Categories: Current Happenings

SFAR Water Year Forecast March 2018

Dry: March rafting flows of 1,300-1,500 cfs Friday through Tuesday. With only 200 cfs guaranteed on Wednesdays and Thursdays.

South Fork American River water year forecast as of March 8, 2018: Dry

The March Water Forecast Report for 2018 was released Thursday March 8th, with the Water-Year forecasted for Dry. Specifically it’s forecasting the Full Natural Flow (FNF) of the American River below Folsom at 1,350 million acre-feet (MAF), with Dry being considered 900 MAF to less than 1,700 MAF. This is a significant drop from the previous month’s forecast of 1,750. The biggest change will be no guaranteed Thursday water in the coming months, with no significant change to summer flows1.

The current forecast has an 80% probability range of 1,080 – 1,605 MAF, which would keep us firmly in a Dry water year. However, for comparison, the previous period had an 80% probability range of 1,350 – 2,280 MAF. The forecast should be more accurate as we move forward, but the change from 1,750 to 1,350 speaks to some significant differences between the previous models and what actually happened in February.

The February forecast had predicted 230 MAF of unimpaired runoff during February, however we only saw 98 MAF. That’s only 42.6% of what had been forecasted. For March they’ve only made a small adjustment down, predicting 299 MAF versus the previous expectation of 340 MAF for March unimpaired runoff.

Guaranteed Minimum Flows

The March forecast will be used to determine minimum guaranteed flows from March 11th through Mid-April2

March through Mid-April

  • Monday, Tuesday, Friday: 1,300 cfs 9am-12pm
  • Saturday & Sunday: 1,500 cfs 9am-12pm
  • No guaranteed raftable flows Wednesday or Thursday
  • Minimum Flow: 200 cfs

Forecasted based on Current Data

If the forecast of Dry holds, the flows will be as follows.

April-May

  • Monday, Tuesday, Friday: 1,300 cfs 9am-12pm
  • Saturday & Sunday: 1,500 cfs 9am-12pm
  • No guaranteed raftable flows Wednesday or Thursday
  • Minimum Flow: 200 cfs March , 250 cfs April & May

Summer Flows – May 28th through September 3rd

  • Monday, Tuesday, Thursday, Friday: 1,300 cfs 9am-12pm
  • Saturday & Sunday: 1,500 cfs (7am-12pm Sat, 8am-pm Sun)
  • No guaranteed raftable flows Wednesday
  • Minimum Flow: 250 cfs May & June. 200 cfs July through September

September 4-30, 2018

  • Friday, Saturday, Sunday: 1,300 cfs 9am-12pm
  • No guaranteed raftable flows Monday, Tuesday, Wednesday, Thursday
  • Minimum Flow: 200 cfs

October through early February

  • Saturday & Sunday: 1,500 cfs 9am-12pm
  • No guaranteed raftable flows Monday through Friday
  • Minimum Flow: 200 cfs

The next forecast should be released around April 10th, with it taking effect 3 days after issuance.



Extra Details for Geeks

600 MAF: Full Natural Flow of American River below Folsom Lake from October through February 20183. Through March 4th: 625 MAF4.

Reservoir Storage Amounts of Interest

Here are some reservoirs in the Upper American River Project (UARP) that might be of interest.

As of March 9th – 8 AM

  • Loon Lake – 6,384.94 ft (spills at 6,418 ft) with 38,510 AF (spills at ~76,500 AF)
  • Ice House – 5,425.10 ft (spills at 5,436.5 ft Nov-April 1, then April 2-15: 5,445 ft max , April 16-Oct: 5,447 ft max5)
  • Union Valley – 4,833.45 ft (spills at 4,855 ft Nov-April 1, then April 2-15: 4,865 ft max , April 16-Oct: 4,867 max6)
  • Slab Creek -1,845.48 ft (spills at 1,850 ft) with 12,182 AF (spills at ~13,350 AF)7
  • Chili Bar –  995.49 ft (spills at 997.5 ft) with a max capacity of only about 1,340 AF.

Related Links and References

About the Forecasts

  • Forecasts are issued February through May around the 2nd Tuesday or the 8th of each month. They take effect 3 days after being released.
  • The final forecast is issued in May, and is used to determine the flow schedule through October.
  • The final (actual) water year report is released in October and is used to determine flows for November until 3 days after the February forecast is released.
  • While some bias might slip in, as much as possible the above information is purely based on the forecasts and not based on gut intuition or predictions.

Note: This page is updated with current forecasts as they become available. Previous forecasts are archived for reference.

Show 7 footnotes

  1. Just a reduction from 6 hours on Saturday and Sunday to only 5 hours of flow
  2. The new forecast takes effect 3 days after it is issued. Each forecast is typically issued around the 8th
  3. Full Natural Flow for the American River below Folsom Lake
  4. Estimated via FNF CFS as reported by Daily Folsom Lake Data and multiplied by 1.98. (Full calculation to derive 1.98 conversion: 1 Day of FNF in AF = (Daily Average FNF CFS) * 60sec/min * 60min/hr * 24hours / 43,560cf/af)
  5. April-Oct these are the maxes, but they can choose to spill earlier.
  6. April-Oct are the maxes, but they can choose to spill earlier by opening the Ogee gate.
  7. Official data claims 1,870 ft for spill and 16,600 AF of storage, but historical data shows 1,850 ft and 13,350 AF to be more realistic – I’m looking into it.
Jansen Wendlandt

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