EOF analysis of water level variations for microtidal and mangrove-covered Frog Creek system, westcentral Florida

At present, little is known about the mechanisms that control water level variations for the Frog Creek system, westcentral Florida, which is a mangrove-covered, microtidal, shallow estuary. Although connected with open waters, topographic and roughness constraints can alter expected variability of water levels in this area. Water level variations are especially important for the Frog Creek system, because the increases or decreases in subtidal water level change the present tidal environments. In this paper, 1-year measurements of water levels were examined using the empirical orthogonal function (EOF) method. Water level data from eight stations were first filtered by employing a Lancz6-squared filter because of its ability to eliminate diurnal tidal signals while inducing minimal attenuation in variations with periods of 2 to 3 days, which are usually caused by wind. The filtered water levels were then analyzed using the EOF method to determine relative contributions of various forcing factors. The first three EOF modes explained 64.13, 33.25, and 1.55 of total variances and were interpreted by calculating correlation coefficients and coherences. We attributed EOF mode 1 to the variations of coastal subtidal water levels introduced by physical processes with longer periods, such as spring-neap tidal variability and seasonal freshwater river discharge variability. Next, EOF mode 2 was related to the effects of the local N-S wind component, which introduces short-timescale variations of the subtidal water level with typical periods ranging from 2 to 6 days. The effect of river discharge on water level was captured by EOF mode 3. Overall, EOF analysis provides an insightful tool to examine water level disturbances.