Evapotranspiration Water Losses of Rodman Reservoir
An Information, Opinion, & Sources Report
Compiled by Ocklawahaman Paul Nosca
With the assistance of Captain Erika Ritter
Revised: 25 October 2013
NOTE: Florida Department of Environmental Protection (FDEP) provides its official management plans for Rodman Reservoir as "Appendix 4: Documents Related to Management of Rodman Reservoir" at http://www.dep.state.fl.us/gwt/cfg/Plan_PDF/CFG_LMP_apps.pdf . "Seasonal Water Level Plan and Basis for Rodman Reservoir Elevation Management June 15, 2006" is one of the available online papers.
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..."
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.
USGS Stream-Flow Gages of the Ocklawaha River Basin
USGS has maintained certified records from its many Ocklawaha River Basin stream-flow gages going as far back as 1931.
Ocklawahaman finds it very interesting to examine the archived river discharge numbers--expressed as CFS of water--from both before & after the 9/30/1968 completion of Rodman Dam (a.k.a. Kirkpatrick Dam since 1998). Online data obtainable from these gage stations agrees very much with Marjorie Carr’s water loss argument--especially so in severe drought years.
Four stations, in particular, provide a good accounting of the source INFLOW TO & the subsequent OUTFLOW FROM the so-called Rodman Reservoir. These four USGS gages proceeding downriver (north then east) along with their general web-pages are:
(1) "USGS 02240000 OCKLAWAHA RIVER NEAR CONNER" at the SR-40 Bridge (just downstream of the massive inflow of the Silver River from Silver Springs & about 39 river miles upstream of Rodman Dam)
(2) "USGS 02240500 OCKLAWAHA RIVER AT EUREKA" at the CR-316 Bridge (about 21 river miles upstream of Rodman Dam)
(3) "USGS 02243960 OCKLAWAHA RIVER AT RODMAN DAM"
(4) "USGS 02244032 CROSS FLORIDA BARGE CANAL AT BUCKMAN LOCK" (about 5 miles to the northeast of Rodman Dam)
Adding the discharges of "Ocklawaha River at Rodman Dam" to the "Cross Florida Barge Canal at Buckman Lock" would equal the Ocklawaha River Basin's total recorded CFS water contribution to the St. Johns River.
A river system normally would gain extra water as it flows downstream because of four quite obvious natural reasons: (1) inflow from additional tributary streams (2) inflow of added groundwater including artesian springs (3) inflow of runoff from a larger watershed & (4) actual rainfall upon the miles of water surface in the expanded area itself.
In the 18 river miles of the "Middle" Ocklawaha River from the SR-40 Bridge near Conner to the CR-316 Bridge at Eureka, four tributary creeks (with road bridges over them) & at least five springs plus 18 river miles worth of runoff as well as rainfall enter the basin.
Downriver over the next 21 river miles commonly known as Rodman Reservoir from the CR-316 Bridge at Eureka to the Rodman Dam, six named tributary creeks (with road bridges over them) & some 20 springs plus 21 river miles worth of runoff as well as rainfall enter the basin.
The six named tributary creeks that provide additional flow into Rodman Pool are:
(1) MILL CREEK (no USGS gage);
(2) USGS 02243000 ORANGE CREEK AT ORANGE SPRINGS
(3) USGS 02243400 LITTLE ORANGE CREEK NR ORANGE SPRINGS
& also USGS 02243300 LITTLE ORANGE CREEK NEAR JOHNSON
(4) USGS 02243530 BRUNTBRIDGE BROOK AT KENWOOD
(5) USGS 02243800 DEEP CREEK NR RODMAN
& also USGS 02243609 DEEP CREEK NR KENWOOD
(6) CAMP BRANCH (no USGS gage but enters upstream of Buckman Lock).
Rodman Reservoir Inflow-Outflow Dynamics During Drought Years 2000-2001
Now let's examine the annual average river discharge figures, measured in CFS of water, from those aforementioned (several paragraphs above) Rodman Reservoir inflow-outflow "accounting" gages (Conner SR-40, Eureka CR-316, Rodman Dam, & Buckman Lock) during the severe drought years of 2000 & 2001:
The average discharge numbers for the water year 2000 show: "Ocklawaha River near Conner" 607 CFS, "Ocklawaha River at Eureka" 644 CFS, "Ocklawaha River at Rodman Dam" 562 CFS, & "Cross Florida Barge Canal at Buckman Lock" 13 CFS.
The average discharge numbers for the water year 2001 show: "Ocklawaha River near Conner" 491 CFS, "Ocklawaha River at Eureka" 569 CFS, "Ocklawaha River at Rodman Dam" 519 CFS, & "Cross Florida Barge Canal at Buckman Lock" zero CFS.
These figures show that during both 2000 & 2001, the Ocklawaha Basin had LESS total flow of water below Rodman Dam than it had 21 river miles upstream at Eureka! In those same two years, notice that the Eureka gage at CR-316 discharged more CFS of water than the Conner gage 18 river miles further upstream at SR-40. The increase between Conner & Eureka is expectedly normal but the decrease between Eureka & Rodman Dam is not. Keep in mind that Rodman Pool not only receives the inflow of the Ocklawaha River from Eureka but also is the recipient of the waters from 6 named tributaries plus some 20 springs. Could Rodman Pool's EVAPOTRANSPIRATION be responsible for much of this water loss? There should be little doubt that Rodman "Reservoir" provides less of the freshwater that the St. Johns River tidal estuary needs--than a restored free-flowing Ocklawaha would.
REFERENCE AS: Nosca, P. 2013. "Evapotranspiration water losses of Rodman Reservoir" webpage report. "Ocklawahaman" website. Paul Nosca, Eureka, FL.