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Hydrological models and the data derived from the Gravity Recovery and Climate Experiment (GRACE) satellite mission have been widely used to study the variations in terrestrial water storage (TWS) over large regions. However, both GRACE products and model results suffer from inherent uncertainties, calling for the need to make a combined use of GRACE and models to examine the variations in total TWS and their individual components, especially in relation to natural and human-induced changes in the terrestrial water cycle. In this study, we use the results from two state-of-the-art hydrological models and different GRACE spherical harmonic products to examine the variations in TWS and its individual components, and to attribute the changes to natural and human-induced factors over large global river basins. Analysis of the spatial patterns of the long-term trend in TWS from the two models and GRACE suggests that both models capture the GRACE-measured direction of change, but differ from GRACE as well as each other in terms of the magnitude over different regions. A detailed analysis of the seasonal cycle of TWS variations over 30 river basins shows notable differences not only between models and GRACE but also among different GRACE products and between the two models. Further, it is found that while one model performs well in highly-managed river basins, it fails to reproduce the GRACE-observed signal in snow-dominated regions, and vice versa. The isolation of natural and human-induced changes in TWS in some of the managed basins reveals a consistently declining TWS trend during 2002–2010, however; significant differences are again obvious both between GRACE and models and among different GRACE products and models. Results from the decomposition of the TWS signal into the general trend and seasonality indicate that both models do not adequately capture both the trend and seasonality in the managed or snow-dominated basins implying that the TWS variations from a single model cannot be reliably used for all global regions. It is also found that the uncertainties arising from climate forcing datasets can introduce significant additional uncertainties, making direct comparison of model results and GRACE products even more difficult. Our results highlight the need to further improve the representation of human land-water management and snow processes in large-scale models to enable a reliable use of models and GRACE to study the changes in freshwater systems in all global regions. [ABSTRACT FROM AUTHOR]

Aquifer overexploitation could significantly impact crop production in the United States because 60% of irrigation relies on groundwater. Groundwater depletion in the irrigated High Plains and California Central Valley accounts for &sim:50% of groundwater depletion in the United States since 1900. A newly developed High Plains recharge map shows that high recharge in the northern High Plains results in sustainable pumpage, whereas lower recharge in the central and southern High Plains has resulted in focused depletion of 330 km3 of fossil groundwater, mostly recharged during the past 13,000 y. Depletion is highly localized with about a third of depletion occurring in 4% of the High Plains land area. Extrapolation of the current depletion rate suggests that 35% of the southern High Plains will be unable to support irrigation within the next 30 y. Reducing irrigation withdrawals could extend the lifespan of the aquifer but would not result in sustainable management of this fossil groundwater. The Central Valley is a more dynamic, engineered system, with north/south diversions of surface water since the 1950s contributing to &sim:7x higher recharge. However, these diversions are regulated because of impacts on endangered species. A newly developed Central Valley Hydrologie Model shows that groundwater depletion since the 1960s, totaling 80 km3, occurs mostly in the south (Tulare Basin) and primarily during droughts. Increasing water storage through artificial recharge of excess surface water in aquifers by up to 3 km3 shows promise for coping with droughts and improving sustainability of groundwater resources in the Central Valley. [ABSTRACT FROM AUTHOR]

