs provides valuable insights for decision-makers, facilitating compromise and cooperation between different water users. The integration of thermal regimes into environmental flow management adds a crucial dimension to the study's contribution. These findings are especially relevant in the context of climate change, where water scarcity and increased hydrologic variability demand efficient water resource management strategies.

This study demonstrates that allocating a portion of reservoir inflow and storage capacity for environmental management is far more efficient than solely relying on pass-through flows to meet ecosystem objectives. The findings highlight the advantages of an environmental water budget, particularly in terms of maintaining cold-water pools and mitigating trade-offs with other water users. Future research could focus on refining the modeling framework to incorporate more complex ecological interactions and water quality parameters, and applying this methodology to other large river basins with different hydrological characteristics.

The study's model is simplified, using a generalized representation of a large reservoir. While this allows for broad comparisons and understanding of fundamental principles, it might not capture the nuances of specific reservoir systems. The simplification of water demands and the exclusion of some factors (e.g., hydropower, recreation) may also limit the generalizability of the results. Furthermore, the model focuses primarily on Chinook Salmon and might not fully reflect the needs of other species and ecosystem components.