Land Cover Dynamics on the Lower Ganges–Brahmaputra Delta: Agriculture–Aquaculture Transitions, 1972–2017

Abstract

Aquaculture in tropical and subtropical developing countries has expanded in recent years. This practice is controversial due to its potential for serious economic, food security, and environmental impacts—especially for intensive operations in and near mangrove ecosystems, where many shrimp species spawn. While considerable effort has been directed toward understanding aquaculture impacts, maps of spatial extent and multi-decade spatiotemporal dynamics remain sparse. This is in part because aquaculture ponds (ghers) can be challenging to distinguish from other shallow water targets on the basis of water-leaving radiance alone. Here, we focus on the Lower Ganges–Brahmaputra Delta (GBD), one of the most expansive areas of recent aquaculture growth on Earth and adjacent to the Sundarbans mangrove forest, a biodiversity hotspot. We use a combination of MODIS 16-day EVI composites and 45 years (1972–2017) of Landsat observations to characterize dominant spatiotemporal patterns in the vegetation phenology of the area, identify consistent seasonal optical differences between flooded ghers and other land uses, and quantify the multi-decade expansion of standing water bodies. Considerable non-uniqueness exists in the spectral signature of ghers on the GBD, propagating into uncertainty in estimates of spatial extent. We implement three progressive decision boundaries to explicitly quantify this uncertainty and provide liberal, moderate, and conservative estimates of flooded gher extent on three different spatial scales. Using multiple extents and multiple thresholds, we quantify the size distribution of contiguous regions of flooded gher extent at ten-year intervals. The moderate threshold shows standing water area within Bangladeshi polders to have expanded from less than 300 km2 in 1990 to over 1400 km2 in 2015. At all three scales investigated, the size distribution of standing water bodies is increasingly dominated by larger, more interconnected networks of flooded areas associated with aquaculture. Much of this expansion has occurred in immediate proximity to the Bangladeshi Sundarbans.

Deep Learning Methods of Satellite Image Processing for Monitoring of Flood Dynamics in the Ganges Delta, Bangladesh

“…The following 10 land cover types were identified around the Ganges River Delta, Bangladesh: (1) water, (2) wetlands and mudflats, (3) mangrove forests, (4) sandy areas, (5) forests, (6) croplands, (7) grasslands, (8) urban settlements, (9) orchards and (10) aquaculture. Information on land cover types was derived from Rahman et al [112], Sousa and Small [113] and Paszkowski et al [114] and applied to the extent of the current study area. The presented maps of floods can be used as a broad base for environmental applications and flood hazard assessment.…”

Assessing 30-Year Land Use and Land Cover Change and the Driving Forces in Qianjiang, China, Using Multitemporal Remote Sensing Images

“…They found that the aquaculture areas experienced a rapidly increasing period from 1990 to 2011, a stable period from 2011 to 2017, and a sharply shrinking period after 2017. Sousa and Small analyzed land cover dynamics on the lower Ganges-Brahmaputra Delta from 1972 to 2017 using a combination of MODIS composites and Landsat images [17]. Results showed that standing water area within Bangladesh has expanded from less than 300 km 2 in 1990 to over 1400 km 2 in 2015, mainly caused by the increased interconnected networks of flooded areas associated with aquaculture.…”