| Sacramento Valley Water Year Hydrologic Classification: | ||
|---|---|---|
| W | Wet year | Greater than or equal to 9.2 |
| AN | Above normal year | Greater than 7.8, and less than 9.2 |
| BN | Below normal year | Greater than 6.5, and less than 7.8 |
| D | Dry year | Greater than 5.4, and less than 6.5 |
| C | Critical year | Less than or equal to 5.4 |
The 'reconstructed' index is based on observed runoff. Official year-types are based on May 1 forecasts of future runoff.
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. Official year types are set by first of month forecasts beginning in February. Final determination is based on the 50% exceedence forecast of future runoff issued on May 1. The reconstructed index is based on observed runoff.
Current water year indices (defined in SWRCB Decision 1641 and used operationally since 1995) based on forecast runoff are posted on CDEC's water supply page and published in DWR Bulletin 120.