The application of MODIS Aqua/Terra surface reflectance product to estimating the surface turbidity in Darwin Harbour, Australia

Turbidity is an indicator of the quality of water and usually exhibits variability associated with changing hydrodynamic conditions, which can be reflected in the sediment dynamics in coastal regions. The conventional method of measuring turbidity is water sampling at fixed stations. However, conventional methods can be costly and may not properly capture the spatial patterns in turbidity. The use of satellite data to retrieve turbidity in coastal areas is an optimal approach to detect spatial patterns in surface turbidity. Darwin Harbour is a typical macro-tidal environment influenced by industries, human activities and natural factors including winds, currents, river discharges, waves and tides. Because of the many activities that influence Darwin Harbour and the complexity of its estuarine environment, and the lack of research using remote sensing methods in this domain, Darwin Harbour was chosen as the case study to investigate the variability in turbidity. In this study, a correlation model between turbidity and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua/Terra surface reflectance is used to retrieve turbidity in Darwin Harbour. MODIS data from November 2012 to October 2017 are used to examine the seasonal, spring-neap and intra-tidal turbidity variations, and explore the factors controlling these variabilities in the study domain. A two-dimensional hydrodynamics model is also developed to study the formation of a turbidity maximum zone (TMZ) observed in the coastal area outside of Darwin Harbour. The results of this study illustrate that the seasonal turbidity variation is mainly influenced by the wind waves. The dredging campaigns in 2013 and 2014 wet seasons contributed to the rise of turbidity in Darwin Harbour. The action of tidal currents appears to be the dominant factor controlling the turbidity pattern in a spring-neap cycle and the turbidity intra-tidal variation. In addition, based on the results of the modelling study, the TMZ near Charles Point is formed by tidal current convergence. The techniques, data and results presented in this study can be applied to other coastal areas to detect and monitor water turbidity for better management of turbid harbours, estuarine and other coastal waters