Changes In Continental Water Storage Research Articles

Improved Hydrological Loading Models in South America: Analysis of GPS Displacements Using M-SSA

Environmental loading, in particular from continental water storage changes, induces geodetic station displacements up to several centimeters for the vertical components. We investigate surface deformation due to loading processes in South America using a set of 247 permanent GPS (Global Positioning System) stations for the 2003–2016 period and compare them to loading estimates from global circulation models. Unfortunately, some of the hydrological components, and in particular surface waters, may be missing in hydrological models. This is especially an issue in South America where almost half of the seasonal water storage variations are due to surface water changes, e.g., rivers and floodplains. We derive river storage variations by rerouting runoffs of global hydrology models, allowing a better agreement with the mass variations observed from GRACE (Gravity Recovery and Climate Experiment) mission. We extract coherent seasonal GPS displacements using Multichannel Singular Spectrum Analysis (M-SSA) and show that modeling the river storage induced loading effects significantly improve the agreement between observed vertical and horizontal displacements and loading models. Such an agreement has been markedly achieved in the Amazon basin. Whilst the initial models only explained half of the amplitude of GPS, the new ones compensate for these gaps and remain consistent with GRACE.

Estimation of daily hydrological mass changes using continuous GNSS measurements in mainland China

Surface displacements measured by the Global Navigation Satellite System (GNSS) integrate the elastic response of the solid Earth to regional and local hydrologic loading signals and provide an opportunity for near-real-time monitoring of terrestrial water storage variations. Here, we estimate the spectral information of hydrology-induced vertical surface deformation based on spherical Slepian basis functions and recover the daily changes in continental water storage over mainland China from January 2010 to December 2019. Our inversion results, depending on a sparsely-distributed GNSS network, indicate the largest seasonal fluctuation of hydrological mass loads in southwestern China. The GNSS-inferred equivalent water height has an annual amplitude of up to 314 mm, which is greater than 243 mm from the Gravity Recovery and Climate Experiment (GRACE) and 150 mm from the Global Land Data Assimilation System (GLDAS). We also find notable seasonal water oscillations of 153–166 mm in the middle and lower Yangtze River Basin. This feature appears in the GNSS inversion model and GRACE Mascon solutions but is invisible in the GLDAS model, mostly indicating the existence of significant changes in surface water storage that is unmodeled in the GLDAS model. There is board agreement in the water estimates derived from multiple data sets in South China, in contrast to weak temporal coherence of various water estimates in North China. We also demonstrate the ability of GNSS for tracking the drought and wet periods, which can serve as an independent means to characterize hydrological extremes. Our study emphasizes that a sparse GNSS network can still be considered as a complementary tool to map large-scale spatiotemporal variations of water storage and benefit the hydrological community for assessing water storage changes and hydrological dynamics.

Lake and reservoir volume variations in South America from radar altimetry, ICESat laser altimetry, and GRACE time-variable gravity

Thanks to 25-years of radar altimetry measurements from TOPEX/POSEIDON, Jason 1, 2 and 3, ENVISAT and others, water level changes of major lakes and rivers can be measured regularly, remotely, with unprecedented precision, facilitating monitoring of continental water storage variations. In addition, the 18 laser altimeter measurement campaigns over 6 years by the Ice, Cloud and Land Elevation Satellite (ICESat) satellite complement these radar altimetry measurements and also validate them, expanding our monitoring capabilities. We extract derived water height estimates for various lakes in South America from radar and laser altimetry measurements. Using available imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS), we estimate time-variable surface extent of these water bodies, and together with the altimetry, we derive corresponding estimates of relative volume variations for a number of lakes and reservoirs in South America. We see good agreement between our water extent and available products, with some differences when snow covered area are present. We find good agreement between the temporal variations of lake water heights obtained from ICESat, radar altimetry, and in situ measurements, when available. Lake-dependent biases still exist, likely due to differences in reference datum, choice of geoid model, and differences in data processing schemes. The is a good agreement in volume changes obtained from imagery and the various altimetry techniques. Furthermore, using the latest one-degree global iterated mascon solutions from NASA Goddard Space Flight Center, we successfully compare them to independent volume changes assessments for these water bodies, derived from mass variations from time-variable gravity measurements from 15 years of Gravity Recovery And Climate Experiment (GRACE) observations. The agreement in water volume changes from GRACE with those from altimetry and imagery is higher for lakes in arid and semi-arid regions than on mountainous areas. GRACE volume estimates remain limited to the spatial resolution and the quality of the hydrology models used to correct for soil-moisture and snow contributions. Overall, studies like the one presented here further validate the power of leveraging these data synergies to monitor continental water storage changes from space, taking advantage of what each of the techniques has to offer. We show that water level measurements from laser and radar altimeters have similar precision, of a few centimeters over inland water bodies, and prove that GRACE global mascons allow to retrieve small lake relative volume changes, even for small bodies, thanks to their higher sensitivity to small wavelength mass variations.

