Groundwater storage dynamics in the world’s large aquifer systems from GRACE: uncertainty and role of extreme precipitation

Under variable and changing climates groundwater storage sustains vital ecosystems and enables freshwater withdrawals globally for agriculture, drinking water, and industry. Here, we assess recent changes in groundwater storage (ΔGWS) from 2002 to 2016 in 37 of the world's large aquifer systems using an ensemble of datasets from the Gravity Recovery and Climate Experiment (GRACE) and land surface models (LSMs). Ensemble GRACE-derived ΔGWS is well reconciled to in situ observations (r=0.62–0.86, p value 0.5, p value 90th percentile, 1901–2016) precipitation and is inconsistent with prevailing narratives of global-scale groundwater depletion at the scale of the GRACE footprint (∼200 000 km^{2}). Substantial uncertainty remains in estimates of GRACE-derived ΔGWS, evident from 20 realisations presented here, but these data provide a regional context to changes in groundwater storage observed more locally through piezometry

Spatio-temporal changes in terrestrial water storage in the Himalayan river basins and risks to water security in the region: a review

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

Groundwater storage dynamics in the world's large aquifer systems from GRACE: uncertainty and role of extreme precipitation

Under variable and changing climates groundwater storage sustains vital ecosystems and enables freshwater withdrawals globally for agriculture, drinking water, and industry. Here, we assess recent changes in groundwater storage (ΔGWS) from 2002 to 2016 in 37 of the world's large aquifer systems using an ensemble of datasets from the Gravity Recovery and Climate Experiment (GRACE) and land surface models (LSMs). Ensemble GRACE-derived ΔGWS is well reconciled to in situ observations (r=0.62–0.86, p value 0.5, p value 90th percentile, 1901–2016) precipitation and is inconsistent with prevailing narratives of global-scale groundwater depletion at the scale of the GRACE footprint (∼200 000 km2). Substantial uncertainty remains in estimates of GRACE-derived ΔGWS, evident from 20 realisations presented here, but these data provide a regional context to changes in groundwater storage observed more locally through piezometry

Climate change driven disaster risks in Bangladesh and its journey towards resilience

Globally, disasters from natural and anthropogenic hazards or humanitarian crises can reverse development gains and weaken resilience. In recent years, some countries have made significant progress towards building resilience to disaster risks, including those driven by the climate crisis. Bangladesh is a leading example as it is well-known as one of the most vulnerable countries for its multifaceted hazard risks projected to intensity under climate change. Today, the scale of loss of human life from both rapid and slow-onset disasters (e.g. cyclone, flood and drought) is significantly lower than in the 1970s. This remarkable achievement was made possible by independence and the government’s proactive investment in development and societal changes through education, technologies and reduction in poverty and inequalities. However, the climate crisis is threatening these development and disaster risk reduction gains. In addition, disaster displacement is a major challenge. The COVID-19 pandemic has unveiled both strengths and weaknesses in our societies. The article argues that disaster management plans need to adapt to the climate crisis and human displacement and reduce migrants’ vulnerability while responding to infectious disease transmission

Groundwater dynamics and arsenic mobilisation in Bangladesh: a national-scale characterisation

Elevated arsenic (As) concentrations in groundwater-fed drinking water supplies in Bangladesh are a major public health problem but the hydrogeological conditions that give rise to the mobilisation and regional-scale distribution of As in shallow groundwater remain unknown. Published hypotheses developed from highly localised case studies are, to date, untested regionally and contradictory. My doctoral thesis makes a novel and substantial contribution to knowledge of the relationship between groundwater dynamics and As mobilisation in the Bengal Basin by (1) characterising national-scale groundwater storage dynamics and recharge processes in the shallow aquifer of Bangladesh and (2) relating statistically static and dynamic hydrogeological factors to the observed variation of As concentrations in groundwater. After constructing a national database of shallow groundwater levels from a network of 1267 monitoring stations, robust statistical techniques are applied to characterise long-term (1985 to 2005) trends and seasonality in groundwater levels, net recharge, and groundwater storage; the latter is supported by analysis of remotely sensed data derived from GRACE (Gravity Recovery and Climate Experiment). These characterisations highlight the critical influence of groundwater abstraction on net recharge to the shallow aquifer. Net annual recharge in Bangladesh has increased in response to intensive abstraction challenging conventional definitions of "safe yield". To examine the national-scale variability in groundwater As concentrations generalised regression models were constructed using geology and hydrological factors. Crucially, these models reveal that areas of increasing groundwater-fed irrigation and net recharge are associated with lower As concentrations. These findings are inconsistent with current hypotheses that contend irrigation-induced recharge mobilises groundwater As in shallow aquifers. Inverse associations between As concentrations and both mean annual recharge and trends in groundwater-fed irrigation suggest that As has been actively flushed from the shallow aquifer as a result of recently increased net recharge induced by intensive irrigation in Bangladesh

