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A few decades ago, hydroelectric projects were considered a clean source of energy. The present study aims to assess the biomass and soil carbon (C) stock in the hydroelectric catchment in India and further, its implications to mitigate GHG emissions. The estimation depicts the low 'C' stock in the reservoir's catchment in comparison to reservoirs located globally in the same eco-region which will ultimately result in lower GHG emission in the future. Moreover, estimated total carbon density of the forest species was found as 54.92 ± 18.07 MgC.ha−1 while mean SOC was 79.64 ± 19.32 MgC.ha−1. The average total 'C' stock of Tehri hydroelectric reservoir's catchment was estimated as 134.56 MgC.ha−1. Moreover, ratios (total flooded carbon/installed capacity) comprised between 4 and304 kgC/W from the other reservoirs reported globally. These ratios are higher than the Tehri reservoir (0.28 kgC/W) as flooded area of Tehri reservoir during impoundment was 52 km2. Thus, Tehri reservoir was found to emit less GHG in time to come. The present study helps the hydropower industries, policymakers, and environmentalists in designing the catchment area treatment plan to minimize the negative impact of GHG on climate change. [ABSTRACT FROM AUTHOR]

Inland water bodies (particularly ponds) emit a significant amount of greenhouse gases (GHGs), particularly methane (CH4), carbon dioxide (CO2), and a comparatively low amount of nitrous oxide (N2O) to the atmosphere. In recent decades, ponds (<10,000 m2) probably account for about 1/3rd of the global lake perimeter and are considered a hotspot of GHG emissions. High nutrients and waterlogged conditions provide an ideal environment for CH4 production and emission. The rate of emissions differs according to climatic regions and is influenced by several biotic and abiotic factors, such as temperature, nutrients (C, N, & P), pH, dissolved oxygen, sediments, water depth, etc. Moreover, micro and macro planktons play a significant role in CO2 and CH4 emissions from ponds systems. Generally, in freshwater bodies, the produced N2O diffuses in the water and is converted into N2 gas through different biological processes. There are several other factors and mechanisms which significantly affect the CH4 and CO2 emission rate from ponds and need a comprehensive evaluation. This study aims to develop a decisive understanding of GHG emissions mechanisms, processes, and methods of measurement from ponds. Key factors affecting the emissions rate will also be discussed. This review will be highly useful for the environmentalists, policymakers, and water resources planners and managers to take suitable mitigation measures in advance so that the climatic impact could be reduced in the future. [ABSTRACT FROM AUTHOR]

Hydropower is one of the widely used renewable energy sources and contributing a huge share in overall electricity generation in developing countries like India. A few decades ago, hydropower reservoirs were found to be a clean source of renewable energy, but after the pre-industrial era (1990s), these reservoirs were emitting a significant amount of greenhouse gases (GHG). The GHG are contributing a huge share to the regional/global carbon budget. The current study represents the first regional review and focuses on: present scenarios of Indian hydropower, environmental issues, constraints in dam construction, identified key predictors (i.e., age, depth of reservoir) impacting GHG potential, GHG risk to reservoirs, and suggested mitigation measures to the way forward for the sustainable planning of hydropower development. The review reveals that most Indian hydropower reservoirs are coming under the low to medium GHG risk of reservoirs. Small hydropower projects (SHPs) are deliberated as a worthy alternative for the sustainable management of hydropower projects. There are still many issues associated with SHPs, which cannot be neglected completely. This study provides baseline information to the hydropower industries, policy-makers, environmentalists, and water managers to take corrective measures before impoundment of a reservoir at the regional and global scale. [Display omitted] • This is the first review summarises GHG emission from Indian Hydropower. • Indian hydropower are coming under the low to medium GHG risk of reservoirs. • Small hydro is deliberated as a worthy alternative for the sustainable management of hydropower projects. • Research is urgently needed in tropical eco-regions to know the GHG risk to reservoirs. • Proposed mitigation strategies for carbon deduction in future dam constructions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water pollution severely impacts the GHG emissions from riverine ecosystems. • GHG emissions from Riverine ecosystem follows: CO 2 > CH 4 > N 2 O. • High nutrient loading and eutrophication enhanced the GHG emissions in rivers. • Riverine CTLI values were positively correlated with CO 2 and CH 4 fluxes. Many recent studies have revealed that polluted rivers are emerging as potent emitters of greenhouse gases (GHGs). However, the scientific understanding of GHG emissions is still limited. The present review aims to provide a systematic analysis of GHG emissions from polluted rivers in various eco-regions. The analysis highlighted that high nutrient loadings such as total nitrogen (TN), organic carbon (OC), total phosphorous (TP), and the anoxic condition of rivers are the major influencing factors for CO 2 , N 2 O, and CH 4 emissions. Moreover, a strong positive correlation of trophic level index (TLI) with CH 4 and CO 2 fluxes was observed. The state-of-art suggested that based on land use and water quality, GHG fluxes from different rivers vary considerably. For instance, the reported N 2 O fluxes ranged between 0.007 and 3.721 mmol m-2d-1 for urban-impacted rivers and 0.009–0.049 mmol m-2d-1 for agricultural rivers. The CH 4 fluxes ranged between 0.001 and 458 mmol m-2d-1, 0.05–1.02 mmol m-2d-1, and 1.40–14.30 mmol m-2d-1 for urban impacted, agricultural and natural rivers, respectively. Further reported CO 2 fluxes range between 19.94 and 895 mmol m-2d-1 for urban-impacted rivers. Overall the in-depth analysis of the existing state of the art revealed that water pollution (i.e. eutrophication) enhances the GHG emissions from rivers; therefore, incorporating effective restoration measures can mitigate the intensity of GHG emissions from rivers and assist in achieving sustainable development goals (SDGs). [ABSTRACT FROM AUTHOR]