Observations of changes in terrestrial water storage (TWS) obtained from the satellite mission GRACE (Gravity Recovery and Climate Experiment) have frequently been used for water cycle studies and for the improvement of hydrological models by means of calibration and data assimilation. However, due to a low spatial resolution of the gravity field models, spatially localized water storage changes, such as those occurring in lakes and reservoirs, cannot properly be represented in the GRACE estimates. As surface storage changes can represent a large part of total water storage, this leads to leakage effects and results in surface water signals becoming erroneously assimilated into other water storage compartments of neighbouring model grid cells. As a consequence, a simple mass balance at grid/regional scale is not sufficient to deconvolve the impact of surface water on TWS. Furthermore, non-hydrology-related phenomena contained in the GRACE time series, such as the mass redistribution caused by major earthquakes, hamper the use of GRACE for hydrological studies in affected regions. In this paper, we present the first release (RL01) of the global correction product RECOG (REgional COrrections for GRACE), which accounts for both the surface water (lakes and reservoirs, RECOG-LR) and earthquake effects (RECOG-EQ). RECOG-LR is computed from forward-modelling surface water volume estimates derived from satellite altimetry and (optical) remote sensing and allows both a removal of these signals from GRACE and a relocation of the mass change to its origin within the outline of the lakes/reservoirs. The earthquake correction, RECOG-EQ, includes both the co-seismic and post-seismic signals of two major earthquakes with magnitudes above Mw9. We discuss that applying the correction dataset (1) reduces the GRACE signal variability by up to 75 % around major lakes and explains a large part of GRACE seasonal variations and trends, (2) avoids the introduction of spurious trends caused by leakage signals of nearby lakes when calibrating/assimilating hydrological models with GRACE, and (3) enables a clearer detection of hydrological droughts in areas affected by earthquakes. A first validation of the corrected GRACE time series using GPS-derived vertical station displacements shows a consistent improvement of the fit between GRACE and GNSS after applying the correction. Data are made available on an open-access basis via the Pangaea database (RECOG-LR: Deggim et al., 2020a, 10.1594/PANGAEA.921851; RECOG-EQ: Gerdener et al., 2020b, 10.1594/PANGAEA.921923). [ABSTRACT FROM AUTHOR]

We develop a Matlab program named LAPS (Lagrangian Advection of Particles at Sea) to simulate the advection of suspended particles in the global ocean with a minimal user effort to install, set and run the simulations. LAPS uses the 3D sea current velocity fields provided by ECCO2 to track the fate of suspended particles injected in the ocean, at specific places and times, during a period of time. LAPS runs with a short configuration file set by the user and returns the distribution of the particles at the end of the advection. A continuous tracking option is also available to record the complete trajectory of the particles throughout the entire period of advection. The effect of water waves, or Stokes drift, which alter sea surface current velocities, can also be taken into account. The principle and usage of the program is detailed and then applied to three case studies. The first two cases studies are applied to suspended sediment transport. We show how LAPS simulations can be used to investigate the spatio-temporal distribution of fine particles observed by satellites in the upper ocean. We also estimated sediment deposit areas on the seafloor as a function of sediment grain sizes. The third case study simulates the dispersion of microplastic particles during a tropical cyclone, and shows how the Stokes drift, which is significant during storm events, alters the particles trajectories compared to the case where the Stokes drift is neglected. [ABSTRACT FROM AUTHOR]

• Geostatistical inversion of head and flux data recorded under steady-state conditions. • For few observations, flux data is more informative than head data. • For many observations, the inversion results are comparable regardless of data type. • For equal number of observations, individual and joint inversion perform similarly. • The pumping borehole boundary condition affects individual, but not joint inversion. Hydraulic tomography is a state-of-the-art method for inferring hydraulic conductivity fields using head data. We employed geostatistical inversion using synthetically generated head and flux data individually and jointly in a steady-state experiment. We designed 96 inversion scenarios to better understand the relative merits of each data type. For the typical case of a small number of observation points, we find that flux data provide a better resolved hydraulic conductivity field compared to head data when considering data with similar signal-to-noise ratios. This finding is further confirmed by a resolution analysis. When considering a high number of observation points, the estimated fields are of similar quality regardless of the data type. In terms of borehole boundary conditions, the best setting for flux and head data are constant head and constant rate, respectively, while joint inversion results are insensitive to the borehole boundary type. When considering the same number of observations, the joint inversion of head and flux data does not offer advantages over individual inversions. When considering the same number of observation points and, hence, twice as many observations, the joint inversion performs better than individual inversions. The findings of this paper are useful for future planning and design of hydraulic tomography tests comprising flux and head data. [ABSTRACT FROM AUTHOR]

