Differences in the lag characteristics of saltwater intrusion in the sub-estuaries of the Pearl River Estuary and their implications for water allocation

Abstract

The water dispatch during the dry season of the Pearl River Estuary(PRE) has effectively responded to the adverse effects brought by the saltwater intrusion for many years, however, in recent years the saltwater intrusion of the estuary is still further intensified, seriously affected the water supply safety of the estuary and its irradiated area The Guangdong-Hong Kong-Macao Greater Bay Area. In order to manage the water dispatch for preventing the saltwater intrusion more reasonably, it is urgent to study the spatial-temporal characteristics of the saltwater intrusion in the sub-estuaries of the PRE. The wavelet analysis and cross wavelet method used and the results show: there is obvious diurnal, semi-diurnal and semi-lunar cycle of the salinity in most waters of the PRE. The changes in salinity at each sub-estuary are not synchronized with the tidal processes. The semi-monthly periodic changes in salinity typically precede the changes in tidal range, with the salt front appearing 3–4 days before spring tides. The diurnal and semi-diurnal periodic changes in salinity usually lag behind the changes in tidal levels, with the salt front occurring 2–3 hours after high tide. Only at the Hongqi Men sub-estuary does the salt front appear 1 day before spring tides and 8 hours after high tide. The main reason for the differences in lag characteristics may lie in the varying mixing states of saltwater and freshwater in different estuaries. Consequently, the optimal timing for freshwater releases to control salinity intrusion also varies among the sub-estuaries. In the practice of suppressing saltwater intrusions, it is important to choose appropriate timing for water releases based on these lag characteristics and to scientifically manage water allocation to achieve maximum benefits with minimal water usage. We hope that the experience gained from the PRE will be helpful to other coastal regions around the world.