European Gravity Service for Improved Emergency Management (EGSIEM)—from concept to implementation

Earth observation satellites yield a wealth of data for scientific, operational and commercial exploitation. However, the redistribution of mass in the system Earth is not yet part of the standard inventory of Earth Observation (EO) data products to date. It is derived from the Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-On mission (GRACE-FO). Among many other applications, mass redistribution provides fundamental insights into the global water cycle. Changes in continental water storage impact the regional water budget and can, in extreme cases, result in floods and droughts that often claim a high toll on infrastructure, economy and human lives. The initiative for a European Gravity Service for Improved Emergency Management (EGSIEM) established three different prototype services to promote the unique value of mass redistribution products for Earth Observation in general and for early-warning systems in particular. The first prototype service is a scientific combination service to derive improved mass redistribution products from the combined knowledge of the European GRACE analysis centres. Second, the timeliness and reliability of such products is a primary concern for any early-warning system and therefore EGSIEM established a prototype for a near real-time service that provides dedicated gravity field information with a maximum latency of 5 d. Third, EGSIEM established a prototype of a hydrological/early warning service that derives wetness indices as indicators of hydrological extremes and assessed their potential for timely scheduling of high-resolution optical/radar satellites for follow-up observations in case of evolving hydrological extreme events.

Hydrologically-driven crustal stresses and seismicity in the New Madrid Seismic Zone

The degree to which short-term non-tectonic processes, either natural and anthropogenic, influence the occurrence of earthquakes in active tectonic settings or ‘stable’ plate interiors, remains a subject of debate. Recent work in plate-boundary regions demonstrates the capacity for long-wavelength changes in continental water storage to produce observable surface deformation, induce crustal stresses and modulate seismicity rates. Here we show that a significant variation in the rate of microearthquakes in the intraplate New Madrid Seismic Zone at annual and multi-annual timescales coincides with hydrological loading in the upper Mississippi embayment. We demonstrate that this loading, which results in geodetically observed surface deformation, induces stresses within the lithosphere that, although of small amplitude, modulate the ongoing seismicity of the New Madrid region. Correspondence between surface deformation, hydrological loading and seismicity rates at both annual and multi-annual timescales indicates that seismicity variations are the direct result of elastic stresses induced by the water load.

Mapping probabilities of extreme continental water storage changes from space gravimetry

AbstractUsing data from the Gravity Recovery And Climate Experiment (GRACE) mission, we derive statistically robust “hot spot” regions of high probability of peak anomalous—i.e., with respect to the seasonal cycle—water storage (of up to 0.7 m one‐in‐five‐year return level) and flux (up to 0.14 m/month). Analysis of, and comparison with, up to 32 years of ERA‐Interim reanalysis fields reveals generally good agreement of these hot spot regions to GRACE results and that most exceptions are located in the tropics. However, a simulation experiment reveals that differences observed by GRACE are statistically significant, and further error analysis suggests that by around the year 2020, it will be possible to detect temporal changes in the frequency of extreme total fluxes (i.e., combined effects of mainly precipitation and floods) for at least 10–20% of the continental area, assuming that we have a continuation of GRACE by its follow‐up GRACE Follow‐On (GRACE‐FO) mission.