[Display omitted] • Distributed Temperature sensing is used to monitor borehole flows in fractured media. • Fast analytical solutions are used to invert the temperature profile into flow profile • The potential of DTS data for monitoring the dynamics of fractured media is shown. Fractured aquifers are known to be very heterogeneous with complex flow path geometries. Their characterization and monitoring remain challenging despite the importance to better understand their behavior at all spatial and temporal scales. Heat and correspondingly temperature data have gained much interest in recent years and are often used as a tracer for characterizing groundwater flows. In the current work, a fast computer code is developed using Ramey and Hassan and Kabir analytical solutions which converts the temperature profile to the flow rate profile along the borehole. The method developed is validated through numerical simulations. A global sensitivity study recognizes the media thermal properties as the most influential parameters. For testing the method in the field, fiber-optic distributed temperature sensing (FO-DTS) data were used to monitor the dynamic behavior of fractured aquifers at the borehole scale at the Ploemeur-Guidel field site in Brittany, France. DTS data are used to infer the flow rates in the different sections of a fractured wellbore (flow profile) and calculate the contribution of each fracture to the total flow. DTS data were acquired for about three days in three different hydraulic conditions corresponding to two different ambient flow conditions and one pumping condition. Flow profiling using distributed temperature data matches satisfactorily with results from heat-pulse flow metering performed in parallel for cross-checking. Moreover, flow profiling reveals the daily variations of ambient flow in this fractured borehole. Furthermore, it shows that during ambient flowing conditions, shallow and deep fractures contribute roughly equally to the total flow while during the pumping condition, the deepest fractures contribute more to the total flow, suggesting a possible reorganization of flow and hydraulic heads depending on the hydraulic conditions. Thus, although the proposed method (DTS data and proposed framework) may be costlier and is based on indirect characterization through temperature measurements, it provides real-time monitoring of complex fracture interactions and recharge processes in fractured media. Thus, this method allows for a full analysis of the temporal behavior of the system with a simple and fast analytical model. Furthermore, thanks to its narrow width, DTS can be used and installed in boreholes for long-term monitoring while heat-pulse flow metering may lead to head losses in the borehole and may not be always possible depending on some borehole conditions. One of the limitations of the approach proposed is the proper knowledge of the thermal properties of media required to infer the flow rate from the temperature. Nevertheless, surface rate measurement can be useful to constrain these properties and reduce the flow profiling uncertainty. Thus, the method proposed appears to be an interesting and complementary method for characterizing borehole flows and groundwater dynamics in fractured media such as for instance, monitoring the recharge dynamic. [ABSTRACT FROM AUTHOR]

Observations of changes in terrestrial water storage obtained from the satellite mission GRACE (Gravity Recovery and Climate Experiment) have frequently been used for water cycle studies and for the improvement of hydrological models by means of calibration and data assimilation. However, due to a low spatial resolution of the gravity field models spatially localized water storage changes, such as those occurring in lakes and reservoirs, cannot properly be represented in the GRACE estimates. As surface storage changes can represent a large part of total water storage, this leads to leakage effects and results in surface water signals becoming erroneously assimilated into other water storage compartments of neighboring model grid cells. As a consequence, a simple mass balance at grid/regional scale is not sufficient to deconvolve the impact of surface water on TWS. Furthermore, non-hydrology related phenomena contained in the GRACE time series, such as the mass redistribution caused by major earthquakes, hamper the use of GRACE for hydrological studies in affected regions. In this paper, we present the first release (RL01) of the global correction product RECOG (REgional COrrections for GRACE), which accounts for both the surface water (lakes & reservoirs, RECOG-LR) and earthquake effects (RECOG-EQ). RECOG-LR is computed from forward-modelling surface water volume estimates derived from satellite altimetry and (optical) remote sensing and allows both a removal of these signals from GRACE and a re-location of the mass change to its origin within the outline of the lakes/reservoirs. The earthquake correction RECOG-EQ includes both the co-seismic and post-seismic signals of two major earthquakes with magnitudes above 9 Mw. We can show that applying the correction dataset (1) reduces the GRACE signal variability by up to 75 % around major lakes and explains a large part of GRACE seasonal variations and trends, (2) avoids the introduction of spurious trends caused by leakage signals of nearby lakes when calibrating/assimilating hydrological models with GRACE, even in neighboring river basins, and (3) enables a clearer detection of hydrological droughts in areas affected by earthquakes. A first validation of the corrected GRACE time series using GPS-derived vertical station displacements shows a consistent improvement of the fit between GRACE and GNSS after applying the correction. Data are made available as open access via the Pangea database (RECOG-LR: Deggim et al. (2020a) https://doi.org/10.1594/PANGAEA.921851; RECOG-EQ: Gerdener et al. (2020b, under revision), https://doi.pangaea.de/10.1594/PANGAEA.921923). [ABSTRACT FROM AUTHOR]

