Most of Big O’s water loss is to evaporation

OKEECHOBEE — Weather patterns are on track to drop Lake Okeechobee to 12.5 feet by the start of the rainy season, according to a forecast shared at the last meeting of the 16 County Coalition for Responsible Management of Lake Okeechobee, the St. Lucie and Caloosahatchee Estuaries, and Lake Worth Lagoon. Not all forecasters agree with the prediction – and of course the meteorologists have no control over Mother Nature.

It will rain … or not … no matter what they predict.

The forecasters also have no control over how rapidly water leaves the lake. While much of the water that enters the lake is controlled by the South Florida Water Management District’s locks, culverts and other water control structures, there are no human controls on most of the water that leaves the lake. That’s because most of the water that leaves Lake Okeechobee does not go east, west or south; it goes up into the air.

Evaporation and evapotranspiration estimates are based on historical data. While some water from the lake has been released south since the start of the dry season, and some is released west to prevent saltwater intrusion in the Caloosahatchee River, no water has been released east this year. The water released east and west is minimal compared to the lake’s total volume. The lake level is dropping due to the warm temperatures and sunny skies.

If the level does drop to 12.5 or lower, it will be a good thing for the lake, according to biologists. Lake Okeechobee’s ecology needs regular periods of low water. The habitat for the fish and the wading birds depend on a healthy system of marshes.

“Ideally, Lake Okeechobee should never rise above 15.5 feet and should drop to about 12 feet most dry seasons,” states “Lake Okeechobee: A Synthesis of Information and Recommendations for its Restoration,” by P. N. Gray, C. J. Farrell, M. L. Kraus, and A. H. Gromnicki. The study, published in 2005 by Audubon of Florida, was funded in part by the Batchelor Foundation and by the Everglades Foundation.

What’s the difference between evaporation and evapotranspiration? Webster’s Dictionary explains that evaporation is the movement of water to the air from sources such as the soil, canopy interception, and water bodies. Evapotranspiration is the sum of evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere.

Evaporation and evapotranspiration rates are difficult to measure. Scientists use different models. On average, they estimate Lake Okeechobee drops from around 4.3 ft. to more than 5 ft. each year due to the movement of water into the air.

A study conducted in Florida in 1995 by the United States Geological Survey found there are many factors in play including the temperature of the air, temperature of the water, depth of the water, temperature of the soil under the water, horizontal wind speed, vertical wind speed. Water evaporates more quickly in open water, less quickly in marshy areas.

Soil composition and types of vegetation affect evapotranspiration rates. Evaporation rates in open water increase on a sunny, cloudless day.

“Evaporation Estimation for Lake Okeechobee in South Florida,” by Wossenu Abtew, published in the June 2001 edition of “Journal of Irrigation and Drainage Engineering” analyzed meteorology data from 1993-1997 and estimated a loss of 132 centimeters per year (about 4.3 feet) from evaporation.

“Evapotranspiration Estimate for South Florida” published in 2003 by Wossenu Abtew, Jayantha Obeysekera, Michelle Irizzary-Ortiz, Danielle Lyons and Anna Reardon, found that average evaporation rates in the South Florida Water Management District range from 122 cm (about 4 ft.) in the north part of the SFWMD to 137 cm (about 4.5 ft) in the south.

“Evaporation and evapotranspiration are functions of solar radiation, temperature, wind speed, vapor pressure deficit, atmospheric pressure, characteristics of the surrounding environment and type and condition of vegetation. South Florida is an area of high rainfall, high humidity and generally low wind speed. Air temperature is high with relatively warm winter months. Solar radiation is abundant with seasonal and daily variation,” the authors state. Annual lake evaporation in the continental United States ranges from 51 cm in the extreme northeast to 218 cm in Southern California, the authors note.

The 2005 Audubon study estimated water loss to evaporation around 5 ft. per year — usually higher during dry periods.

“In an average year, Lake Okeechobee receives enough rain and inflow to raise it about 7.5 feet (note: this discussion of the water budget uses rounded numbers for simplicity). This water does not come in at once, but is spread over the entire year, thus the lake does not actually rise 7.5 feet each year. Roughly 5 feet of this water will evaporate, leaving 2.5 feet of water in the lake. On average, irrigation and municipal demands use 1 to 2 feet of lake water per year, leaving an “excess” of about one foot of water that must be discharged,” the 2005 Audubon report explains.

While water is coming in through direct rainfall into the lake, and runoff from the north of the lake throughout the year, water is also leaving the lake through evaporation all year, the researchers explain.

Nothing about water management is simple, but in simple terms that additional about one foot of water is what went east and west in 2016, helping to disrupt the salinity levels in the estuaries and contributing to the conditions that spawned a massive algal bloom. That extra one foot of water is what the environmentalists hope to send south through the Central Everglades Planning Project, and what they want use proposed Everglades Agricultural Reservoir and accompanying stormwater treatment areas to store, clean and move south to Everglades National Park and Florida Bay.

That scenario – the option of sending the excess water south – will also depend on how much water is already south of the lake from direct rainfall. In 2017, there was so much water in the water conservation areas south of the lake that 8.7 billion gallons of water had to be backpumped into the lake to protect the Everglades wildlife and also protect the east coast protection levy. (Not that this made much difference to the lake. One inch of the lake is about 12 billion gallons of water.) Had the EAA reservoir been in place in 2017, it likely would have been filled by direct rain and local runoff with no capacity left to store water from the lake.

Politicians and water managers often remind the public that when it comes to issues of water quality and quantity in South Florida there are no “silver bullets,” no easy solutions. Understanding the scope of the issues involved is a start.

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