Abstract
Nearly one-fifth of the Earth's accessible freshwater is stored in the densely-populated, alluvial floodplains of the Brahmaputra, Ganges, Indus, Irrawaddy and Meghna River Systems in the Himalayan region where extreme hydrological conditions exist due to the seasonal variability in terrestrial water storage (TWS). Groundwater storage (GWS) – a hidden resource underneath the land surface, plays a critical role in sustaining irrigated agriculture in these river basins, particularly during the dry season when rice crops are generally grown in irrigated lands across South Asia. Although monitoring of groundwater levels has been operational in the region for a number of decades, a basin-wide comprehensive assessment of GWS is lacking in most river basins. The NASA's Gravity Recovery and Climate Experiment (GRACE) twin satellites offer an opportunity to map basin-wide changes in GWS where in-situ observations are limited in time and space. GRACE-derived assessments of GWS vary substantially in these basins and have not been reconciled with in-situ observations in most cases. Recent declining trends in GWS over the Himalayan river basins are attributed primarily to over-abstraction of groundwater due to dry-season irrigation. Seasonal variability in terrestrial water is likely to increase or become unpredictable in the future as a result of increased climate variability. The consequent impacts may potentially threaten the security of water supply and food in the region, where there is currently a growing demand for food grains from irrigated agriculture, energy, and domestic and industrial water supplies.
Highlights
Freshwater represents ~2.5% of the Earth's total water content
In the Himalayan river basins in South and Southeast Asia have been a focus of many recent studies that primarily depend on remote-sensing data such as Gravity Recovery and Climate Experiment (GRACE) gravity observations to map trends in terrestrial water storage (TWS) and its critical components such as groundwater storage (GWS)
Auxiliary information on soil moisture storage in disaggregating GRACE-derived TWS for GWS comes primarily from global-scale land surface models that are rarely calibrated with any ground-based observations
Summary
Freshwater represents ~2.5% of the Earth's total water content. Approximately 30% of this freshwater is stored as groundwater and the rest is stored in glaciers and ice caps (68.6%), snow and ice (0.95%), surface-water bodies (0.3%) (i.e., rivers, lakes and wetlands), soil (~0.05%), and atmosphere (~0.03%) [1]. Fresh groundwater and other individual freshwater stores combined with an additional 1% saline water store (i.e. saline lakes and groundwater) collectively represent the total terrestrial water storage (TWS) on the Earth's surface. Intensive and unsustainable use of groundwater in South Asia, in northern India, Pakistan, and central and northwestern Bangladesh, has led to rapid depletion of groundwater storage in recent. Despite the importance of having reliable estimates of TWS, the knowledge about the spatial and temporal variations and its individual components is generally lacking This is true at large, global to regional scales due to the absence of any dedicated globalscale monitoring network of individual water stores (i.e., surface water, soil moisture, and groundwater). We analyse precipitation and GRACE satellite datasets over the Himalayan river basins in order to provide an updated picture of changes in terrestrial water storage of the region
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