Rain, Rain, Where are you?
By Clell Ford
The level of
Lake Istokpoga, at the time this was written, was at least 8 inches below
where it should be at this time of year. Why, you ask. Well, the answer is
simply too little rain and too much evaporation. According to Clell
Ford, we lose between 1/4" to 1/5" per day. Read on for the rest of
To determine the lake level You essentially need to plug evaporation and
rainfall values into a hydrologic model for Istokpoga that accounts for all
losses of water from the lake and all gains of water to the lake. Sounds
In the case of Istokpoga, evaporation is likely the main loss of water from
the lake (except for the insignificant amounts lost around the edges of the
structures, especially given that SFWMD "is not releasing water from
Istokpoga"). Evaporation in Highlands County is estimated to
average 50 inches per year, or ~6 inches per month or more this time of year-
perhaps a more conservative (on the high side) estimate would put it between
1/4 and 1/5 inch per day. For Istokpoga, that represents ~500 acre-feet of
water per day, 2,178,000 cubic feet per day or 252 cubic feet per second (cfs).
Just to evaporation. There is also a small flow into the lake: USGS reported
23 cfs down Josephine Creek today, and there may be 5 to 10 times more
than that flowing down Arbuckle creek today. Make it 10X and the flow into
Istokpoga (Josephine + Arbuckle Creeks ~253 cfs) might be keeping up with the
evaporative losses. This time of year the shallow groundwater seepage into the
lake is somewhat small; since I do not have a good number to estimate it, I
will ignore it for this back of the envelope
Thus ends the easy part.
The hard part is how much rainfall it will take to raise the level of the
depends far less on how much water actually falls on the lake,
than how much
water falls in the 603 square mile (385,952 acre) watershed that drains to the
lake. Soil moisture conditions, lake levels on the Ridge,
potentiometric surface (fancy name for elevation - compare it with the land
surface elevation to predict springs) of the shallow and intermediate aquifer
and the actual location of the rainfall in the watershed all factor into the
rate of runoff to Istokpoga. There is probably a whole host of other
incredibly important factors that I have completely ignored in
the evaporation loss and stream flow balance, I do not have good estimates for
the rainfall input function is (for obvious reasons).
One thing is
1/5" to 1/4" of rain every day on Lake Istokpoga will be hard
pressed to bring the lake up to its dry season level. It will depend on
consistent rainfall and creek flows throughout the watershed..
to Clell Ford for this simplified explanation of the evaporation problem we
are experiencing on our lake.