Water Storage Trends in High Mountain Asia

Changes in terrestrial water storage (TWS) in High Mountain Asia (HMA) could havemajor
societal impacts, as the region’s large reservoirs of glaciers, snow, and groundwater
provide a freshwater source to more than one billion people. We seek to quantify and
close the budget of secular changes in TWS over the span of the GRACE satellitemission
(2003–2016). To assess the TWS trend budget we consider a new high-resolution mass
trend product determined directly from GRACE L1B data, glacier mass balance derived
from Digital Elevation Models (DEMs), groundwater variability determined from confined
and unconfined well observations, and terrestrial water budget estimates from a suite of
land surface model simulations with the NASA Land Information System (LIS). This effort
is successful at closing the aggregated TWS trend budget over the entire HMA region, the
glaciated portion of HMA, and the Indus and Ganges basins, where the full-region trends
are primarily due to the glacier mass balance and groundwater signals. Additionally,
we investigate the closure of TWS trends at individual 1-arc-degree mascons (area
≈12,000 km2); a significant improvement in spatial resolution over previous analyses
of GRACE-derived trends. This mascon-level analysis reveals locations where the TWS
trends are well-explained by the independent datasets, as well as regions where they are
not; identifying specific geographic areas where additional data andmodel improvements
are needed. The accurate characterization of total TWS trends and its components
presented here is critical to understanding the complex dynamics of the region, and
is a necessary step toward projecting future water mass changes in HMA.