Multi-faceted analyses of seasonal trends and drivers of land surface variables in Indo-Gangetic river basins

Abstract

High mountain Asia holds the largest glacier mass on Earth outside the polar regions. In combination with monsoonal rainfall, glacial meltwater provides the downstream Indo-Gangetic river basins with indispensable water resources. These rivers supply 1.2 billion people with water and their water resources are essential for many sectors including the world's largest connected irrigated cropland and domestic needs. In this context, the quantification of land surface dynamics and driving factors is of particular interest to enhance the understanding of interplays between multiple spheres. This study investigates multivariate time series including climatic, hydrological, and remote sensing-based land surface and anthropogenic variables. For the first time, we generate dense time series of multiple remote sensing-based land surface variables at biweekly intervals and employ statistical time series analyses at seasonal and annual scale. We analyze trends and controlling variables in consideration of temporal lags using causal networks between December 2002 and November 2020. The results show increasing trends in vegetation greenness being accelerated during the second half of the study period. Furthermore, a mismatch between trends in surface water area and terrestrial water storage was identified. Considering snow cover area, trends remain mostly insignificant at basin scale suggesting no change during the 18 years. At seasonal and grid scale, analyses indicate negative trends over the Upper Ganges and Brahmaputra river basins particularly during the monsoon and post-monsoon season. Furthermore, the causal networks revealed so far unexplored interactions among multivariate feature spaces at seasonal scale. For example, vegetation greenness is constrained by water availability, surface water area is influenced by river discharge, snow cover, and temperature, whereas snow cover area is largely controlled by precipitation and temperature with spatial and temporal variations. To summarize, these findings provide valuable insights with respect to the trends and interactions between various spheres in the Indo-Gangetic river basins.