Evaluation of satellite algorithms for Chlorophyll-a concentration in
the Northeastern Arabian Sea: A validation approach
Abstract
Primary productivity of an aquatic system like Arabian Sea is broadly
determined by the concentration of Chlorophyll-a (Chl-a/Ca) pigment. The
present study is essaying to validate the Chl-a data set of prominent
ocean color sensors (OC3M - MODIS, OC-OCM2, and OC3V-VIIRS) with sea
truth data, collected from 204 stations for three year period
(2015-2017). The in-situ concentrations of Chl-a depicts the geographic
region under the mesotrophic and eutrophic spans
((0.1>Ca>1.0 mg m-3). The ratio of
CaOCM2/CaIn-situ is 0.97±0.27 mg m-3 (n=199), in unsimilarity with
CaVISSR/CaIn-situ is 1.75±0.79 mg m-3 (n=170) and CaMODIS/CaIn-situ is
2.53±1.42 mg m-3 (n=158). The regression analysis proclaims a moderate
significant relationship for MODIS (r2 = 0.36; p<0.001),
followed OCM2 (r2 = 0.32; p<0.001) and VISSR (r2 = 0.19;
p<0.001) with evident overestimation (MODIS and VIRRS) and in
tune (OCM2) with the satellite-derived datasets. The global ocean color
missions aimed to set RMSE error at 0.35, the OCM2 shown the lowest RMSE
as 0.13, which is relatively lower than the reference error limit. In
overall performance among three algorithms, the OCM2 will provide a
better estimation of Chl-a with a prediction of 32% accuracy and 34.37
% of bias. The log bias values for MODIS (0.35) and VIIRS (0.20)
algorithms indicating the overestimation of Chl-a with in-situ
concentrations, but the OCM2 algorithm is suitable in the region with a
negligible bias of -0.03. The biogeochemical processes and ecosystem
characteristics are dynamic from region to region, as yet in its urgent
need to validate global and formulate regionally tuned algorithms
periodically.