Measuring surface water salinity of Pearl River Estuary by MODIS 250-m imageries

Abstract

Aim: Moderate resolution imaging spectroradiometer (MODIS) 250-m imageries were used to evaluate the performance for developing the effective model of salinity using field spectra, the absorption coefficient of color dissolved organic matter (aCDOM) collected on October 21and November 2, 2012 in the Pearl River Estuary (PRE), China. Methodology: Remote sensing reflectance of the four simulated MODIS bands (Rrs(Bi)) with aCDOM (λ) data were used for algorithms development, which include least squares regression of the single band, difference between bands, band ratios, sediment index and their varieties with aCDOM (λ) based on the dataset of November 2, 2012 (N=18). Additionally, the model of salinity with aCDOM (λ) data was also calibrated (N=18). Results: Results illustrated the optimum performance of quadratic model of sediment index for estimation of aCDOM (355) (R2= 0.67, N=18, P<0.001), and the reverse linear model of aCDOM (355) is of the best accuracy for estimation of salinity (R2=0.83, N=18, P<0.001). An effective method to estimate surface water salinity from simulated MODIS 250-m bands was calibrated and validated using the independent dataset of October 21, 2012 (RMSE = 1.95‰, MRE = 9.65%, N = 17) in the Lingding Bay of PRE. The preferred models were further applied to retrieve aCDOM (355) and salinity data from the MODIS 250-m data in the coastal and inner Lingding Bay of PRE. The result presented rationality of salinity distribution of PRE. The MODIS 250-m salinity mapping was validated based on in-situ measurements, and presented a good salinity mapping accuracy (RMSE=1.72‰, MRE=8.24%, N=10). The study proved the robustness of these algorithms for aCDOM (355) and salinity estimation in the coastal and inner Lingding Bay of PRE by MODIS remote sensing. Interpretation: Therefore, the effective method can be applicable in detecting variability of CDOM concentration and salinity during tidal cycle in a high frequency (two times during daytime), which proves the detection ability of MODIS 250-m imageries in estuarine and coastal waters. It provides the water supply and conservancy authorities spatially and temporally understand the marine intrusion in the Lingding Bay of PRE.