The accurate quantification of sediment mass redistribution is central to the study of surface processes, yet it remains a challenging task. Here we test a new combination of terrestrial gravity and drone photogrammetry methods to quantify sediment mass redistribution over a 1 km2 area. Gravity and photogrammetry are complementary methods. Indeed, gravity changes are sensitive to mass changes and to their location. Thus, by using photogrammetry data to constrain this location, the sediment mass can be properly estimated from the gravity data. We carried out three joint gravimetry–photogrammetry surveys, once a year in 2015, 2016 and 2017, over a 1 km2 area in southern Taiwan, featuring both a wide meander of the Laonong River and a slow landslide. We first removed the gravity changes from non-sediment effects, such as tides, groundwater, surface displacements and air pressure variations. Then, we inverted the density of the sediment with an attempt to distinguish the density of the landslide from the density of the river sediments. We eventually estimate an average loss of 3.7 ± 0.4 × 10 9 kg of sediment from 2015 to 2017 mostly due to the slow landslide. Although the gravity devices used in this study are expensive and need week-long surveys, new instrumentation currently being developed will enable dense and continuous measurements at lower cost, making the method that has been developed and tested in this study well-suited for the estimation of erosion, sediment transfer and deposition in landscapes. [ABSTRACT FROM AUTHOR]

We investigated the mixing and dynamic of denitrification processes induced by long-term pumping in the crystalline aquifer of Ploemeur (Brittany, France). Hydrological and geochemical parameters have been continuously recorded over 15 boreholes in 5 km 2 on a 25-year period. This extensive spatial and temporal monitoring of conservative as well as reactive compounds is a key opportunity to identify aquifer-scale transport and reactive processes in crystalline aquifers. Time series analysis of the conservative elements recorded at the pumped well were used to determine mixing fractions from different compartments of the aquifer on the basis of a Principal Component Analysis approach coupled with an end-member mixing analysis. We could reveal that pumping thus induces a thorough reorganization of fluxes known as capture, favoring infiltration and vertical fluxes in the recharge zone, and upwelling of deep and distant water at long-term time scales. These mixing fractions were then used to quantify the extent of denitrification linked to pumping. Based on the results from batch experiments described in a companion paper, our computations revealed that i) autotrophic denitrification processes are dominant in this context where carbon sources are limited, that ii) nitrate reduction does not only come from the oxidation of pyrite as classically described in previous studies analyzing denitrification processes in similar contexts, and that iii) biotite plays a critical role in sustaining the nitrate reduction process. Both nitrate reduction, sulfate production as well as fluor release ratios support the hypothesis that biotite plays a key role of electron donor in this context. The batch-to-site similarities support biotite availability and the role by bacterial communities as key controls of nitrate removal in such crystalline aquifers. However, the long term data monitoring also indicates that mixing and reactive processes evolve extremely slowly at the scale of the decade. [ABSTRACT FROM AUTHOR]

