ESTIMATING ESTUARINE FLUSHING AND RESIDENCE TIMES IN CHARLOTTE HARBOR, FLORIDA, VIA SALT BALANCE AND A BOX MODEL

We present a new concept and techniques for processing flow and salinity data and calibrating a box model designed to estimate water residence times in Charlotte Harbor, a large estuary in southwestern Florida. The new concept is that, over many tidal cycles, the tidally averaged "flow" (Q(g)) of water from the Gulf of Mexico with a salinity of 35 parts per thousand can be treated as a constant at any point in the estuary. This flow is used in a simple mixing equation to predict salinity in the estuary at different river inflows, and the predicted salinities are used to compute residence times for water in the estuary. The techniques developed to achieve optimal precision in the relation between river inflow and salinity include a newly derived equation to fit Q(g) by a least-squares method and a procedure to determine the optimal averaging period for river inflow. Results from Charlotte Harbor indicate that, under average (70 m3 s-1) river inflow, 95% of the original water present in the harbor flushes into the gulf in 130 d.