The COEQWAL scenario library How the scenarios are built: strategies, hydroclimates, and the model runs behind them
The scenario library pairs 28 water management strategies with five hydroclimates. Each pairing is one scenario, and one scenario is one CalSim 3 simulation over a 100-year period. Over 100 scenarios make up the library, and every outcome level COEQWAL reports traces to one of these runs.
This brief describes how the scenario library is constructed. It is a reference brief, part of the project's background material, and it carries no modeled results. For what the runs produce, see the summary briefs on the Data page.
§2 What a scenario is
Every scenario pairs one water management strategy with one hydroclimate. The strategy describes the decisions, priorities, and policies that determine how water is allocated: the part of the system that can be controlled. The hydroclimate describes the temperature and the patterns of rain and snow that determine how much water is available, the part that cannot.
The pairing is the unit of analysis. A result belongs to the pair, never to a strategy alone: the same strategy can produce different results under different hydroclimates, and comparing a strategy across hydroclimates is one of the central readings the library is structured to support.
§3 The 28 strategies
The strategies span the main levers of Central Valley water management. The website organizes them in five groupings, and the matrix in §5 lists all 28 under the same five headings.
Baselines. Six strategies that hold water management at or close to today's rules: Current operations, its reintroduction variant, and four variants that change the agency rule set, the land-use dataset behind agricultural demand, or the drought provisions. Current operations is the strategy every other strategy is compared against. Current operations with reintroduction is listed under this heading because the library groups it there; it is not a baseline, because reintroduction has not occurred.
Farms and groundwater. Four strategies that limit groundwater pumping, two in the San Joaquin Valley and two across the Central Valley. In each pair, one strategy limits pumping directly and one reaches the limit by reducing crop acreage.
Rivers and salmon. Nine strategies that change instream flow requirements: No flow requirements, Functional environmental flows, Winter-run refuge flows, a reduced-crop-acreage version of each of the last two, and the four reintroduction variants that pair with them.
Community water systems. Two strategies that change where community water systems sit in the allocation order: Prioritizing human health water deliveries to community water systems, and Prioritizing full demands of community water systems.
The Delta. Seven strategies that change conditions in and around the Delta: four Delta outflow targets, running from Reduce Delta outflows (35% unimpaired flow target) to Increase Delta outflows (65% unimpaired flow target), together with Increase Shasta carry-over storage, Relax Delta salinity standards, and the Delta Conveyance Project.
A strategy's public identity is its descriptive name, for example Functional environmental flows or Groundwater pumping limits in the San Joaquin Valley. Each strategy is described in its own documentation brief, linked from its row in §5.
§4 The five hydroclimates
Each strategy is simulated once under each of five hydroclimates: a historical hydroclimate, and four projected climates named by their level of stress on the water system. The four projections are a sample of possible futures drawn from state planning studies and downscaled global climate models. They are not predictions, and they are not stages of a single timeline. Each is a complete simulation period in its own right.
| Hydroclimate | Conditions |
|---|---|
| Historical hydroclimate | Temperature, precipitation, and flow patterns reflect historical conditions |
| Moderate climate stress | Slightly warmer and moderately wetter, +3.5% flow change |
| Moderate-high climate stress | Moderately warmer, with little change in precipitation, −1% flow change |
| High climate stress | Much warmer and moderately drier, −6.5% flow change |
| Extreme climate stress | Much warmer and much drier, −19.2% flow change |
Two things the table does not show. The historical hydroclimate is not the observed record: it is the 100-year climate and hydrologic record built for CalSim 3, adjusted for recent climate change so that overall time trends are removed while the observed variability of wet years, dry years, and multi-year patterns is preserved. And each of the four flow changes is an average: the average change in total annual flow across California's major water supply basins, over a 30-year period centered on 2043.
The five hydroclimates, their source data, and the modeling behind them are documented in full in the companion reference brief, The five COEQWAL hydroclimates.
§5 The scenario matrix
Every scenario in the library appears below. Rows are the 28 strategies, grouped under the five headings of §3 and listed in the order the website uses; each strategy name links to its documentation brief. Columns are the five hydroclimates, in order of increasing climate stress. Each cell holds the identifier of the CalSim 3 run for that pairing, the same identifier that appears in the references note of each brief and in the machine-readable records behind the website.
† Each marked strategy adds one action that is not a water management action: reintroduction of winter-run Chinook salmon to cold-water habitat upstream of Shasta Dam. The marked strategy and the strategy directly above it share the same CalSim 3 simulation, which is why the two rows carry identical run identifiers. Reintroduction has not occurred, so it is not part of any baseline: the comparison reference for every scenario in the library is current operations under the same hydroclimate, without reintroduction. Reintroduction enters the modeling through the winter-run life cycle model, not through CalSim 3, so modeled results differ from the partner strategy only for the winter-run salmon key outcome.
§6 Current operations and the baseline
One strategy, Current operations, represents the current operating rules for the major Central Valley water systems: the operational logic of the California Department of Water Resources' 2023 Delivery Capability Report, with agricultural demands set by the 2020 LandIQ land-use dataset and Temporary Urgency Change Petition (TUCP) actions active. It is simulated under all five hydroclimates, like every other strategy, and it is the reference every other strategy is compared against.
Comparisons are always matched by hydroclimate: an alternative strategy under a given hydroclimate is compared with current operations under that same hydroclimate. Current operations under the historical hydroclimate is the baseline, the scenario that represents the lived status quo. Current operations under a projected hydroclimate is a climate-projected baseline: an independent model run showing where current management would land if the climate changed but the rules did not. It is a full simulation, not an adjustment applied to the historical run.
Current operations with reintroduction is listed with the baselines in §5 because the library groups it there, and it is not a baseline: reintroduction has not occurred, so the comparison reference for every scenario is current operations under the same hydroclimate, without reintroduction.
§7 What this doesn't say
The library is a sample, not a census. The 28 strategies do not cover every possible way of managing Central Valley water, and the five hydroclimates do not cover every possible climate; the absence of a combination from the library is not evidence about that combination. No scenario is a prediction of what will happen, and the library does not rank the strategies or single one of them out. Strategies as modeled are idealized representations in CalSim 3, not policy proposals or implementation plans.
§8 Where to go next
For what the project is and why the library exists, see the companion reference brief, The COEQWAL project. For any single strategy, see its documentation brief, linked from that strategy's row in the matrix in §5. For modeled results, see the summary briefs on the Data page.
References — model runs
Every scenario is one CalSim 3 run. The run identifier for each strategy under each of the five hydroclimates is listed in the matrix in §5, in the references note of that strategy's documentation brief, and in the machine-readable records that accompany the summary briefs on the Data page, so any result can be traced to a specific model run.