Long-term distribution patterns of remotely sensed water quality variables in Pearl River Delta, China

Abstract

Pearl River Delta (PRD) is the third largest and one of the most productive Deltas in south China, where long-term monitoring and its assessment of water quality are necessary to understand the distribution patterns to infer the coastal zone management. The studies on long-term monitoring in water quality variable concerning the changing climate and anthropogenic effects can be made possible by continuous coverage of Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. This study attempts to study the long-term trends, quantitative measurements, and assessment of water quality variables including total suspended solids (TSS), diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd (490)), and sea surface temperature (SST) using 8-day composite and monthly data products of MODIS data for the period between July 2002 and August 2018 in the turbid waters of the PRD. Monthly products are used to quantify the variability of these water quality properties on long-term climatology, seasonal and as well as spatial distributions to interannual timescales. We observed a decreasing trend of TSS and Kd (490) between 2002 and 2018, though not significant, whereas, an increasing trend of SST has recorded for the PRD region during the same time. Seasonal patterns of TSS, Kd (490) and SST showed significant differences between the seasons. The high values of TSS (>15 gm−3) and Kd (490) (>2 m−1) were found in summer, autumn followed by winter, and low values (<0.5 gm −3 and 0.1 m-1) in spring. Consequently, high SSTs were noticed in the in summer-Autumn season and low SSTs in winter. The spatial concentrations of TSS, Kd (490) and SST were found high in the northern region and low values in the offshore and the southern part of the PRD. The increased values of TSS and Kd (490) in summer can be attributed to the high amount of precipitation, and strong monsoonal winds lead to the significant amounts of highly turbid waters in the water column. Based on the MODIS derived time series of TSS and Kd (490), a significant correlation with wind speed was found between these variables, suggesting a critical role of precipitation, river discharge and wind forcing in the variability of ocean variables in the PRD region.