A spaceborne multisensor approach to monitor the desiccation of Lake Urmia in Iran

Lake Urmia, a hypersaline lake in northwestern Iran, is under threat of drying up. The high importance of the lake's watershed for human life demands a comprehensive monitoring of the watershed's behavior. Spaceborne sensors provide a number of novel ways to monitor the hydrological cycle and its interannual changes. The use of GRACE gravity data makes it possible to determine continental water storage changes and to assess the water budget on monthly to multi-annual time scales. We use satellite altimetry data from ENVISAT and CryoSat-2 to monitor the lake water level. Moreover, we employ optical satellite imagery to determine the surface water extent of the lake repeatedly and at an appropriate time interval.Our altimetry results indicate that, on average, the lake has lost 34±1cm of its water level every year from 2002 to 2014. The results from satellite imagery reveal a loss of water extent at an average rate of 220±6km2/yr, which indicates that the lake has lost about 70% of its surface area over the last 14years. By combining water level from altimetry, surface water extent from satellite imagery and local bathymetry, we ascertain the changes in lake volume. Results indicate that the lake volume has been decreasing at an alarming rate of 1.03±0.02km3/yr. The water volume of the lake behaves differently from the water storage of the whole basin captured by GRACE. Our results show that the onset of a drought in 2007 over this region together with an increase in the rate of groundwater depletion caused a new equilibrium level for water storage of the whole basin. Comparing the results from GRACE and the obtained water volume in the lake with in situ groundwater level data reveals the anthropogenic influences on an accelerated lake desiccation. In fact, our monitoring approach raises critical issues regarding water use in the basin and highlights the important role of spaceborne sensors for any urgent or long-term treatment plan.

Increased water storage in North America and Scandinavia from GRACE gravity data

Changes in continental water storage have been difficult to constrain from space-borne gravity data in regions experiencing both ice melting and glacial isostatic adjustment. Separation of the hydrologic and isostatic signals reveals increases in water storage in both North America and Scandinavia over the past decade.

Evaluation of the ISBA-TRIP continental hydrologic system over the Niger basin using in situ and satellite derived datasets

Abstract. During the 1970s and 1980s, West Africa has faced extreme climate variations with extended drought conditions. Of particular importance is the Niger basin, since it traverses a large part of the Sahel and is thus a critical source of water for an ever-increasing local population in this semi arid region. However, the understanding of the hydrological processes over this basin is currently limited by the lack of spatially distributed surface water and discharge measurements. The purpose of this study is to evaluate the ability of the ISBA-TRIP continental hydrologic system to represent key processes related to the hydrological cycle of the Niger basin. ISBA-TRIP is currently used within a coupled global climate model, so that the scheme must represent the first order processes which are critical for representing the water cycle while retaining a limited number of parameters and a simple representation of the physics. To this end, the scheme uses first-order approximations to account explicitly for the surface river routing, the floodplain dynamics, and the water storage using a deep aquifer reservoir. In the current study, simulations are done at a 0.5 by 0.5° spatial resolution over the 2002–2007 period (in order to take advantage of the recent satellite record and data from the African Monsoon Multidisciplinary Analyses project, AMMA). Four configurations of the model are compared to evaluate the separate impacts of the flooding scheme and the aquifer on the water cycle. Moreover, the model is forced by two different rainfall datasets to consider the sensitivity of the model to rainfall input uncertainties. The model is evaluated using in situ discharge measurements as well as satellite derived flood extent, total continental water storage changes and river height changes. The basic analysis of in situ discharges confirms the impact of the inner delta area, known as a significant flooded area, on the discharge, characterized by a strong reduction of the streamflow after the delta compared to the streamflow before the delta. In the simulations, the flooding scheme leads to a non-negligible increase of evaporation over large flooded areas, which decreases the Niger river flow by 15% to 50% in the locations situated after the inner delta as a function of the input rainfall dataset used as forcing. This improves the simulation of the river discharge downstream of the delta, confirming the need for coupling the land surface scheme with the flood model. The deep aquifer reservoir improves Niger low flows and the recession law during the dry season. The comparison with 3 satellite products from the Gravity Recovery and Climated Experiment (GRACE) shows a non negligible contribution of the deeper soil layers to the total storage (34% for groundwater and aquifer). The simulations also show a non negligible sensitivity of the simulations to rain uncertainties especially concerning the discharge. Finally, sensitivity tests show that a good parameterization of routing is required to optimize simulation errors. Indeed, the modification of certain key parameters which can be observed from space (notably river height and flooded zones height changes and extent) has an impact on the model dynamics, thus it is suggested that improving the model input parameters using future developments in remote sensing technologies such as the joint CNES-NASA satellite project SWOT (Surface Water Ocean Topography), which will provide water heights and extentat land surface with an unprecedented 50–100 m resolution and precision.

