An Integrated Assessment of Carbon and Water Use Efficiencies over India using Remote Sensing

Abstract

Carbon use efficiency (CUE) and water use efficiency (WUE) are essential characteristics of ecosystem functioning to understand the strength of carbon allocated by the vegetation as biomass from the sequestered atmospheric carbon and the strength of the linkages between the terrestrial carbon and water cycles, respectively. Knowledge of CUE and WUE at a fine spatiotemporal scale is vital for sustainable management, especially in a country like India, which exhibits diverse ecological and geographical gradients. Remote sensing offers continuous temporal monitoring and a spatially comprehensive understanding of CUE and WUE, making it a valuable tool for understanding the CUE and WUE at finer scales. Most previous studies have attempted to understand the impacts on the Indian ecosystem's carbon and water cycles using CUE and WUE separately. However, there is a lack of research on integrating CUE and WUE in a single framework. Such an integrated approach holds prominence in interpreting the tradeoffs and synergies between ecosystem carbon sequestration and water loss under variable climate and anthropogenic stress conditions prominent in the Indian context. This thesis aims to assess the spatiotemporal variability of CUE and WUE using remote sensing, focusing on comprehending the trends and drivers.

The knowledge of sub-annual scale CUE is limited at a finer spatial scale due to a lack of insights on the net primary productivity. The thesis includes the development of a sub-annual (8-day scale) CUE product using an empirical framework that significantly correlates with in-situ-based data across India. Furthermore, the spatial patterns, role of drivers, sustainability, and resilience of vegetation carbon stocks are identified using CUE as a proxy. The results indicate a substantial control of soil moisture-temperature interplay and land management practices on CUE across India. A focus on the impact of the water table depth (WTD) dynamics on India's CUE and WUE is necessary, especially in the background of groundwater depletion across Indian landscapes. The standalone impact of WTD on CUE and WUE is considered, and the findings reveal that a combination of the regional climate, aquifer geology, and land management practices determine the WTD responses to the CUE and WUE. Moreover, a holistic understanding of India's long-term [1982-2018] ecosystem health is performed by combining the trends and resilience of CUE and WUE and examining the influences of water deficit on CUE and WUE. The results indicate an improvement in India's ecosystem health after the start of the 21st century, possibly due to a paradigm shift in irrigation technologies across India; however, a string of drought events resulted in exacerbated ecosystem health. The insights gained from this thesis can aid ecosystem managers and policymakers in identifying the hotspots of ecosystem degradation and providing sustainable management practices to optimize the carbon uptake at the cost of the most negligible water loss for India.