Assessing reliability of global models is critical because of increasing reliance on these models to address past and projected future climate and human stresses on global water resources. Here, we evaluate model reliability based on a comprehensive comparison of decadal trends (2002-2014) in land water storage from seven global models (WGHM, PCR-GLOBWB, GLDAS NOAH,MOSAIC, VIC, CLM and CLSM) to trends from three Gravity Recovery and Climate Experiment (GRACE) satellite solutions in 186 river basins (∼60% of global land area). Medians of modeled basin water storage trends greatly underestimate GRACE-derived large decreasing (⩽-0.5 km3/y) and increasing (⩾0.5 km3/y) trends. Decreasing trends from GRACE are mostly related to human use (irrigation) and climate variations, whereas increasing trends reflect climate variations. For example, in the Amazon, GRACE estimates a large increasing trend of ∼43 km3/y, whereas most models estimate decreasing trends (-71 to 11 km3/y). Land water storage trends, summed over all basins, are positive for GRACE (∼71-82 km3/y) but negative for models (-450 to -12 km3/y), contributing opposing trends to global mean sea level change. Impacts of climate forcing on decadal land water storage trends exceed those of modeled human intervention by about a factor of 2. The model-GRACE comparison highlights potential areas of future model development, particularly simulated water storage. The inability of models to capture large decadal water storage trends based on GRACE indicates that model projections of climate and humaninduced water storage changes may be underestimated. [ABSTRACT FROM AUTHOR]

Proliferation of different total water storage (TWS) change products from the Gravity Recovery and Climate Experiment (GRACE) satellites, including the newly released mascon solution, warrants detailed analysis of their uncertainties and an urgent need to optimize different products for obtaining an elevated understanding of TWS changes globally. The three cornered hat method is used to quantify uncertainties in TWS changes from GRACE observations, land surface models, and global hydrological models, indicating that the WaterGap Global Hydrological Model (WGHM)-based TWS changes show the lowest uncertainty over sixty basins covering a range of climate settings and levels of human activities globally. Bayesian model averaging (BMA) using WGHM TWS output for training (2003–2006) is subsequently used to merge TWS changes from various GRACE products. Results indicate that the BMA-based TWS changes show the highest consistency with the WGHM output for the validation period (2007–2009) in terms of the highest medium of the Nash-Sutcliffe Efficiency (NSE) coefficient of 0.714 among all TWS change products for the sixty basins. The mascon solution shows a medium of NSE of 0.682, higher than other GRACE TWS change products. Analysis of spatiotemporal variability in BMA-based TWS changes and the mascon solution indicates that higher depletion rates for the 13-year period (Apr 2002–Mar 2015) occurred over major aquifers due to groundwater withdrawals for irrigation (e.g., Tigris, Central Valley, Ganges, upper Arkansas, and Indus), basins subject to great glacier and snow melting (e.g., Yukon, Fraser, and eastern Ganges), the north Caspian Sea (e.g., Don and Ural), and the Caspian Sea. Significant increasing trends in TWS are found over west (e.g., Gambia and Niger) and South Africa (e.g., Zambezi), South America (e.g., Essequibo), North America (e.g., Koksoak and Missouri), central India (e.g., Narmada and Godavari), the north Tibetan Plateau, and the middle Yangtze River basin. Empirical Orthogonal Function decomposition is used to investigate spatiotemporal variations in the GRACE mascon solution-based TWS changes during the study period, showing a detailed pattern of increasing and decreasing long-term trends, interannual and seasonal variations in TWS over the global land surface. [ABSTRACT FROM AUTHOR]