Constrained Regional Recovery of Continental Water Mass Time-variations from GRACE-based Geopotential Anomalies over South America

We propose a “constrained” least-squares approach to estimate regional maps of equivalent-water heights by inverting GRACE-based potential anomalies at satellite altitude. According to the energy integral method, the anomalies of difference of geopotential between the two GRACE vehicles are derived from along-track K-Band Range-Rate (KBRR) residuals that correspond mainly to the continental water storage changes, once a priori known accelerations (i.e. static field, polar movements, atmosphere and ocean masses including tides) are removed during the orbit adjustment process. Newton's first law merely enables the Difference of Potential Anomalies from accurate KBRR data and the equivalent-water heights to be recovered. Spatial constraints versus spherical distance between elementary surface tiles are introduced to stabilize the linear system to cancel the effects of the north-south striping. Unlike the “mascons” approach, no basis of orthogonal functions (e.g., spherical harmonics) is used, so that the proposed regional method does not suffer from drawbacks related to any spectrum truncation. Time series of 10-day regional maps over South America for 2006–2009 also prove to be consistent with independent data sets, namely the outputs of hydrological models, “mascons” and global GRACE solutions.

Tackling mass redistribution phenomena by time-dependent GRACE- and terrestrial gravity observations

Time variable gravity field models derived from the satellite mission GRACE have been demonstrated to be consistent with water mass variations in the global hydrological cycle. Independent observations are provided by terrestrial measurements. In order to achieve a maximum of reliability and information gain, ground-based gravity observations may be deployed for comparison with the gravity field variations derived from the GRACE satellite mission. In this context, the data of the network of superconducting gravimeters (SG) of the ‘Global Geodynamics Project’ (GGP) are of particular interest. This study is focused on the dense SG network in Central Europe with its long-term gravity observations. It is shown that after the separation and reduction of local hydrological effects in the SG observations especially for subsurface stations, the time-variable gravity signals from GRACE agree well with the terrestrial observations from the SG station cluster.Station stability of the SG sites with respect to vertical deformations was checked by GNSS based observations. Most of the variability can be explained by loading effects due to changes in continental water storage, and, in general, the stability of all stations has been confirmed.From comparisons based on correlation and coherence analyses in combination with the root mean square (RMS) variability of the time series emerges, that the maximum correspondence between the SG and GRACE time series is achieved when filtering the GRACE data with Gaussian filters of about 1000km filter length, which is in accordance with previous publications.Empirical Orthogonal Functions (EOF) analysis was applied to the gravity time series in order to identify common characteristic spatial and temporal patterns. The high correspondence of the first modes for GRACE and SG data implies that the first EOF mode represents a large-scale (Central European) time-variable gravity signal seen by both the GRACE satellites and the SG cluster.

Estimating geoid height change in North America: past, present and future

The forthcoming GRAV-D gravimetric geoid model over the United States is to be updated regularly to account for changes in geoid height. Its baseline precision is to be at the 10–20 mm level over non-mountainous regions. The aim of this study is to provide an estimate of the magnitude, time scale, and spatial footprint of geoid height change over North America, from mass redistribution processes of hydrologic, cryospheric and solid Earth nature. Geoid height changes from continental water storage changes over the past 50 years and predicted over the next century are evaluated and are highly dependent on the used model. Groundwater depletion from anthropogenic pumping in regional scale aquifers may lead to geoid changes of 10 mm magnitude every 50–100 years. The GRACE time varying gravity fields are used to (i) assess the errors in a glacial isostatic adjustment model, which, if used to correct the GRAV-D model, may induce errors at the 10 mm geoid height level after ~20 years, (ii), evaluate geoid height change over ice mass loss regions of North America, which, if they remain unchanged in the future, may lead to geoid height changes at the 10 mm level in under a decade and (iii), compute sea level rise and its effect on the geoid, which is found to be negligible. Coseismic gravitational changes from past North American earthquakes are evaluated, and lead to geoid change at the 10-mm level for only the largest thrust earthquakes. Finally, geoid change from volcanic processes are assessed and found to be significant with respect to the GRAV-D geoid model baseline precision for cataclysmic events, such as that of the 1980 Mt. St. Helens eruption. Recommendations on how to best monitor geoid change in the future are given.

