Apr 14, 20174 min read

Archive: SFAR Water Year Forecast April 2017

tl;dr: Very Very Wet. Total rainfall might exceed 1982-1983, but current snowpack is much lower at this point in time.

South Fork American River water year forecast as of April 10, 2017: Wet

The April Water Report was released Monday April 10th, with the Water-Year forecast expectedly solidly in the Wet range. Specifically it’s forecasting the American River below Folsom at 7.435 million acre-feet (MAF), with 3.500 or greater being considered Wet[1. An increased forecast of 0.005 MAF more than forecasted in March 2017]. In fact, we’ve already had 5.028 MAF of unimpaired runoff, so even if the heaven’s remained dry through the end of this water year, we’re already locked in on it being a Wet classification.

Rainfall vs Runoff vs Snowpack

There has been a lot of talk in the media as of late, of different regions setting all time rainfall records. There likely will be more of those reports to come as more and more stations beat previous records with more rain likely to fall prior to the end of this water year on September 30th. While we will likely see significant spring and early summer flows, to draw comparisons to the 1982-1983 water year are, in my opinion, inflammatory and more hype than fact. I’m not saying we won’t see higher flows than we did in those years, I’m saying that to say “we will” is an oversimplification and not a certainty.

Some important differences between the 1982-1983 water year and the 2016-2017 water year:

While this year will likely go down as the highest water year on record, much of that water has already moved into the ocean. Specifically, the unimpaired runoff for the American River below Folsom from Oct-March is 5.028 MAF this year versus only 3.335 MAF for the 1982-83 water year.[1. April 1983 Bulletin 120 page 9 vs April 2017 B120 page 5]

This can be most clearly seen by looking at the current California snow water content:

The South Fork of the American River is part of the Central California breakdown. As you’ll notice, we currently have significantly less water on the mountains above us than we did in ’82-’83. But we do have a bit more than we did in 1997-1998.

Comparing 7 snow station readings in the April report for the American River in 1983 had 451.2 inches of water equivalent versus 2017 having 313.7 equivalent inches of water. That’s 30% less water on the mountain for April 1st than we had in ’83[1. April 1983 Bulletin 120 page 6 and April 2017 B120 page 14 – Note: 1983 was technically measured on April 4th (I don’t know why), while 2017 was measured on March 31st (due to April 1 being a Saturday).].

For comparison, the unimpaired runoff for the American River below Folsom in 1983 in 1,000 Acre-Feet was: 605 April, 983 May, 942 June[1. B120 October 1983 1. Table 6]. Forecast for 2017 are predicting 725 April, 910 May, 560 June[1. B120 April 2017 page 5]. While 1998 had 582 April, 699 May, 787 June[1. American River at Folsom Full Natural Flow]. So if those predictions hold, we should see similar flows in May as we did in 1983, however June will likely drop to smaller flows than we saw in June of 1998.

Guaranteed Minimum Flows

While the April forecast is technically only valid until around May 12th, it’s a reasonable assumption that the following will not change barring an act of congress[1. I’m not intending to be flippant. It really would take a literal act of Federal Congress to change things.].

It’s worth a reminder that how quickly or slowly the snowpack melts could result in significantly higher flows during the remainder of Spring.

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April-May

Monday – Friday: 1,500 cfs 9am-12pm
Saturday & Sunday: 1,750 cfs 7am-1pm
Minimum Flow: 300 cfs April, 500 cfs May

Summer Flows – May 27th through September 4th

Monday – Friday: 1,500 cfs 8am-12pm
Saturday & Sunday: 1,750 cfs 7am-1pm
Minimum Flow: 500 cfs May & June, 350 cfs July, 300 cfs August, 250 cfs September

September 5-30, 2017

No guaranteed raftable flows Monday, Tuesday, Wednesday
Thursday, Friday, Saturday, Sunday: 1,500 cfs 9am-12pm
Minimum Flow: 250 cfs

October Flows

No guaranteed raftable flows Tuesday, Wednesday, Thursday
Mondays & Fridays: 1,300 cfs 9am-12pm
Saturdays & Sundays: 1,500 cfs 9am-12pm
Minimum Flow: 250 cfs

November through early February

No guaranteed raftable flows Monday-Friday
Saturdays & Sundays: 1,500 cfs 9am-12pm
Minimum Flow: 250 cfs

The next water year forecast should be released around May 9th, with it taking effect 3 days after issuance.

Reservoir Storage – Of Interest

With a high snow pack and reservoirs on the Upper American River Project (UARP) filling up, here are some[1. There are many more reservoirs in the system, these are just the most impactful.] quick water storage related numbers[1. For reservoir storage, current elevation is a hard number, while current storage is a rough calculation based on estimated capacity at that elevation. Natural sediment deposits over time, and from high runoff/melt conditions, can modify (sometimes greatly) actual capacities.] that might be of interest.

As of April 14, 2017 – 11 AM

Loon Lake – 6402 ft (spills at 6,418 ft) with 58,257 AF (spills at ~76,500 AF)
Ice House – 5,436 ft (spills at 5,436.5 ft Nov-April 1, then April 2-15: 5,445 ft max , April 16-Oct: 5,447 ft max[1. April-Oct these are the maxes, but they can choose to spill earlier.])
Union Valley – 4861 ft (spills at 4,855 ft Nov-April 1, then April 2-15: 4,865 ft max , April 16-Oct: 4,867 max[1. April-Oct are the maxes, but they can choose to spill earlier by opening the Ogee gate.])
Slab Creek – 1,852 ft (spills at 1,850 ft) with 13,523 AF (spills at ~13,350 AF)[1. 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.]
Chili Bar – 1,000 ft (spills at 997.5 ft) with a max capacity of only about 1,340 AF.