WSI.2006 (05/08/06 1700)
Department of Water Resources
California Cooperative Snow Surveys
2006 Water Year Hydrologic Classification Indices
2006 Water Year Forecast as of May 1, 2006
SACRAMENTO RIVER UNIMPAIRED RUNOFF (SACRAMENTO RIVER INDEX)
Probability of Exceedence
Forecast Date 99% 90% 75% 50% 25% 10%
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Dec 1, 2005 6.7 (35%) 8.9 (47%) 11.6 (61%) 15.5 (81%) 20.5(108%) 25.5(134%)
Jan 1, 2006 14.8 (78%) 16.8 (88%) 19.3(101%) 23.0(121%) 27.8(146%) 32.4(170%)
Feb 1, 2006 16.6 (87%) 18.3 (96%) 20.1(105%) 22.8(120%) 26.3(138%) 30.5(160%)
Mar 1, 2006 18.9 (99%) 20.0(105%) 21.4(112%) 23.1(121%) 25.4(133%) 28.2(148%)
Apr 1, 2006 25.2(132%) 26.0(136%) 26.7(140%) 27.5(144%) 28.7(151%) 30.3(159%)
May 1, 2006 29.9(157%) 30.6(161%) 31.2(164%) 31.8(167%) 32.5(171%) 33.2(174%)
Sacramento River Runoff is the sum of unimpaired flow in million acre-feet at:
Sacramento River above Bend Bridge
Feather River at Oroville (aka inflow to Lake Oroville)
Yuba River near Smartville
American River below Folsom Lake
Also known as the "Sacramento River Index", this index was previously used
to determine year type classifications under SWRCB Decision 1485.
Also previously referred to as the "4 River Index" or "4 Basin Index".
Water Year Runoff through end of last month:
2006 (current year) = 24.6 MAF 176% of average
2005 (last year) = 10.6 MAF 76% of average
Previous Water Year Total Runoff:
2005 =18.4 MAF 97% of average
1977 (Min) = 5.1 MAF 27% of average
1983 (Max) =37.7 MAF 198% of average
1951-2000 average =19.1 MAF
SACRAMENTO VALLEY WATER YEAR TYPE INDEX (40-30-30)
Probability of Exceedence
Forecast Date 99% 90% 75% 50% 25% 10%
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Dec 1, 2005 4.6 5.3 6.2 7.5 9.2 10.8
Jan 1, 2006 7.2 7.9 8.7 9.9 11.5 13.0
Feb 1, 2006 7.7 8.2 8.8 9.8 11.0 12.4
Mar 1, 2006 8.4 8.8 9.3 9.8 10.6 11.6
Apr 1, 2006 10.5 10.8 11.1 11.4 11.8 12.4
May 1, 2006 12.4 12.7 12.8 13.0 13.3 13.5
Index = 0.4 * Current Apr-Jul Runoff (1)
+ 0.3 * Current Oct-Mar Runoff (1)
+ 0.3 * Previous Year's Index (2)
Notes:
(1) Runoff is the sum of unimpaired flow in million acre-feet at:
Sacramento River above Bend Bridge
Feather River at Oroville (aka inflow to Lake Oroville)
Yuba River near Smartville
American River below Folsom Lake
(2) Maximum 10.0 for previous year index term
Previous Water Year Indices:
2005 = 8.5 MAF 99% of average
1977 (Min) = 3.1 MAF 36% of average
1983 (Max) = 15.3 MAF 179% of average
1951-2000 average = 8.5 MAF
Year Type Classification: Index based on flow in million acre-feet:
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
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.
SAN JOAQUIN VALLEY WATER YEAR TYPE INDEX (60-20-20)
Probability of Exceedence
Forecast Date 99% 90% 75% 50% 25% 10%
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Dec 1, 2005 1.5 1.9 2.3 3.0 3.7 4.8
Jan 1, 2006 2.2 2.6 3.0 3.6 4.3 5.2
Feb 1, 2006 2.7 3.3 3.6 4.0 4.6 5.3
Mar 1, 2006 2.8 3.2 3.5 3.8 4.3 4.8
Apr 1, 2006 3.9 4.2 4.4 4.5 4.8 5.1
May 1, 2006 5.1 5.4 5.5 5.6 5.7 5.9
Water Year Index based on flow in million acre feet
Index = 0.6 * Current Apr-Jul Runoff (1)
+ 0.2 * Current Oct-Mar Runoff (1)
+ 0.2 * Previous Year's Index (2)
Notes:
(1) Runoff is the sum of unimpaired flow in million acre-feet at:
Stanislaus River below Goodwin Reservoir (aka inflow to New Melones Res.)
Tuolumne River below La Grange (aka inflow to New Don Pedro Reservoir)
Merced River below Merced Falls (aka inflow to Lake McClure)
San Joaquin River inflow to Millerton Lake
(2) Maximum 4.5 for previous year index term
Previous Water Year Indices:
2005 = 4.8 MAF 143% of average
1977 (Min) = 0.8 MAF 25% of average
1983 (Max) = 7.2 MAF 217% of average
1951-2000 average = 3.3 MAF
Year Type Classification: Index based on flow in million acre-feet:
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
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.
SACRAMENTO VALLEY & SAN JOAQUIN 8 RIVER INDEX
Previous month: April 8520 TAF
This index is the sum of the previous month's unimpaired runoff for the 8 rivers
that are included in the above SACRAMENTO RIVER UNIMPAIRED RUNOFF
and the SAN JOAQUIN VALLEY WATER YEAR TYPE INDEX
A listing of reconstructed indices based on historical observed runoff is posted
at http://cdec.water.ca.gov/water_supply.html
The official year types are based on May 1 forecasts, not the observed runoff.
For more information please contact:
Stephen Nemeth at (916) 574-2634 nemeth@water.ca.gov
David Rizzardo at (916) 574-2617 daver@water.ca.gov
John King at (916) 574-2637 kingjj@water.ca.gov
