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 the story.
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 simple. Right?
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
calculation. Thus ends the easy part.
The hard part is how much rainfall it will take to raise the level of the
lake. That 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,
the 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
developing this "model".
Unlike 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 certain, simply
receiving 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..

Note: Thanks to Clell Ford for this simplified explanation of the evaporation problem we are experiencing on our lake.