Decomposing land surface total water storage in the Indus, Ganges, and Brahmaputra basins

Alexander A Prusevich^1*Richard Lammers¹Danielle Sarah Grogan¹Shantar Zuidema¹David D Meko²Isabella Velicogna^3,4

• ¹Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire, United States
• ²Laboratory of Tree-Ring Research, College of Science, University of Arizona, Tucson, Arizona, United States
• ³Department of Earth System Science, School of Physical Sciences, University of California, Irvine, Irvine, California, United States
• ⁴Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States

We compare changes in the macroscale hydrology of three important High Mountain Asian drainage basins: the Indus, Ganges and Brahmaputra through seasonal and long-term trends in total water storage (TWS). Using statistical time series analysis of nine individual TWS components modeled by a Water Balance Model that simulates both on and below the land surface water storage terms we find major drivers and causes of TWS decline. Even though long term TWS trends may look somewhat similar in the study basins, we find that the drivers and causes of the trends and its seasonal variability are fundamentally different in each basin. TWS decline in the Indus and Ganges watersheds are primarily driven by the depletion of aquifers (67 and 76 % respectively) due to irrigated land expansion and overuse of their water, which is partly offset by reaching well depth limits, and therefore an even greater decline of TWS in these basins is averted. The Brahmaputra has a lesser aquifer water use stress and the TWS drop is mostly due to the melting of glaciers (75 %) which exhibit the highest rate over all three basins. The Ganges and Brahmaputra have a quasi-monotonic decline of TWS without a sign of leveling up, and the Indus basin has a non-monotonic trend line of TWS due to different stages of its aquifer depletion relevant to aquifer water accessibility limited by the well depth thresholds. Seasonal variability is primarily controlled by soil moisture saturation, shallow groundwater levels, reservoir storage in the lowlands, and snow accumulation in the mountains for the Ganges and Brahmaputra basins, but the Indus is driven by high mountain cold region storage of snow and glaciers in the headwaters. The headwaters of the Indus and Brahmaputra rivers originate from sides of the Himalayan Mountain range (northwest and southeast) leading to distinct patterns in their TWS decline and seasonal variability. The Ganges basin, situated between these two regions, shows intermediate characteristics in TWS over time and in what influences these changes, reflecting a blend of the features seen in the Indus and Brahmaputra areas.