Satellite gravimetry is a unique measurement technique for observing mass transport processes in the Earth system on a global scale, providing essential indicators of both subtle and dramatic global change. Although past and current satellite gravity missions have achieved spectacular science results, due to their limited spatial and temporal resolution as well as limited length of the available time series numerous important questions are still unresolved. Therefore, it is important to move from current demonstration capabilities to sustained observation of the Earth's gravity field. In an international initiative performed under the umbrella of the International Union of Geodesy and Geophysics, consensus on the science and user needs for a future satellite gravity observing system has been derived by an international panel of scientists representing the main fields of application, i.e., continental hydrology, cryosphere, ocean, atmosphere and solid Earth. In this paper the main results and findings of this initiative are summarized. The required target performance in terms of equivalent water height has been identified as 5 cm for monthly fields and 0.5 cm/year for long-term trends at a spatial resolution of 150 km. The benefits to meet the main scientific and societal objectives are investigated, and the added value is demonstrated for selected case studies covering the main fields of application. The resulting consolidated view on the required performance of a future sustained satellite gravity observing system represents a solid basis for the definition of technological and mission requirements, and is a prerequisite for mission design studies of future mission concepts and constellations. [ABSTRACT FROM AUTHOR]

Between November 2018 and January 2020, a continuous multi-parameter survey, using nine types of sensors, was carried out on a coastal chalk cliff in Sainte-Marguerite-sur-Mer (Normandy, France) with the objective of gaining a deeper understanding of the forcing agents and processes that lead to cliff fatigue and failure. This paper will present the survey instrumentation, the results on the internal characteristics of the chalk massif, initial results (from November 2018 to March 2019) of the thermal subsurface behaviour along the cliff face, and the analyses of the observed cliff-top ground motion and the movement of existing fractures on the cliff face in relation to forcing agents. Our main results show that 1) the magnitude of cliff-top displacement on this coastal chalk cliff is consistent with prior studies conducted in different settings showing rather high displacement amplitudes (up to 50 μm in relatively calm conditions) likely to be related to chalk elasticity; 2) the displacement on existing fractures is partly controlled by the tidal amplitude, with a threshold response, but not only. Statistical analyses help the identification of other controls. The processing of the entire dataset from November 2018 to January 2020 with a combined analysis of multiple sensors' output is expected to provide further insight on cliff fatigue and failure. • A monitoring system was implemented along the cliff coast to better understand the agents that lead to fatigue and failure. • For 13 months, 9 types of sensors recorded cliff mechanical response, marine agents and subaerial agents. • Cliff-top displacements on this coastal chalk cliff showed rather high amplitudes, likely to be related to chalk elasticity. • The displacement on existing fractures on the cliff face was partly controlled by the tidal amplitude but was multifactorial. [ABSTRACT FROM AUTHOR]

Droughts and floods alternately occur over a large karst plateau (Yun–Gui Plateau) in Southwest China. Here we show that both the frequency and severity of droughts and floods over the plateau are intensified during the recent decade from three-decade total water storage anomalies (TWSA) generated by Gravity Recovery and Climate Experiment (GRACE) satellite data and artificial neural network (ANN) models. The developed ANN models performed well in hindcasting TWSA for the plateau and its three sub-regions (i.e., the upper Mekong River, Pearl River, and Wujiang River basins), showing coefficients of determination ( R 2 ) of 0.91, 0.83, 0.76, and 0.57, respectively. The intensified climate extremes are indicative of large changes in the hydrological cycle and brought great challenges in water resource management there. The TWSA of the plateau remained fairly stable during the 1980s, and featured an increasing trend at a rate of 5.9 ± 0.5 mm/a in the 1990s interspersed extreme flooding in 1991 and during the second half of the 1990s. Since 2000, the TWSA fluctuated drastically, featuring severe spring droughts from 2003 to 2006, the most extreme spring drought on record in 2010, and severe flooding in 2008. The TWSA of the upper Mekong has decreased by ~ 100 mm (~ 15 km 3 ) compared with that at the end of the 1990s. In addition to hindcasting TWSA, the developed approach could be effective in generating future TWSA and potentially bridge the gap between the current GRACE satellites and the GRACE Follow-On Mission expected to launch in 2017. [ABSTRACT FROM AUTHOR]