Monitoring groundwater storage changes in the highly seasonal humid tropics: Validation of GRACE measurements in the Bengal Basin

International audienceSatellite monitoring of changes in terrestrial water storage provides invaluable information regarding the basin-scale dynamics of hydrological systems where ground-based records are limited. In the Bengal Basin of Bangladesh, we test the ability of satellite measurements under the Gravity Recovery and Climate Experiment (GRACE) to trace both the seasonality and trend in groundwater storage associated with intensive groundwater abstraction for dry-season irrigation and wet-season (monsoonal) recharge. We show that GRACE (CSR, GRGS) datasets of recent (2003 to 2007) groundwater storage changes (ΔGWS) correlate well (r = 0.77 to 0.93, p value < 0.0001) with in situ borehole records from a network of 236 monitoring stations and account for 44% of the total variation in terrestrial water storage (ΔTWS) highest correlation (r = 0.93, p value < 0.0001) and lowest root-mean-square error (<4 cm) are realized using a spherical harmonic product of CSR. Changes in surface water storage estimated from a network of 298 river gauging stations and soil-moisture derived from Land Surface Models explain 22% and 33% of ΔTWS, respectively. Groundwater depletion estimated from borehole hydrographs (-0.52 ± 0.30 km3 yr-1) is within the range of satellite-derived estimates (-0.44 to -2.04 km3 yr-1) that result from uncertainty associated with the simulation of soil moisture (CLM, NOAH, VIC) and GRACE signal-processing techniques. Recent (2003 to 2007) estimates of groundwater depletion are substantially greater than long-term (1985 to 2007) mean (-0.21 ± 0.03 km3 yr-1) and are explained primarily by substantial increases in groundwater abstraction for the dry-season irrigation and public water supplies over the last two decades

Climate–groundwater dynamics inferred from GRACE and the role of hydraulic memory

Groundwater is the largest store of freshwater on Earth after the cryosphere and provides a substantial proportion of the water used for domestic, irrigation and industrial purposes. Knowledge of this essential resource remains incomplete, in part, because of observational challenges of scale and accessibility. Here we examine a 14-year period (2002–2016) of Gravity Recovery and Climate Experiment (GRACE) observations to investigate climate–groundwater dynamics of 14 tropical and sub-tropical aquifers selected from WHYMAP's (Worldwide Hydrogeological Mapping and Assessment Programme) 37 large aquifer systems of the world. GRACE-derived changes in groundwater storage resolved using GRACE Jet Propulsion Laboratory (JPL) mascons and the Community Land Model's land surface model are related to precipitation time series and regional-scale hydrogeology. We show that aquifers in dryland environments exhibit long-term hydraulic memory through a strong correlation between groundwater storage changes and annual precipitation anomalies integrated over the time series; aquifers in humid environments show short-term memory through strong correlation with monthly precipitation. This classification is consistent with estimates of groundwater response times calculated from the hydrogeological properties of each system, with long (short) hydraulic memory associated with slow (rapid) response times. The results suggest that groundwater systems in dryland environments may be less sensitive to seasonal climate variability but vulnerable to long-term trends from which they will be slow to recover. In contrast, aquifers in humid regions may be more sensitive to climate disturbances such as drought related to the El Niño–Southern Oscillation but may also be relatively quick to recover. Exceptions to this general pattern are traced to human interventions through groundwater abstraction. Hydraulic memory is an important factor in the management of groundwater resources, particularly under climate change

Groundwater recharge from heavy rainfall in the southwestern Lake Chad Basin: evidence from isotopic observations

We examine groundwater recharge processes and their relationship to rainfall intensity in the semi-arid, southwestern Lake Chad Basin of Nigeria using a newly compiled database of stable isotope data (δ2H, δ18O) from groundwater and rainfall. δ18O signatures in groundwater proximate to surface waters are enriched in 18O relative to regional rainfall and trace focused groundwater recharge from evaporated waters via ephemeral river discharge and Lake Chad; groundwater remote from river channels is comparatively depleted and associated with diffuse recharge, often via sand dunes. Stable isotope ratios of O and H (δ2H, δ18O) in groundwater samples regress to a value along the local meteoric waterline that is depleted relative to weighted mean composition of rainfall, consistent with rainfall exceeding the 60th percentile of monthly precipitation intensity. The observed bias in groundwater recharge to heavy monthly rainfall suggests that the intensification of tropical rainfall under global warming favours groundwater recharge in this basin