Historical changes in water-management practices to accommodate a large and rapidly growing urban population along the Atlantic Coast of south Florida as well as intensive agricultural activities have resulted in a highly managed hydrologic system with canals, levees, and pumping stations. These structures have altered the hydrology of the Everglades ecosystem on coastal and interior lands. Surface-water flows south of Lake Okeechobee have been regulated by an extensive canal network, begun in the 1940’s, to provide for drainage, flood control, saltwater intrusion control, agricultural requirements, and various environmental needs. Much of the development and subsequent monitoring of canal and river discharge south of Lake Okeechobee have traditionally focused on the eastern coastal areas of Florida. Recently, increased emphasis has been placed on providing a more accurate accounting of canal flows in interior regions of south Florida.

As part of its Place-Based Studies Program, the U.S. Geological Survey (USGS) is presently conducting a study to: (1) evaluate approaches for quantifying freshwater flows to and from Native American Lands, and (2) provide hydrologic data to support various other federal, state, and tribal hydrologic investigations. The implementation and development of strategically placed streamflow and water-quality gaging sites in the interior have provided vital information for determining future surface-water flow requirements in the internal canal system. Subsequent studies, based on accurate flow determinations at these sites, have been used for computation of nutrient loadings in the canal system. Providing continuous- flow data from selected impact points for internal basins complements the data from the eastern flow canal discharge network. This has resulted in increased accuracy for timed water deliveries to specific locations.

Figure 1. South Florida Water Management District/Seminole Agreement water-quality and flow sampling sites. Click for larger image.

During 1996-97, the USGS constructed, instrumented, and calibrated three streamflow monitoring sites south of Lake Okeechobee (fig. 1) in an effort to accurately gage flows in canals entering and exiting Tribal Lands, Big Cypress National Preserve, and Water Conservation Area 3A in south Florida. The L-28U site is used to monitor freshwater flows to and from Seminole and Miccosukee Indian Tribal Lands. The L-28IN site is used to monitor freshwater flows from Seminole Indian Tribal lands to Big Cypress National Preserve and ultimately to Miccosukee Tribal Lands. The L-28IS site, discontinued in September 1999, was used to monitor flows from Seminole Indian Tribal Lands and Big Cypress National Preserve to Miccosukee Indian Tribal Lands. This site also was instrumental in bracketing and quality assuring the flow calibration conditions for the upstream L-28IN site.

Acoustic instrumentation, in lieu of standard methods for field data collection and flow computations, is used to gage flows in the canals. With the acoustic velocity meter (AVM) and the acoustic doppler current profiler (ADCP), it is possible to more accurately gage flows in this type of environment because they can quickly measure low or rapidly changing water velocities. The ADCP calibration of the in situ AVM index velocities is ongoing. A sum of least squares regression has been developed for data processing at all sites and continues to be refined.

Velocity data collected during the dry season have displayed a phenomenon known as acoustic refraction or ray bending. This is produced by thermal stratification in the water column during extended periods of very slow flow. At one site, a point velocity electromagnetic velocity meter and associated velocity index were established in conjunction with the AVM to verify periods when these episodes occur.

Average annual runoff of 70,100 acre-feet was recorded during 1997-99 at L-28U (fig. 2). This represents about twice the inflow amount determined by the South Florida Water Management District (SFWMD) at their upstream U.S. Sugar Outflow (USSO) site located on the northwestern border of the Seminole Indian Tribal Lands.

An average annual runoff of 53,770 acre-feet has been recorded at L-28IN since its inception in 1997, and an average annual runoff of 49,070 acre-feet was recorded during 1997-99 at L-28IS (fig. 3). The lesser discharge recorded at the more southerly site of the two on the Interceptor canal was likely due to the heavy influence of the S-140 pump station where losses could be attributed to heavy operational pumping periods.

These flows also are being monitored as part of a multiagency effort to calculate nutrient loads in the canals that cross or border

Figure 3. Comparison of L28IN and L2BIS flows with S190 flow, May 1998 to April 1999. Click for larger image.

Tribal Lands. The SFWMD installed flow-weighted samplers at the gaging sites for nutrient analysis in conjunction with the streamflow monitoring; the flow-weighted samplers have been serviced by the Seminole and Miccosukee Indian Tribes, respectively. Real-time telemetry programming assistance and phosphorus and nitrogen load calculations have been provided by the SFWMD. The nutrient load data from all three sites is used by water managers for resource planning and management.

The implementation of strategically placed streamflow and water-quality gaging sites in the interior of south Florida - in conjunction with four entities to collect, analyze, and distribute useful information to help water managers determine future surface-water flow requirements in the interior canal system - has been a success. Ongoing flow-weighted nutrient loads require accurate flow data collection combined with a highly coordinated nutrient collection and analysis SFWMD/Seminole Working Group. The SFWMD is in the process of documenting the protocol used for collection, computation, and processing of “flow-weighted” nutrient loads in the interior canal system and a future quality assurance/ quality control document will be forthcoming. Another future effort is being considered to co-locate an in situ side-looking, acoustic Doppler, continuous recording flowmeter with the existing AVM at the L-28IN site to more accurately monitor flows along with the potential ability to provide auxiliary nutrient information at little cost.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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