Using Hydrologic Correlation As A Tool To Estimate Flow At Non-Instrumented Estuarine Creeks In Northeastern Florida Bay

Poster presented April 2003, at the Greater Everglades Ecosystem Restoration Conference

INTRODUCTION

[larger version]

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, including East Highway Creek and Oregon Creek, 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 and Oregon Creek were instrumented to quantify the accuracy of the estimated discharge at non-instrumented sites. A test case is presented here for East Highway Creek and Oregon Creek comparing computed and estimated discharge.

Oregon Creek [larger version] West Highway Creek [larger version]

 

East Highway Creek [larger version]

METHODS

Initially, discharge was estimated at East Highway Creek and Oregon Creek using hydrologic correlation. Acoustic Doppler Current Profiler (ADCP) measured discharge was related to the instantaneous discharge computed from 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 and Oregon 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).

Example of remote monitoring stations and acoustic Doppler instruments. [click image for larger version]

 

RESULTS

East Highway Creek

Differences in total discharge volume, wet season discharge volume, and dry season discharge volume were evaluated. 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.

Oregon Creek

Differences in total discharge volume, wet season discharge volume, and dry season discharge volume were evaluated. From August 29, 2002 to March 13, 2003, computed and estimated discharge volume equaled 2,863 acre-feet (mean = 8.0 ft³/s) and 4,038 acre-feet (mean = 11.3 ft³/s), respectively. During the 181-day comparison, the estimated discharge volume was 41 percent greater 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 552 acre-feet (mean = 2.7 ft³/s) and 1,414 acre-feet (mean = 6.9 ft³/s), respectively. During the 103-day dry season comparison, the estimated discharge volume was 156 percent greater than the computed discharge volume. Computed and estimated wet season discharge volume equaled 2,312 acre-feet (mean = 15 ft³/s) and 2,625 acre-feet (mean = 17 ft³/s), respectively. During the 78-day wet season comparison, the estimated discharge volume was 13.5 percent greater than the computed discharge volume.

FINDINGS

Although wet season discharges were slightly overestimated at East Highway Creek and Oregon Creek, the discharge trend was reproduced. Dry season discharges were underestimated at East Highway Creek and overestimated at Oregon Creek. The dry season flows 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 discharge record is considered critical since only five flow conditions have been measured. Improvements to the computed discharge record are possible with additional ADCP measurements at low flow conditions.

Additional Instrumented Stations Joe Bay 8W [larger version] Joe Bay 5C [larger version] Joe Bay 1E [larger version] Joe Bay 2E [larger version]

 

For More Information: Mark Zucker mzucker@usgs.gov Clinton Hittle cdhittle@usgs.gov U.S. Geological Survey Center for Water and Restoration Studies 9100 NW 36th St. Suite 107 Miami, Fl. 33178 303-717-5800 SOFIA South Florida Information Access http://sofia.usgs.gov