Analytical model captures intratidal variation of salinity in a convergent, well-mixed estuary

Knowledge of the processes governing salt intrusion in estuaries is important since it influences the eco-environment of estuaries as well as its water resource potential in many ways. Analytical models of salinity variation offer a simple and efficient method to study salt intrusion in estuaries. In this paper, an unsteady analytical solution is presented to simulate the spatial-temporal variation of salinity in convergent estuaries. It is derived from a one-dimensional advection-diffusion equation for salinity adopting a constant mixing coefficient and a single-frequency tidal wave. Compared with steady-state solutions, it can directly reflect the influence of the tide and the interaction between the tide and runoff. The deduced analytical solution is illustrated with an application to the Humen estuary of the Pearl River Delta (PRD), and proves to be an efficient and accurate approach to predicate the salt intrusion in convergent estuaries. The unsteady analytical solution is tested against six surveys made at six study sites, to validate its capability of predicating salt intrusion variation. The results show that the proposed unsteady analytical solution can be successfully used to reproduce the spatial distribution and temporal processes governing salinity dynamics in convergent, well-mixed estuaries. Meanwhile, this predictive equation provides a quick and convenient approach to decide upon water fetching works to make good use of water resources.