Precipitation minus evapotranspiration, the net flux of water between the atmosphere and Earth's surface, links atmospheric and terrestrial water budgets and thus represents an important boundary condition for both climate modeling and hydrological studies. However, the atmospheric-terrestrial flux is poorly constrained by direct observations because of a lack of unbiased measurements. Thus, it is usually reconstructed from atmospheric reanalyses. Via the terrestrial water budget equation, water storage estimates from the Gravity Recovery and Climate Experiment (GRACE) combined with measured river discharge can be used to assess the realism of the atmospheric-terrestrial flux in models. In this contribution, the closure of the terrestrial water budget is assessed over a number of European river basins using the recently reprocessed GRACE release 05 data, together with precipitation and evapotranspiration from the operational analyses of high-resolution, limited-area NWP models [Consortium for Small-Scale Modelling, German version (COSMO-DE) and European version (COSMO-EU)] and the new COSMO 6-km reanalysis (COSMO-REA6) for the European Coordinated Regional Climate Downscaling Experiment (CORDEX) domain. These closures are compared to those obtained with global reanalyses, land surface models, and observation-based datasets. The spatial resolution achieved with the recent GRACE data allows for better evaluation of the water budget in smaller river basins than before and for the identification of biases up to 25 mm month−1 in the different products. Variations of deseasoned and detrended atmospheric-terrestrial flux are found to agree notably well with flux derived from GRACE and discharge data with correlations up to 0.75. Finally, bias-corrected fluxes are derived from various data combinations, and from this, a 20-yr time series of catchment-integrated water storage variations is reconstructed. [ABSTRACT FROM AUTHOR]

West African countries have been exposed to changes in rainfall patterns over the last decades, including a significant negative trend. This causes adverse effects on water resources of the region, for instance, reduced freshwater availability. Assessing and predicting large-scale total water storage (TWS) variations are necessary for West Africa, due to its environmental, social, and economical impacts. Hydrological models, however, may perform poorly over West Africa due to data scarcity. This study describes a new statistical, data-driven approach for predicting West African TWS changes from (past) gravity data obtained from the gravity recovery and climate experiment (GRACE), and (concurrent) rainfall data from the tropical rainfall measuring mission (TRMM) and sea surface temperature (SST) data over the Atlantic, Pacific, and Indian Oceans. The proposed method, therefore, capitalizes on the availability of remotely sensed observations for predicting monthly TWS, a quantity which is hard to observe in the field but important for measuring regional energy balance, as well as for agricultural, and water resource management. Major teleconnections within these data sets were identified using independent component analysis and linked via low-degree autoregressive models to build a predictive framework. After a learning phase of 72 months, our approach predicted TWS from rainfall and SST data alone that fitted to the observed GRACE-TWS better than that from a global hydrological model. Our results indicated a fit of 79 % and 67 % for the first-year prediction of the two dominant annual and inter-annual modes of TWS variations. This fit reduces to 62 % and 57 % for the second year of projection. The proposed approach, therefore, represents strong potential to predict the TWS over West Africa up to 2 years. It also has the potential to bridge the present GRACE data gaps of 1 month about each 162 days as well as a-hopefully-limited gap between GRACE and the GRACE follow-on mission over West Africa. The method presented could also be used to generate a near-real-time GRACE forecast over the regions that exhibit strong teleconnections. [ABSTRACT FROM AUTHOR]

Highlights: [•] Investigating impacts of climate change and groundwater pumping on wetland extension. [•] Simple model to understand surface-subsurface interaction and wetland vulnerability. [•] Climate change has a greater impact with loss of wetland area by 5.3–13.6%. [•] The impact of groundwater abstraction would lead to a maximum decrease of 3.7%. [•] Effects of climate and pumping could be reduced by stop pumping in peat exploitation. [Copyright &y& Elsevier]

