Current Science

Arvind Sahay ¹, Anurag Gupta ¹, Gunjan Motwani ¹, Mini Raman ¹, Syed Moosa Ali ¹, Meghal Shah ², Shard Chander ¹, Pradipta R. Muduli ³, R. N. Samal ³

Affiliations
1 Marine Ecosystem Division, Biological and Planetary Sciences and Applications Group, Earth, Ocean, Atmosphere, Planetary Sciences and Applications Area, Space Applications Centre, Ahmedabad 380 015, IN
2 Department of Botany, University of Gujarat, Ahmedabad 380 009, IN
3 Wetland Research and Training Centre, Chilika Development Authority, Department of Forest and Environment, Government of Odisha, Balugaon 752 030, IN

Abstract

Coloured dissolved and detrital matter (CDM) forms a significant fraction of the total dissolved organic matter (DOM) in water bodies. It absorbs light strongly in the ultraviolet and blue domains of the electromagnetic spectrum. The present study maps CDM absorption of the entire Chilika lagoon, Odisha, India (an optically complex water body) using hyperspectral data of AVIRIS-NG. This study takes advantage of hyperspectral data which use SWIR bands for the estimation of remote sensing reflectance in highly turbid waters of Chilika lagoon (northern sector, which otherwise is masked using standard atmospheric correction schemes). During 24–27 December 2015, we have collected in situ bio-optical data over waters of Chilika lagoon, for studying the distribution of CDM. AVIRIS-NG data have also been acquired synchronous to in situ measurements over the study area. CDM absorption coefficient is retrieved using quasi analytical algorithm and the distribution of CDM is discussed in detail in three different sectors of Chilika lagoon (southern, central, northern) and at the outer channel. The variability of CDM absorption at 412 nm shows that in the north sector of Chilika lagoon, CDM absorption is quite high compared to other sectors (5.5 m^–1 with a standard deviation of 0.06 m^–1). In the southern sector and at the outer channel it is 1.8 m^–1 with a standard deviation of 0.02 m^–1 and in the central sector it is 3.76 m^–1 with a standard deviation of 0.22 m^–1. High CDM in the northern sector is attributed to the inflow of terrestrial organic matter. The advantage of hyperspectral data is that it gives CDM absorption contiguous in the range of 375–425 nm, where the absorption by CDM is strong and away from chlorophyll-a absorption.

Keywords

Dissolved Organic Matter, Hyperspectral Data, Lagoon, Optically Complex Waters.

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R. P. Singh ¹, Shard Chander ¹, Ratheesh Ramakrishnan ¹, Ashwin Gujrati ¹, Rohit Pradhan ¹, Chirag Wadhwa ¹, A. S. Rajawat ¹, Raj Kumar ¹

Affiliations
1 Space Applications Centre, Indian Space Research Organisation, Ahmedabad 380 015, IN

Abstract

In this study we identify a region of submarine groundwater discharge (SGD) near Okha coast, Gujarat, India using thermal infrared remote sensing technique. Observations of brightness temperature (BT) in the thermal infrared spectral region (10.6–11.19 μm) from Landsat-8 satellite in the coastal region showed unique localized cooling in the Arabian Sea during winter. We observed lowering of BT in the range 0.6°– 2.3°C in the coastal region associated with SGD in comparison to sea surface temperature of the ocean during low-tide conditions. Consistent geographical pattern of thermal contrast was observed near the same location (lat. 22°26′54.43″N, long. 69°00′41.67″E) when multi date (11 datasets) thermal data were analysed between 2015 and 2019 in winter. Generally, low-tide conditions show more cooling of ocean surface at the SGD site compared to high-tide conditions, which indicates the process of SGD. Satellite-based assessment was further validated using field- and ship-based measurements.

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