Salinity response to seasonal runoff in a complex estuarine system (Cochin estuary, west coast of India)

Abstract

The hydrographic conditions in Cochin estuary are investigated using longitudinal measurements of salinity profiles during a 1-year period. The estuary transformed from salt wedge conditions during the wet season to a partially or wellmixed state during the dry season. During high runoff months (June–September) and moderate runoff months (October– December and May), the salinity structure is a strong function of variations in river runoff, and the estuary responds coherently to episodic inputs of freshwater. However, during the period from January to April, the dependence of the salinity field on runoff was weakened by axial diffusivity. The salt transport was a balance between seaward flux induced by river runoff and tidally induced diffusive landward flux. The latter dominated in almost all sections along the estuary except at the upstream end, increasing the salt content of the estuary. The computed horizontal diffusion coefficients ranged from 5 to 704 m²s^-1 . The flushing time of Cochin estuary ranged from 1 to 2.5 days during the wet season. During the dry season, the tide-driven flushing time was 8.7 days. The results represent an important benchmark against which numerical models should be measured and will be useful for developing a better understanding of biological and chemical distributions within the estuary