Jim Winterle, P.G.

EAA Modeler Supervisor

August 27, 2013

Considerations VISPO triggered for 2014 if J-17 water level is below

635.0 feet above mean sea level on October 1, 2013

Given that J-17 is now below 635 ft in late August, the water level changes during September will determine whether VISPO is triggered

The following analysis of J-17 historical water level responses for the month of September is used to estimate likelihood of remaining below 635 feet on October 1, 2013

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Historical September Response in J-17

September is historically the month with the highest gains in J-17 water levels: average gain = 4.34 ft since 1947

Generally attributable to reduced drawdown following end of irrigation season for summer crops

Average water level at start of September is 656.4 ft3

Historical September Response in J-17

Approximately 1 in 5 are losing years

Often can go 10 years or more without decrease in J-17 water level during September

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Statistical Distribution

Average gain is 4.34 ft

Median gain is 4.6 ft

Normal distribution fits data reasonably well

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Probability of VISPO Trigger for 2014

Need a water level rise in J-17 of ~2 feet or more to avoid VISPO trigger on October 1

Probability plot indicates ~67% probability of gaining at least 2 ft in September, assuming random-normal process

Need to check for other non-random processes that might affect probability?

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Does wet or dry prior month affect probability?

Comparison of August water level gain/loss to September shows no significant correlation

Therefore, wet or dry August, does not affect likelihood of a wet or dry September

r = −0.078

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Effect of Water Levels at Start of the Month

With exception of 1954 and 1956, every losing year began the month with J-17 water levels above the monthly average of 656.4 ft

Higher than average water levels mean higher losses to spring flows

1956

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1954

Does J-17 water level at start of month affect probability?

September gain/loss shows moderate negative correlation to J-17 water levels at the start of month

Therefore, below average water levels at start of month increases probability of gain

Only during lowest recharge years of the drought of record did J-17 fail to gain at least 3 ft when water levels were below average at start of September

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1954, 1955, & 1956

Current Weather Forecast

Current rainfall is not producing much runoff in the contributing zone, but probably reducing demand

No tropical cyclones heading our way at present No indication that the current 7-day forecast will

significantly help or hurt September rebound

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Conclusions Current J-17 water levels and weather forecast suggest

we will need a water level rise of at least 2 feet in J-17 during September to avoid VISPO trigger

Historical data suggests less than 1 in 3 chance of VISPO being triggered on September 1

Probability is likely lower due to fact that current J-17 water level is below historical average

Only during the peak of the drought of record did J-17 fail to rebound at least 3 feet when the water level was below the average of 656.4 at the start of September

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