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

Dams and reservoirs degrade freshwater ecosystems by blocking access to upstream habitats and altering downstream hydrology, geomorphology, and biogeochemistry. To counteract these impacts, environmental flows are often released to augment flows, maintain water quality, and support aquatic species. This creates a paradox in which the infrastructure contributing to ecosystem decline also offers the means for recovery. The central question posed is whether reservoir storage can be explicitly allocated and managed to revive river health. Traditional prescriptions focus on the effectiveness of environmental flows, often treating them as constraints on multipurpose operations rather than priority objectives. The efficiency of environmental water—achieving objectives with minimal water, time, and cost—has been overlooked. The authors propose an environmental water budget (EWB): allocating water and storage capacity for environmental uses to elevate ecological objectives to operational priorities alongside human uses. There is limited precedent in the USA (e.g., Marble Bluff Dam, Truckee River), with broader experience in Australia under the Commonwealth Water Act allowing purchase and storage of environmental entitlements, including carryover and trading. California’s Water Storage Investment Program similarly envisions storage for public benefits, including environmental water. The study evaluates these ideas in the context of California’s system, particularly as state policies consider pass-through of a share of unimpaired flows versus negotiated or managed approaches, and focuses on ecological objectives relevant to Sacramento River Chinook salmon, including flow and temperature targets.

The paper situates its work within: (1) environmental flow science emphasizing effectiveness of prescribed flows to achieve ecological outcomes; (2) policy precedents for environmental water storage and entitlements, notably Australia’s Commonwealth Water Act enabling purchase, storage, carryover, and trading of environmental water; and (3) emerging California policies (e.g., Lower San Joaquin River 40% unimpaired flow pass-through proposal; San Joaquin River Restoration Program’s managed percentage of unimpaired inflow) and programs funding storage for public environmental benefits. Prior studies highlight the need for functional/designer flows that mimic key hydrograph components, and the importance of temperature management for salmonids. The authors also reference sophisticated system models (CalLite/CalSim, optimization frameworks) and temperature modeling platforms used in California and other basins, indicating that their simplified model complements, rather than replaces, these tools.

The authors developed a simplified, priority-based monthly water balance operations model coupled with a one-dimensional reservoir temperature model (WQRRS), representing a large multipurpose California reservoir with capacity 5.55 Bm³ (modeled after Shasta Reservoir). Simulation period: water years 1996–2021. Key components:

• Environmental objectives: (1) Environmental baseflows meeting minimum instream flow and water quality standards (averaging ~8% of inflow, varying by month and water year type); (2) Flow shaping volumes for a fall pulse, winter pulse, and spring recession (averaging ~14% of inflow), with manager-defined timing/magnitude within each year; (3) Optimal downstream temperature targets: <11.5 °C (Jun–Dec), <12.8 °C (Dec–Apr), and <15 °C year-round to support different salmon life stages.
• Other demands: (a) wildlife refuges (senior priority, fixed seasonal volumes by water year type), (b) in-basin urban and agricultural (senior, seasonal, larger in dry years), (c) system water for Delta salinity maintenance (senior), (d) out-of-basin exports (junior). Hydropower and recreation were excluded.
• Priorities and storage accounting: Environmental, refuge, in-basin, and system water are senior; exports are junior. The model tracks separate storage accounts for each demand group with capacity allocations, allows carryover subject to flood control rules (carryover spills first), and assigns flood control releases in proportion to storage above each group’s capacity allocation. Delivery shortfalls occur when storage is insufficient for monthly demand. Some runs enforce a minimum reservoir storage constraint of 1.54 Bm³ to preserve a cold-water pool.
• Management alternatives:
  1. Pass-through: allocate 10%, 20%, 30%, or 40% of inflow to environmental pass-through, with and without minimum storage constraint (8 runs total).
  2. Environmental Water Budget (EWB): allocate equal shares of inflow and storage capacity to environment at 10%, 20%, 30%, or 40%, with and without minimum storage constraint (8 runs total).
• Temperature modeling: WQRRS (1-D mechanistic) run daily, averaged monthly. Inputs: average monthly inflow and temperature (Sacramento River at Delta station), meteorology (air temperature, wind, humidity from RAWS), constant atmospheric pressure, estimated cloud cover. Reservoir represented with 90 vertical layers (2 m each). Outflow temperature controlled via a simplified selective withdrawal structure with three intake elevations and a spillway. Reservoir stratification and selective withdrawals simulate cold-water pool management and release temperatures.
• Analysis: Performance assessed for dry years (critically dry, dry, below-normal) and wet years (above-normal, wet). Metrics include percentage of months meeting environmental objectives, annual shortages for baseflows and flow shaping, summer (Jul–Sep) release temperatures, and delivery performance for other demand groups. A 2019–2021 drought case study compares 30% pass-through vs 30% inflow+30% storage EWB under a 1.54 Bm³ minimum storage constraint.

