Characteristics Of Water Age And Residence Time In Hangzhou Bay

Time scale (age and residence time) analysis in Hangzhou Bay plays an important role in revealing the impact of river (Yangtze and Qiantang river) discharge and tidal current on the aquatic environment in Hangzhou Bay. In this study, the CART (Constituent-oriented Age and Residence-time Theory) theory is applied to study the characteristics of water age (age of Yangtze and Qiantang freshwater) in Hangzhou Bay and residence time in the northern and southern regions in the bay. Based on ROMS (Regional Ocean Modeling System) model, the simulation of two years’(year 2007-2008) period is conducted and the results of the second year are used for analysis. The results of tidal elevation, tidal current, salt and the residue current in different hydrodynamic conditions (flood and drought season, spring and neap tide) are validated and analyzed. The main results of age and residence time are in the following:(1) The distribution of Yangtze freshwater concentration shows that the much higher (about 0.7) value in the northern part of Hangzhou Bay is greatly affected by Yangtze river discharge, which indicates that the Yangtze river flow can go through the northern region to Hangzhou Bay. Qiantang freshwater concentration in the upstream of Hangzhou Bay is high (e.g.0.86 in Ganpu station in flood season) and its concentration is sensitive to Qiantang river discharge.(2) The mean age distribution of Yangtze freshwater in Hangzhou Bay is low in northern region (e.g.80 days in flood season in Luchaogang station), nearly high (e.g. the mean age of this region in drought season is 205 days) in middle and southern regions and a much higher value in upstream region (e.g. the mean age of this region in drought season is 350 days). The vertical Yangtze freshwater age distribution is greatly affected by tidal current:the spring tide can well mix water, leading to the vertical age distribute uniformly and the neap tide together with river discharge may cause differences in vertical age distributions.(3) The age distribution of Qiantang freshwater in Hangzhou Bay can be concluded that the age grows seaward and it reaches 400 days in Luchaogang station. Age in the west of Longitude 121.1′ is negative related to Qiantang river discharge. Affected by the strong tide, the vertical Qiantang freshwater age distribution is uniform in the middle, north and upstream of Hangzhou Bay even in neap tide condition. In the southern part of Hangzhou Bay, the vertical Qiantang age distribution is much different in neap tide condition due to the impact of archipelagos.(4) Residence time in the northern control domain in the bay is greatly affected by Yangtze river discharge. When the discharge is 28000m3/s (flood season), residence time in the control domain ranges from 0 to 18 days. When the discharge is only 10000 m3/s (drought season), residence time is between 12 and 32 days. Residence time in the southern control domain is controlled by the flood-ebb tide so that it decreases eastward in the control domain. The distribution of residence time in this control domain is similar in both spring and neap tide conditions and they share the values that ranges from 6 to 26 days (June). The time series of residence time shows that the value in spring tide condition is 0.5 to 6.5 days smaller than that in neap tide condition.