Sunday, January 15, 2012

Definition of "full pool condition"

Our battle to save our lakes has been fraught with learning new technical terms and complex hydrological explanations (this is why we hired a hydrologist as a consultant to aid our cause). The following exchange between Terry Brant and Peter Schreuder clarifies the technical term "full pool condition" (originally published October 5, 2011).

Peter,  I would greatly appreciate  your  advising me if I’m defining a hydrologic term correctly. I have reviewed several hydrologic regimes and defined circumstances and “full pool condition” is always defined as a high water event.  Hence, as you will see, I believe the district assertion below is delusory on its face because it fails to recognize that many other water levels, from MFL to MA for example,  could contribute to recognition and consideration of both recreational and scenic attributes, not just the maintenance of the highest possible water level.  I’m trying to push back on the plan to lower the Geneva MFL by 15.2 ‘.

The SJRWMD MFL Lake Geneva Draft addressing aesthetic and scenic attributes (Rule 62.40.473(1) (f), F.A.C.), continues:  “Lake Geneva naturally fluctuates over a very large range. The water levels are not considered seasonal like those of a riverine system. Thus, the lake can be extremely full during some years/decades (e.g., 1940s, 50s, 60s, and early 70s) and extremely low during other years/decades (e.g., 1990s and 2000s). The lake cannot easily (emphasis added) be maintained in a full pool condition to maximize aesthetic and scenic attributes because there is not sufficient water in the watershed during most years to allow such lakes levels to be maintained. Further, minimum levels allow for a decrease in the number of high water events and an increase in the number of low water events per century, on average. The recovery time for aesthetic and scenic attributes is considered short, probably less than a year. Therefore, the aesthetic and scenic attributes WRV and associated criteria are not considered to be limiting at Lake Geneva.”

I would like to counter the point by saying the following:
To conclude variously that “fish passage” provisions and general statements, including the cobbling together of decades of varying lake levels from the 1940’s to the present date, somehow equals compliance with all other separate nonconsumptive criteria, is unsupported, speculative and raises several legitimate questions. 

Saying that the lake cannot “easily (emphasis added) be maintained in a full pool condition to maximize aesthetic and scenic attributes because there is not sufficient water in the watershed,” is delusory inasmuch as this condition refers to a typical elevation that would occur only during high flow events, not  any others, such as average, minimum or even the current set MFL level (98.5’) on Lake Geneva, which could protect both scenic attributes and Recreation as well as “fish Passage” and other consumptive uses.  

Based on your understanding of the definition “full pool condition,”  do you agree with my use of the term as a high flow event or a high water event – a level higher than a MFL or a fish passage of 0.7”  at the current Geneva MFL of 98.5’?  If you see any other argument  - especially in the sweeping decades-long generalizations regarding full Vs. low levels, please comment.

Thanks, Terry


John and I are not entirely sure what the exact SJRWMD definition of “full pool condition” is.  Our professional opinion is that “full pool condition” refers to the elevation in any surface water retention area including lakes, when the surface water flows out of this retention area as overland flow to a downstream water body. In the case of Lake Geneva, we believe that “full pool condition” should be tied to the elevation of the outflow invert of Lake Geneva. Looking at the USGS Keystone Heights 7.5 minute quadrangle map, this invert is approximately 105 to 108 ft NGVD and it located on the south-east section of Lake Geneva.

I am personally challenged by their statement that “minimum levels allow for a decrease in the number of high water events” . It seems that if one assumes that the wide fluctuations in rainfall are the primary driving force on the surface water levels of the lake, with ground water seepage losses an so far quantitatively ill defined volume, the maintenance of a minimum level in the lake would assure that the recovery to “full pool condition” would be easier given a similar rainfall event. I would therefore have to disagree. I fail to see the logic of the statement:” and an increase in the number of low water events per century, on average”.

The District needs to better define their definition of “full pool condition”. It most likely occurs somewhere less that the “overflow/outflow invert elevation”. I believe that the fish passage criterion is a rather reasonably defensible environmentally responsible definition. Presuming this to be the case than the minimum level could be and should be established by measuring the highest bottom elevation inside the wetted perimeter of Lake Geneva and adding 0.7’ to that elevation.

Both John and I have been wondering how the District intends to justify statistically based minimum water levels in Lake Geneva, using the potentiometric surface elevations measured in the Floridan Aquifer as a point of departure. This is the more puzzling because neither John or I have been able to extract any reliably measured data on the range in values of the vertical leakance from the Lakes downward into the underlying Floridan Aquifer. This should be a prerequisite in case one uses the long-term elevation values measured in the Floridan Aquifer to establish a minimum surface water level in the Lakes.


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