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Ray76’s data shows there was a 3 month period ending in August 2011 when the lake level stayed above 891’, within 10’ of topping out. This contrasts with 2017, when the lake was above 891’ for just 4 days, and has averaged closer to 850’.

Hence, there was more elevation head on the dam for a longer period in 2011 than so far in 2017. Yet the green area (apparently, as assessed by locals) is larger now than in 2011. Had this green area had stayed “green” through August 2011, correlating with the high lake levels on the opposite side of the dam, someone would have noticed and taken pictures, as you don’t get green grass in California in the summer without irrigation.

This indicates that either the internal piping through the dam has gotten much worse recently, passing a higher flow rate now under less head; or this green area is not a function of lake level, but more of a function of the extent and duration of precipitation.

Hi jpal, one variable that is being overlooked is the internal phreatic line or curvature in a long duration summer heat condition. The surface area of the zone 3 fill and its thermal retention properties form a significant evaporative mechanism. Thus the furthest "reach" of the phreatic line (curve) to the zone 3 surface may be fluctuated by this mechanism since the greenage is a balance of a hysteresis state (hotter crosses an accelerated threshold of loss in vegetative cut-off). A deeper percolation becomes the "unseen unknown" in such a comparative experiment. (precipitation to reservoir only).

Another variable that should be included is the phreatic saturation of the core and fill zones (time at a given level). (to which you properly brought up in a "period" notation).

Another "unseen unknown" variable is if a threshold has been crossed where there may be an increase to the defect or source that has changed from in the past.

The Division of Safety of Dams have been noting this area for years in their inspection reports - for monitoring & determining flow. Only in 2016 was a piezometer sensor (water sensor) test well drilled near this area by DWR. This should have been done long ago. There is nothing like direct evidence in getting to the root cause of something that shouldn't be occurring on an earthen dam. Any "unknown" such as this, is unacceptable as this sign are known signs of a potential failure mode.

To prove the certainty of safety requires removing the uncertainty. DWR knows there is an uncertainty as they have officially requested from FERC to place this test well at 670' in the hillside to try to figure out the cause. DWR wouldn't do this if they felt it wasn't an issue.

You're way ahead of me. :)

I was working on this chart before being interrupted by a Kentucky Derby get together.

I wanted to see when there was low precipitation and high lake levels, and when there was high precipitation and low lake levels. In other words, does the occurrence and size of the green area correlate with precipitation or with raised lake elevation.

Aside from a few spikes in precipitation, between April 2011 and November 2014 there was persistently high lake levels and relatively low precipitation. This is particularly true between January 2013 and January 2014 when there was a drought and high lake levels.

It would be very interesting to examine the status of the green area during this period.