In response to 10, 20, 30, and 40% increases in the boundary concentration, the half-life time increased to 91.5, 168.3, 186.2, and 229.1 d, respectively. Sensitivity experiments showed that the half-life time for water exchange was greatly affected by the concentration of the conservative substance which was used at the open boundary. When rivers were included in the model, the half-life time decreased by 74.6% to 13.9 d.

Numerical experiments showed that wind accounted for an 11.9% reduction in the half-life time to 48.3 d. In a control run driven by the tide, without external inputs and an open boundary concentration of zero, it was estimated that the average half-life time in QZB was 54.8 d. Given the strong tidal current in QZB, a half-life time was calculated for water exchange by filtering the tidal signal from the concentrations of a conservative substance. A three-dimensional model was established to investigate water exchange in coastal waters, and applied to Qinzhou Bay (QZB) in the South China Sea.