Historical Reconstructed and Official HCIs are obtained from WSIHIST (ca.gov). Current year official HCI forecasts are posted on May 1st at WSI (ca.gov).
SacPAS Calculated HCIs are created using the provided equations for Reconstructed Year Type and full natural flows from FNFSUM (ca.gov), and are calculated in early August. The SacPAS Calculated HCIs are available for the current WY until CDWR updates the Historical Reconstructed values published on CDEC.
Direct quote from https://cdec.water.ca.gov/reportapp/javareports?name=WSIHIST. (Accessed 2024-07-15)
Unimpaired runoff represents the natural water production of a river basin, unaltered by upstream diversions, storage, export of water to or import of water from other basins. Sacramento River Runoff is the sum (in maf) of Sacramento River at Bend Bridge, Feather River inflow to Lake Oroville, Yuba River at Smartville, and American River inflow to Folsom Lake. The WY sum is also known as the Sacramento River Index, and was previously referred to as the "4 River Index" or "4 Basin Index". It was previously used to determine year type classifications under State Water Resources Control Board (SWRCB) Decision 1485. Sacramento Valley Water Year Index = 0.4 * Current Apr-Jul Runoff Forecast (in maf) + 0.3 * Current Oct-Mar Runoff in (maf) + 0.3 * Previous Water Year's Index (if the Previous Water Year's Index exceeds 10.0, then 10.0 is used). This index, originally specified in the 1995 SWRCB Water Quality Control Plan, is used to determine the Sacramento Valley water year type as implemented in SWRCB D-1641. Year types are set by first of month forecasts beginning in February. Final determination is based on the May 1 50% exceedence forecast. Sacramento Valley Water Year Hydrologic Classification: Year Type: Water Year Index: Wet Equal to or greater than 9.2 Above Normal Greater than 7.8, and less than 9.2 Below Normal Greater than 6.5, and equal to or less than 7.8 Dry Greater than 5.4, and equal to or less than 6.5 Critical Equal to or less than 5.4 San Joaquin River Runoff is the sum of Stanislaus River inflow to New Melones Lake, Tuolumne River inflow to New Don Pedro Reservoir, Merced River inflow to Lake McClure, and San Joaquin River inflow to Millerton Lake (in maf). San Joaquin Valley Water Year Index = 0.6 * Current Apr-Jul Runoff Forecast (in maf) + 0.2 * Current Oct-Mar Runoff in (maf) + 0.2 * Previous Water Year's Index (if the Previous Water Year's Index exceeds 4.5, then 4.5 is used). This index, originally specified in the 1995 SWRCB Water Quality Control Plan, is used to determine the San Joaquin Valley water year type as implemented in SWRCB D-1641. Year types are set by first of month forecasts beginning in February. Final determination for San Joaquin River flow objectives is based on the May 1 75% exceedence forecast. San Joaquin Valley Water Year Hydrologic Classification: Year Type: Water Year Index: Wet Equal to or greater than 3.8 Above Normal Greater than 3.1, and less than 3.8 Below Normal Greater than 2.5, and equal to or less than 3.1 Dry Greater than 2.1, and equal to or less than 2.5 Critical Equal to or less than 2.1 Eight River Index = Sacramento River Runoff + San Joaquin River Runoff This Index is used from December through May to set flow objectives as implemented in SWRCB Decision 1641. The 'reconstructed' table is based on observed runoff, and does NOT show the official year-types, which are based on May 1 forecasts of future runoff. The current water year indices based on forecast runoff are posted at http://cdec.water.ca.gov/water_supply.html and published in DWR Bulletin 120 (also available at http://cdec.water.ca.gov/snow/bulletin120) These indices have been used operationally since 1995, and are defined in SWRCB Decision 1641 (https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/decision_1641/) This report is updated each fall once the data is available. For more information, contact CDWR Flood Management, Hydrology Branch
This index, originally specified in the 1995 SWRCB Water Quality Control Plan, is used to determine the Sacramento Valley water year type as implemented in SWRCB D-1641. Year types are set by first of month forecasts beginning in February. Final determination is based on the May 1 50% exceedence forecast.