• Pass-through performance:
  • With 10% pass-through, environmental deliveries are insufficient: in dry years, baseflows average 44% of demand and flow shaping 20%; in wet years, baseflows average 32% and flow shaping 30%. Interquartile annual shortages: baseflows 203–780 Mm³/yr; flow shaping 617–762 Mm³/yr.
  • With 40% pass-through, environmental objectives improve: in dry years, baseflows average 90% and flow shaping 76%; in wet years, baseflows 94% and flow shaping 68%. Interquartile shortages: baseflows 12–208 Mm³/yr; flow shaping 153–339 Mm³/yr. However, many months have pass-through volumes exceeding needs, indicating inefficiency without storage.
  • Temperature trade-offs: Increasing pass-through reduces reservoir storage and depletes the cold-water pool. In dry years, optimal temperature targets are met ~68% of months with 10% pass-through, dropping to ~53% with 40%. In wet years, ~73% with 10% vs ~68% with 40%. Imposing a minimum storage constraint provides only marginal temperature protection under pass-through because environmental deliveries are tied to inflow.
  • Other demands: Larger pass-through increases shortages. Senior demands still average >90% deliveries with 40% pass-through in most year types, but in critically dry years shortages increase. Junior exports are most affected (often <30% in dry years). Adding a 1.54 Bm³ minimum storage reduces senior deliveries by ~6–9% and reduces exports further (to ~27% in dry years at 40% pass-through).
• Environmental Water Budget (inflow + storage capacity):
  • With 10% inflow + 10% storage, baseflows are almost always delivered, but flow shaping is largely unmet; interquartile shortages: baseflows 28–375 Mm³/yr; flow shaping 763–888 Mm³/yr.
  • With 30% inflow + 30% storage, average deliveries are 99% of baseflows and 96% of flow shaping across wet and dry years.
  • Temperature performance improves with storage: without minimum storage, temperature objectives are met 64–73% of months in dry years and 71–76% in wet years across all allocation levels. Adding a 1.54 Bm³ minimum storage boosts attainment to ~77–80% of months across alternatives; summer median release temperatures are ~9.5–9.7 °C (IQR ~8.2–10.7 °C) regardless of 10–40% allocation when minimum storage is enforced.
  • Other demands under EWB: At ≥30% inflow+storage in dry years, junior exports face severe cutbacks; with 40% EWB, system and in-basin deliveries average near 80% and refuges >95% in dry years. Minimum storage constraints further reduce water available to other demands by shrinking effective capacity and carryover.
• 2019–2021 case study (with 1.54 Bm³ minimum storage): Compared to 30% pass-through, a 30% EWB stores water during summer low-demand periods, increasing cold-water volume and improving the likelihood of meeting late summer/fall temperature targets and winter/fall flow pulses. Baseflows are fully met; flow-shaping shortages occur as drought persists (e.g., 13% [121 Mm³] in 2020 and 37% [333 Mm³] in 2021). Non-environmental shortages in 2021 range from 12% (refuges) to 92% (exports), averaging 45%, higher than in the pass-through scenario.
• Trade-off curves (Fig. 7): For a given share of inflow, allocating storage to manage environmental water shifts the trade-off toward lower environmental shortfalls with similar or smaller other-demand shortfalls. Example: at 20%, pass-through leaves 48% of environmental flow objectives unmet; EWB reduces this to 21–26% (depending on minimum storage). The analysis identifies a breakpoint around 30% EWB where baseflows and flow shaping are largely satisfied; higher allocations yield diminishing environmental gains and large increases in shortages to other demands.

Allocating both inflow and storage capacity to environmental management is more efficient than mimicking natural flows via pass-through. Storage reduces internal trade-offs among environmental objectives (flows vs temperature) by enabling temporal reallocation, seasonal/interannual carryover, and cold-pool preservation. Minimum storage constraints further improve temperature outcomes but reduce water available to other demands and carryover for non-environmental uses. Pass-through alone can substantially erode cold-water pools, worsening temperature outcomes despite greater flow volumes; thus, pass-through should be avoided where temperature management is an objective. The study is a proof-of-concept that complements, rather than substitutes for, detailed system and temperature models; those tools can test EWB design, quantify benefits, and refine breakpoints in real systems. Governance and policy innovations—environmental water budgets treated operationally as senior priorities, designated trustees, and funding via regulatory allocations, voluntary agreements, purchase, storage investments, markets, and user fees—can reduce regulatory uncertainty and improve reliability for ecosystems. Underground storage and managed aquifer recharge can complement surface storage, capturing spills and augmenting cool baseflows. In the context of increasing hydroclimatic variability and warming, integrating flow and thermal objectives via environmental storage provides resilience for freshwater ecosystems.

The study demonstrates that storing and actively managing a dedicated share of inflow and reservoir capacity for environmental purposes is more efficient and effective than pass-through designed to mimic natural flows. An Environmental Water Budget of roughly 30% inflow plus 30% storage (in this simplified system) largely meets environmental baseflow and flow-shaping objectives, while preserving cold-water conditions more reliably, especially when paired with minimum storage constraints. Beyond ~30%, additional allocations yield diminishing environmental benefits and substantial shortages to other demands. Practical implementation will require integrating environmental assets into reservoir operations as priority objectives, supported by governance structures and funding mechanisms. Future work should apply detailed, basin-specific hydrologic and temperature models to refine EWB design, evaluate breakpoints under climate change, explore conjunctive surface–groundwater strategies, and assess multi-objective benefits and trade-offs across taxa and habitats.

• Simplified, proof-of-concept modeling framework; not intended to set regulatory standards or determine sufficiency of flows for species recovery.
• Monthly timestep water balance and one-dimensional temperature model (WQRRS) may not capture finer-scale operational, hydraulic, or thermal dynamics.
• Hydropower and recreation were excluded; some multipurpose re-use of water (e.g., temperature releases meeting other demands) was simplified.
• Single-reservoir representation (Shasta-like) and stylized demand time series; results may differ in other systems and with more detailed operations and infrastructure constraints.
• Results depend on assumed priorities, storage accounting rules (e.g., carryover spilling first), and temperature control structure idealization.
• Trade-off findings (e.g., ~30% breakpoint) are system- and assumption-specific and should be validated with more comprehensive models and data.