Understanding the quantity, timing, and distribution of freshwater flow to northeastern Florida Bay and other coastal environments is critical for restoring south Florida estuaries. A coastal monitoring network was established in 1996 by the U.S. Geological Survey to provide flow at five estuarine creeks in northeastern Florida Bay. Four additional estuarine creeks were not instrumented and discharge was estimated using hydrologic correlation. This technique assumes that flow from streams near each other correlate because of similarities in geomorphology, rainfall, and distance from source waters. To verify this technique, East Highway Creek, Oregon Creek, and three creeks in Joe Bay were instrumented in July 2001 to quantify the accuracy of the estimated discharge at non-instrumented sites. A test case is presented here for East Highway Creek comparing computed and estimated discharge.

Initially, discharge was estimated at East Highway Creek using hydrologic correlation. Diascharge measured using Acoustic Doppler Current Profiler (ADCP) at East Highway Creek was related to the instantaneous discharge computed using the calibrated velocity meter at West Highway Creek. The equation generated from regression analysis provided the means to estimate discharge over time using computed discharge from West Highway Creek as the explanatory variable. To compute discharge at East Highway Creek, continuous velocity and stage data were collected using an Acoustic Doppler Velocity Meter (ADVM) with an upward acoustic stage sensor. The ADVM velocities were calibrated to the mean channel velocity over a range of velocity conditions using an ADCP. A more detailed discussion of acoustic methods and discharge estimation techniques is provided in Hittle and others (2001).

Differences in discharge volume, wet season discharge volume, and dry season discharge volume were evaluated for the period of record. From February 10, 2002 to September 30, 2002, computed and estimated discharge volume equaled 17,277 acre-feet (mean = 37.5 ft³/s) and 14,788 acre-feet (mean = 32.1 ft³/s), respectively. During the 233-day comparison, the estimated discharge volume was 14.4 percent lower than the computed discharge volume. An evaluation of seasonal discharge volume indicated that dry season estimates were less accurate than wet-season estimates. Computed and estimated dry season flows equaled 4,316 acre-feet (mean = 19.6 ft³/s) and 853 acre-feet (mean = 3.9 ft³/s), respectively. During the 111-day dry season comparison, the estimated discharge volume was 80 percent lower than the computed discharge volume. Computed and estimated wet season discharge volume equaled 12,961 acre-feet (mean = 53.7 ft³/s) and 13,935 acre-feet (mean = 57.7 ft³/s), respectively. During the 122-day wet season comparison, the estimated discharge volume was 7.5 percent greater than the computed discharge volume. Since the regression equation used to estimate discharge at East Highway Creek from 1996 to 2001 was similar to the regression equation used in this analysis, it is probable that these findings apply to prior estimates (fig. 1).

Although wet season discharges were slightly overestimated at East Highway Creek, the discharge trend was reproduced (fig. 2). Dry season discharges were underestimated at East Highway Creek and primarily represent saline exchanges between northeastern Florida Bay and the estuarine creeks, rather than freshwater runoff. However, the exchange of saline water towards upstream wetlands, to ground-water systems, and the impact on local ecology may be important. Concerns over the accuracy of the computed East Highway discharge record is considered critical because only five flow conditions have been measured. Improvements to the computed discharge record are possible with additional ADCP measurements at low flow conditions.

Figure 1. (above) Scatter-plot of computed and estimated daily mean discharge at East Highway Creek including the line of equality. [larger image]

Figure 2. (above) Time-series graph of computed and estimated daily mean discharge at East Highway Creek. [larger image]

REFERENCES

Hittle, C.D., Patino, Eduardo., and Zucker, Mark, 2001, Freshwater Flow from Estuarine Creeks into Northeastern Florida Bay: U.S. Geological Survey Water Resources Investigation Report 01-4164, 32 p.

Contact: Mark Zucker, U.S. Geological Survey, 9100 NW 36 St., Ste. 107 Miami, FL, 33178, Phone: 305-717-5852, Fax: 305-717-5801, mzucker@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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