Sensitivity of superconducting gravimeters in central Europe on variations in regional river and drainage basins

As underpinned by various studies in the last years, temporal changes of the Earth’s gravity field contain a wealth of information on mass redistribution processes in the Earth’s system particularly associated with variations in continental water storage. By combining satellite and terrestrial observations with superconducting gravimeters (SG) a maximum of information can be gained due to the different temporal and spatial sampling. Esp. the cluster of superconducting gravimeters in central Europe is well suited for studies related to spatial and temporal changes in continental water storage. Due to the distribution of SG sites different sensitivities of the instruments are to be expected on changes in the various river and drainage basins which could, if sufficiently pronounced, be deployed to pinpoint areas in which main discrepancies between modelled and actual water storage changes occur and would thus allow us to fine-tune hydrological models. Based on the WaterGap Global Hydrological Model (WGHM), this sensitivity of the SG observations is investigated. One compartment of the WGHM is surface water, thus comprising rivers, flooding areas, and major reservoirs. This contribution is given for the total cell of 0.5° × 0.5° and not localized, e.g. in a riverbed, therefore the question arises to which extent localization or non-localization of this compartment affects the estimate if the respective surface waters are in the vicinity of 50 km around the SG stations. It can be shown, however, that the lateral distribution of the surface water masses plays only a negligible role for the central European stations meaning distributed surface water masses are an acceptable simplification when estimating hydrological effects. It emerges that variations in water storage in regions outside central Europe produce comparable effects on gravity at all sites and the impact of basins within central Europe is clearly distinguishable among the SG stations.

The measurement of Earth’s gravity field after the GOCE mission

GOCE ( Gravity Field and Steady-State Ocean Circulation Explorer), the first Earth Explorer Core Mission in ESA’s Living Planet programme, has been launched on March 17th 2009. Employing an ultra-sensitive gradiometer on a very low altitude orbit in along-track drag-free condition, within about 2 years GOCE will provide a global model of the Earth’s gravity field and of the geoid to unprecedented spatial resolution and accuracy. Meanwhile, the requirement for monitoring the temporal variations of gravity with a spatial resolution similar to GOCE, but over much longer time periods than the lifetime of GOCE, is emerging. It is driven by the need to detect long-term periodicities and trends in geophysical phenomena that involve large mass redistributions (e.g. melting of the polar ice sheets, changes in continental water storage, etc.), which are extremely valuable for a better understanding of climate change. From a series of preparatory studies led by the European Space Agency (ESA) since 2003, it turned out that the most suitable technique for such a next-generation gravity mission is the so-called Low-Low Satellite-to-Satellite Tracking based on laser distance metrology. A first design of this mission and of its laser metrology system has been defined in the studies performed for ESA the team led by Thales Alenia Space Italia (TAS-I).

Comment on “On the steric and mass‐induced contributions to the annual sea level variations in the Mediterranean Sea” by David García et al.

[1] Garcia et al. [2006] (hereafter referred to as GCDVG) present an analysis of mass-induced sea level variation (SLVmass) of the Mediterranean Sea based on satellite altimetry, ocean modeling and Gravity Recovery and Climate Experiment (GRACE) gravity measurements. In particular, GCDVG find consistency between altimeter sea level variations corrected for steric effects and changes in oceanic mass derived from Gravity Recovery and Climate Experiment (GRACE). GCDVG show that the annual cycle of the sea level variability (SLVtotal) is dominated by changes in its steric part (SLVsteric) and that the basin average of mass (SLVmass) is maximum in February. They conclude that, when the sea level is rising (falling) the Mediterranean Sea is actually losing (gaining) mass. [2] GCDVG appear to use an unrealistically large estimate of steric sea level variability (SLVsteric) and neglect changes in continental water storage that can significantly affect GRACE measurements over the Mediterranean Sea. These issues are examined in turn below by reviewing relevant individual basin averages of the Mediterranean Sea in comparison to results of other studies. In particular, while also finding consistency between altimetry and GRACE measurements, Fenoglio-Marc et al. [2006] (hereafter referred to as FKB), in a similar study of the Mediterranean Sea, find steric effects only accounting for about 60% of the total annual sea level variability and the annual cycle of SLVmass having a maximum in November.

