(7.3) WATER DATA - Evapo-transpiration Water Losses of Rodman Reservoir

Evapotranspiration Water Losses of Rodman Reservoir

An Information, Opinion, & Sources Report

Compiled by Ocklawahaman Paul Nosca

Created: 15 May 2012

Moved to this Webpage: 31 May 2015

Last Revised: 09 December 2020

The late Marjorie Harris Carr, for whom the Cross Florida Greenway is named after, often argued that Lake Ocklawaha should never be referred to as Rodman RESERVOIR "because it did not conserve water." She said, "Indeed, due to evaporation, Rodman Pool loses water from the [Ocklawaha/St. Johns] river system."

Ocklawahaman tends to agree with that heroic lady about the 9,200 to 13,000-acre impoundment behind Rodman Dam which causes 21 miles of the mainstream Ocklawaha River to be over-exposed for no legitimate reason to 3 massive evapotranspiration powers: (1) hot Florida sunshine (2) wind & (3) runaway exotic aquatic plant growth. Rodman was designed & built by the US Army Corps of Engineers in the 1960’s for the sole purpose of being a "barge navigation pool" for the defunct (since the 1970’s) Cross Florida Barge Canal project.


Ronald L. Hanson (1991) of the United States Geological Survey (USGS) defined evapotranspiration as: "Evapotranspiration is the water lost to the atmosphere by two processes-evaporation and transpiration. Evaporation is the loss from open bodies of water, such as lakes and reservoirs, wetlands, bare soil, and snow cover; transpiration is the loss from living-plant surfaces."


Excerpted from "U.S. Geological Survey Water-Supply Paper 2300" (1984) "FLORIDA Surface-Water Resources" pages 187-188:


"A large percentage of the rainfall (60 to 88 percent) is lost to evapotranspiration. Annual evaporation from free-water surfaces ranges from 48 inches in the southeast to about 42 inches in the northwest (Farnsworth and others, 1982)."

"Evaporation from Lake Oklawaha [sic] and diversions through Buckman Lock have contributed to the downward trend in average discharge by water year for the Oklawaha River..."

Rohrer, K. 1984. Hydrologic reconnaissance of Marion County. Technical publication SJ 84-6. St. Johns River Water Management District, Palatka, FL.

"Annual lake evaporation for Marion County is estimated to be 45.5 inches."


Evaporation ONLY math calculations

North-central Florida = 45 vertical inches or 3.75 vertical feet annual evaporation per square feet of free-water surface;

3.75 vertical feet X 7.48 gallons (1 cubic foot of water) = 28.05 gallons annual evaporation per square feet of free-water surface;

28.05 gallons X 43,560 square feet (1 acre) = 1,221,858 GALLONS ANNUAL EVAPORATION PER ACRE of free-water surface;

1,221,858 gallons / 365 days per year = 3347.56 GALLONS DAILY EVAPORATION PER ACRE of free-water surface;

3347.56 gallons per acre daily X 10,000 acres = 33,475,562 GALLONS DAILY EVAPORATION PER 10,000 ACRES of free-water surface;

33,475,561 gallons / 1,000,000 = 33.48 MILLION GALLONS PER DAY (MGD) DAILY EVAPORATION PER 10,000 ACRES of free-water surface;


33.48 MGD X 1.55 = 51.89 CUBIC FEET PER SECOND (CFS) EVAPORATION PER 10,000 ACRES OF FREE-WATER SURFACE such as Rodman Reservoir.

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