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.