Quantification of SPM and chlorophyll using OCM data

Quantification of SPM and chlorophyll using OCM data


M. S. Vinaya
M. S. Vinaya* and H. Gangadhara Bhat
Department of Marine Geology
Mangalore University, Mangalagangothri – 574 199, India
Email: [email protected]

A. V. Thomaskutty, A. S. Rajawat, R. M. Dwivedi and S. R. Nayak
Marine Water Resources Group, Remote Sensing Application Area
Space Applications Centre, Ahmedabad – 380 015, India
Email: [email protected]

In the present study, Ocean Colour Monitor (OCM) data have been processed for suspended particulate matter (SPM) and chlorophyll using Tassan’s and Ocean Chlorophyll 2 (OC2) algorithms respectively and their performance in the nearshore waters has been evaluated using the sea truth data collected along the Mangalore region of southern Karnataka coast

The Ocean Color Monitor (OCM) sensor of IRS-P4 satellite, with reasonably high temporal and spatial resolutions, has wide applications in both coastal and oceanic studies of which retrieval of water quality parameters like chlorophyll, suspended particulate matter (SPM) and yellow substance are of great importance. But retrieval of these parameters is not a simple and straightforward process. It needs an effective atmospheric correction of data to obtain the actual water leaving radiance and then, the usage of a suitable and separate bio-optical algorithm for the retrieval of individual parameter. Development of such bio-optical algorithms for the coastal or case II waters is difficult, and a challenging task unlike for the open ocean where phytoplankton control the optical properties. Coastal waters are optically very complex with a greater influence of river inputs and local geological phenomena and are also characterized by the presence of exceptionally high phytoplankton pigments, SPM concentration and dissolved organic matter absorption. Hence, the global color ratio algorithms fail for the coastal and nearshore waters. Thus it has become a basic need to develop site-specific algorithms for the varying coastal waters, which can be updated from time to time.

In the present work, water samples were collected synchronous to the IRS-P4 satellite overpass along the coastal waters (nearshore waters) off Mangalore region of southern Karnataka, India (Fig. 1). and were analyzed for SPM and chlorophyll concentrations. These in-situ measurements were compared with the corresponding satellite derived SPM and chlorophyll. Tassan’s algorithm (1994) was used for SPM estimation and the Ocean Chlorophyll 2 (OC2) algorithm proposed by O’ Reilly et al. (1998) was utilized for chlorophyll retrieval from the OCM data. The results show that both SPM and chlorophyll algorithms underestimate the parameters. The linear regression plots between in-situ and satellite measurements reveal an extremely low value of determination coefficient (r2) in both the cases, i.e. 0.0007 in the case of SPM and 0.0424 in the case of chlorophyll, which show the unsuitability of these algorithms for the Mangalore region. Hence modification of algorithms for both the parameters has been attempted.

1. Introduction
The Indian remote sensing satellite IRS-P4 also known as Oceansat-1 was launched on May 26, 1999 by the Indian Space Research Organization (ISRO). The satellite carried two sensors i.e. Ocean Color Monitor (OCM) and Multifrequency Scanning Microwave Radiometer (MSMR) dedicated for the oceanographic research. The first sensor OCM collects data in eight spectral channels centered at 412, 443, 490, 510, 555, 670, 765, and 865 nm with a spatial resolution of 360 meters, temporal resolution of 2 days, radiometric resolution of 12 bits and a swath width of 1420 kilometers. The last two channels are mainly used to correct the contributions due to atmospheric scattering in the other six shorter wavelengths and thus to estimate the actual water leaving radiances in those bands. Thus derived normalized water leaving radiances in specific wavelengths can be related to the concentrations of chlorophyll and suspended particulate matter and also to the yellow substance absorption by having a good understanding on the spectral behavior of these parameters. Water quality assessment especially for TSM and chlorophyll using satellite data has been carried out by many researchers; Thiemann and Kaufmann (2000), Allee et al. (1999), Forster et al.(1993) and Ritchie et al., (1990).

In the present study OCM data have been processed for SPM and chlorophyll using Tassan’s and OC2 algorithms respectively and their performance in the nearshore waters has been evaluated using the sea truth data collected along the Mangalore region of southern Karnataka coast of India. An attempt has also been made to modify them in order to improve their efficiency for the area under investigation.

Fig. 1: Study area