Global modelling of continental water storage changes – sensitivity to different climate data sets

Abstract. Since 2002, the GRACE satellite mission provides estimates of the Earth's dynamic gravity field with unprecedented accuracy. Differences between monthly gravity fields contain a clear hydrological signal due to continental water storage changes. In order to evaluate GRACE results, the state-of-the-art WaterGAP Global Hydrological Model (WGHM) is applied to calculate terrestrial water storage changes on a global scale. WGHM is driven by different climate data sets to analyse especially the influence of different precipitation data on calculated water storage. The data sets used are the CRU TS 2.1 climate data set, the GPCC Full Data Product for precipitation and data from the ECMWF integrated forecast system. A simple approach for precipitation correction is introduced. WGHM results are then compared with GRACE data. The use of different precipitation data sets leads to considerable differences in computed water storage change for a large number of river basins. Comparing model results with GRACE observations shows a good spatial correlation and also a good agreement in phase. However, seasonal variations of water storage as derived from GRACE tend to be significantly larger than those computed by WGHM, regardless of which climate data set is used.

Estimating Large-Scale Precipitation Minus Evapotranspiration from GRACE Satellite Gravity Measurements

Abstract Currently, observations of key components of the earth's large-scale water and energy budgets are sparse or even nonexistent. One key component, precipitation minus evapotranspiration (P − ET), remains largely unmeasured due to the absence of observations of ET. Precipitation minus evapotranspiration describes the flux of water between the atmosphere and the earth's surface, and therefore provides important information regarding the interaction of the atmosphere with the land surface. In this paper, large-scale changes in continental water storage derived from satellite gravity data from the Gravity Recovery and Climate Experiment (GRACE) project are combined with river discharge data to obtain estimates of areally averaged P − ET. After constructing an equation describing the large-scale terrestrial water balance reflecting the temporal sampling of GRACE water storage estimates, GRACE-derived P − ET estimates are compared to those obtained from a reanalysis dataset [NCEP/Department of Energy (DOE) reanalysis-2] and a land surface model driven with observation-based forcing [Global Land Data Assimilation System (GLDAS)/Noah] for two large U.S. river basins. GRACE-derived P − ET compares quite favorably with the reanalysis-2 output, while P − ET from the Noah model shows significant differences. Because the uncertainties in the GRACE results can be computed rigorously, this comparison may be considered as a validation of the models. In addition to showing how GRACE P − ET estimates may be used to validate model output, the accuracy of GRACE estimates of both the seasonal cycle and the monthly averaged rate of P − ET is examined. Finally, the potential for estimating seasonal evapotranspiration is demonstrated by combining GRACE seasonal P − ET estimates with independent estimates of the seasonal cycle of precipitation.

Climate model biases in seasonality of continental water storage revealed by satellite gravimetry

Satellite gravimetric observations of monthly changes in continental water storage are compared with outputs from five climate models. All models qualitatively reproduce the global pattern of annual storage amplitude, and the seasonal cycle of global average storage is reproduced well, consistent with earlier studies. However, global average agreements mask systematic model biases in low latitudes. Seasonal extrema of low‐latitude, hemispheric storage generally occur too early in the models, and model‐specific errors in amplitude of the low‐latitude annual variations are substantial. These errors are potentially explicable in terms of neglected or suboptimally parameterized water stores in the land models and precipitation biases in the climate models.

GRACE observations of changes in continental water storage

Signatures between monthly global Earth gravity field solutions obtained from GRACE satellite mission data are analyzed with respect to continental water storage variability. GRACE gravity field models are derived in terms of Stokes' coefficients of a spherical harmonic expansion of the gravitational potential from the analysis of gravitational orbit perturbations of the two GRACE satellites using GPS high–low and K-band low–low intersatellite tracking and on-board accelerometry. Comparing the GRACE observations, i.e., the mass variability extracted from temporal gravity variations, with the water mass redistribution predicted by hydrological models, it is found that, when filtering with an averaging radius of 750 km, the hydrological signals generated by the world's major river basins are clearly recovered by GRACE. The analyses are based on differences in gravity and continental water mass distribution over 3- and 6-month intervals during the period April 2002 to May 2003. A background model uncertainty of some 35 mm in equivalent water column height from one month to another is estimated to be inherent in the present GRACE solutions at the selected filter length. The differences over 3 and 6 months between the GRACE monthly solutions reveal a signal of some 75 mm scattering with peak values of 400 mm in equivalent water column height changes over the continents, which is far above the uncertainty level and about 50% larger than predicted by global hydrological models. The inversion method, combining GRACE results with the signal and stochastic properties of a hydrological model as ‘a priori’ in a statistical least squares adjustment, significantly reduces the overall power in the obtained water mass estimates due to error reduction, but also reflects the current limitations in the hydrological models to represent total continental water storage change in particular for the major river basins.

GRACE observations of changes in continental water storage

GRACE observations of changes in continental water storage