# Definitions for the Water Supply and Allocation Toolkit

Version 1.2, 2026-09-22. This text travels with the data. The version line changes whenever a rule on this page changes, and every page in the toolkit shows the version it was read against.

## The water year

A water year runs from October 1 through September 30 and takes the number of the calendar year in which it ends. Water year 2023 is October 2022 through September 2023. The kind of year (wet, above normal, below normal, dry, critical) is the Sacramento Valley classification from the Department of Water Resources, read from CDEC and served by this site's water-year-type API. It is never re-derived here.

## Dayflow terms

Dayflow is the Department of Water Resources' daily accounting of water entering and leaving the Sacramento-San Joaquin Delta. Its terms are used here exactly as Dayflow defines them.

- Inflow: all river flow entering the Delta on a day: the Sacramento River, the Yolo Bypass, the San Joaquin River, and the eastside streams (Cosumnes, Mokelumne, and miscellaneous). Dayflow column TOT.
- Exports: water pumped out of the south Delta by the State Water Project at the Banks Pumping Plant (SWP) and by the Central Valley Project at the Jones Pumping Plant (CVP), plus the Contra Costa Canal and the North Bay Aqueduct. Dayflow column EXPORTS, published from water year 1997. Before that the toolkit adds the CVP and SWP columns, which run from water year 1956, and says so in the `exports_basis` field. Exports serve cities and farms on both projects; the allocation pages split them by contractor class.
- Net Delta outflow: the water that leaves the Delta for Suisun Bay after exports and in-Delta use, as Dayflow computes it (the Net Delta Outflow Index, column OUT).
- In-Delta use: water consumed inside the Delta by crops, channels, and evaporation, less the rain that falls on the Delta. Dayflow column CD (net channel depletion), which is gross channel depletion (GCD) less precipitation (PREC). It is read from Dayflow for every day of the record and is never estimated. In a wet winter month it can be negative, when rain on the Delta exceeds what its crops and channels take; the charts draw such a month at zero and the tables carry the signed value. Dayflow's small miscellaneous diversions (column MISDV) are added to it on the charts. With exports and outflow it closes the Dayflow balance: outflow equals inflow less exports, in-Delta use, and miscellaneous diversions.
- Percent of inflow exported: exports divided by inflow for the period, in percent. It is always shown beside the kind of water year and the floor, never alone.

## The floor

The floor is the minimum net Delta outflow that the State Water Resources Control Board's Water Right Decision 1641 (Table 3) requires for the calendar month and the kind of water year. It exists to keep salt water out of the Delta and its pumps and to hold habitat conditions in the estuary. Outflow above the floor is the only water that could have been diverted without breaking that rule, so it is the measure of surplus used here. D-1641 took effect on March 15, 2000; the toolkit applies its table to earlier years as a reference and counts the days it actually governed in `days_d1641_in_effect`.

## Change log

- 1.2 (2026-09-22): precipitation index and snowpack figure entries added for the This water year page. The average for the precipitation index is the 1991 to 2020 mean of DWR's monthly record, interpolated within the month, the same base period as every published percent; the water-year average stated on the Tulare Basin season summary is not used, because it does not equal the sum of its own monthly averages.
- 1.1 (2026-09-18): in-Delta use is now Dayflow CD read from the record for every water year; the residual wording of 1.0 is withdrawn.
- 1.0 (2026-09-18): first release.

## Binding reason

For each Dayflow day the toolkit records why exports sat below pump capacity. The vocabulary comes from the Delta Export Constraints dataset on this site and is applied in this order:

1. No export value: Dayflow carries no export figure for the day.
2. No surplus outflow: net outflow was at or below the floor, so there was nothing to take without breaking the rule.
3. Pumps at physical capacity: exports were within 250 cfs of the installed design capacity of Banks and Jones together (14,900 cfs when both plants are online).
4. Rule or permit held exports: surplus existed and the pumps had room, and a standing rule, permit term, biological opinion, court order, or real-time action in the constraint timeline limited exports that day.
5. Headroom, no constraint on record: surplus existed, the pumps had room, and nothing in the timeline was active. Most such days fall before 1978.

The classification is a screening aid. It says what the record supports, not what an operator decided on the day. DWR's own daily "controlling factor" for the day is shown beside it on the signed-in daily page.

## Allocations and deliveries

An allocation is the percentage of contract supply that Reclamation (Central Valley Project) or the Department of Water Resources (State Water Project) tells its contractors to expect for the year. The toolkit holds the final announcement for each year and contractor class. An allocation is not a delivery; delivered volumes are a separate record that is not yet on this site, and every page that shows allocations beside exports says so.

## Observed and modeled

Dayflow, CDEC, and the allocation announcements are observed records. CalSim3 and COEQWAL series are model output for scenarios. The toolkit never places a modeled series on the same axis as an observed one; where a scenario appears, its study and scenario are named on the panel.

## Precipitation index

The Department of Water Resources tracks precipitation over the watersheds that fill the project reservoirs with three station indices, published daily on the California Data Exchange Center: the Northern Sierra 8-station index (Mount Shasta City, Shasta Dam, Mineral, Quincy, Brush Creek, Sierraville Ranger Station, Blue Canyon, Pacific House), the San Joaquin 5-station index (Calaveras Big Trees, Hetch Hetchy, Yosemite Headquarters, North Fork Ranger Station, Huntington Lake) and the Tulare Basin 6-station index (Balch Powerhouse, Ash Mountain, Giant Forest, Springville Tule Powerplant, Isabella Dam, Pascoes). Each index is the average of its stations' daily precipitation in inches, accumulated from October 1. The Northern Sierra index is the figure DWR quotes for the Sacramento Valley and the one most often reported. The average for a date is the 1991 to 2020 mean of the monthly record, interpolated within the month; percent of average for the date is the total to date divided by that average. The range of the record is computed across every complete water year of the monthly record (1920 onward for the Northern Sierra, 1913 for the San Joaquin, 1922 for the Tulare Basin), and water years before the daily record (1998 for the Northern Sierra, 2016 for the other two) are drawn from monthly totals.

## Snowpack figure

The snowpack figure is the statewide snow water content as a percent of the April 1 average, from DWR's daily summary of its automated snow sensors, the same figure DWR publishes on its snow survey page. April 1 is the date the snowpack usually peaks, so the figure says how the pack compares with a normal peak, not with a normal value for the date. It is shown from November 1 to June 15, the season in which it carries meaning; outside that window the pages say the snow season runs from November to June and quote no number.

## Public and signed-in pages

Public pages show monthly and water-year aggregates from water year 1996 onward and need no sign-in. The day-by-day record with the binding reason, DWR's controlling factor, and a spreadsheet download is available to signed-in readers.