Spatiotemporal Dynamics and Drivers of Water Use Efficiency in Northeast India: Implications for Forest Ecosystems Under Climate Change

Water Use Efficiency (WUE) is a key metric for evaluating the carbon-water coupling in forest ecosystems, offering crucial insights into how vegetation responds to a changing climate. This study explores the spatiotemporal dynamics of WUE across different forest types in Northeast India from 2001 to 2020, using MODIS Gross Primary Productivity (GPP) and Evapotranspiration (ET) data processed through Google Earth Engine. The long-term average WUE was found to be around $1.78 \mathrm{gC} \mathrm{kg}^{-1} \mathrm{H}_{2} \mathrm{O}$. A combination of Thiel Sen’s Median Trend and MannKendall tests revealed 56.46% of the region to be experiencing an increase in WUE, signalling rising water stress, while 43.54% showed a decreasing trend. Soil moisture emerged as the most significant driver of WUE variability, though Daytime Land Surface Temperature (LST) was dominant in the Himalayan regions. These findings highlight the growing water stress in the region, particularly in the plains, and point to significant shifts in the carbon and water cycles under climate change, emphasizing the need for immediate mitigation efforts.