Subduction zones megathrust faults constitute a considerable hazard as they produce most of the world's largest earthquakes. However, the role in megathrust earthquake generation exerted by deeper subduction processes remains poorly understood. Here, we analyze the 2003 – 2014 space-time variations of the Earth's gravity gradients derived from three datasets of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 Maule earthquake. In all these datasets, our analysis reveals a large-amplitude gravity gradient signal, progressively increasing in the three months before the earthquake, North of the epicentral area. We show that such signals are equivalent to a 60 km 3 water storage decrease over 2 months and cannot be explained by hydrological sources nor artefacts, but rather find origin from mass redistributions within the solid Earth on the continental side of the subduction zone. These gravity gradient variations could be explained by an extensional deformation of the slab around 150-km depth along the Nazca Plate subduction direction, associated with large-scale fluid release. Furthermore, the lateral migration of the gravity signal towards the surface from a low coupling segment around − 32.5 ∘ North to the high coupling one in the South suggests that the Mw 8.8 earthquake may have originated from the propagation up to the trench of this deeper slab deformation. Our results highlight the importance of observations of the Earth's time-varying gravity field from satellites in order to probe slow mass redistributions in-depth major plate boundaries and provide new information on dynamic processes in the subduction system, essential to better understand the seismic cycle as a whole. • An anomalous GRACE gravity gradient signal is detected prior to the Maule earthquake. • This signal cannot be explained by hydrological mass redistributions nor artefacts. • It is consistent with extensional deformation of the subducted slab near 150 km depth. • The earthquake may have originated from the propagation of this deeper deformation. [ABSTRACT FROM AUTHOR]

Abstract: We present an analysis of daily water discharge and suspended sediment concentration measurements for the three main drainage basins in Nepal, on the basis of recent published papers. We first show how precipitation-discharge data can be used to highlight the impact of groundwater storage on the annual hydrological cycle of Himalayan rivers. Then, we show how the concentration of suspended sediment in rivers varies at the year scale depending on the river discharge cycle, as well as how the release of groundwater impacts the concentration of materials in rivers. Finally, we propose a new conceptual model for the mobilization and transportation of material within the monsoonal discharge cycle in the central Himalayas. [Copyright &y& Elsevier]

Abstract: This study explores the sensitivity of tilt measurements to hydrogeologic phenomena in order to use them to characterise aquifers and explain the signals observed on the instruments. The challenge is to separate the hydrogeological contribution from other phenomena that affect this type of measurements. First, we applied the theory propounded by Farell (1972) and Pagiatakis (1990) to simple hydrogeological cases. Then a simulation of the tiltmeter signal obtained in the Seine valley reveals that the order of magnitude is significant: the signal of a load with hydrologic origin therefore represents a significant part of any tiltmeter signal. Moreover, it can be interpreted in hydrogeological terms: it depends directly on the load gradient through a convolution formalism. Consequently, it is possible to imagine a use of the tiltmeter in hydrogeology to characterise, to a certain extent, mass transfer of water in the ground. To cite this article: T. Rerolle et al., C. R. Geoscience 338 (2006). [Copyright &y& Elsevier]

For many environmental applications, the interpretation of fiber-optic Raman distributed temperature sensing (FO-DTS) measurements is strongly dependent on the spatial resolution of measurements, especially when the objective is to detect temperature variations over small scales. Here, we propose to compare three different and complementary methods to estimate, in practice, the "effective" spatial resolution of DTS measurements: The classical "90% step change" method, the correlation length estimated from experimental semivariograms, and the derivative method. The three methods were applied using FO-DTS measurements achieved during sandbox experiments using two DTS units having different spatial resolutions. Results show that the value of the spatial resolution estimated using a step change depends on both the effective spatial resolution of the DTS unit and on heat conduction induced by the high thermal conductivity of the cable. The correlation length method provides an estimate much closer to the value provided by the manufacturers, representative of the effective spatial resolutions along cable sections where temperature gradients are small or negligible. Thirdly, the application of the derivative method allows for verifying the representativeness of DTS measurements all along the cable, by localizing sections where measurements are representative of the effective temperature. We finally show that DTS measurements could be validated in sandbox experiments, when using devices with finer spatial resolution., Competing Interests: The authors declare no conflicts of interest.