GRACE Products Research Articles

From coarse resolution to practical solution: GRACE as a science communication and policymaking tool for sustainable groundwater management

Managing groundwater is challenging because its below-ground existence makes the resource difficult to monitor. GRACE is an important tool as it provides the opportunity to monitor the storage of groundwater. While GRACE-based groundwater studies have focused on many regions across the globe, studies that link scientific research to policymaking and groundwater practices are still limited. This paper aims to provide recommendations for the improved use of GRACE data for the purpose of sustainable groundwater management. To gain insight into this, a policy Delphi survey was conducted to collect opinions from both the academic and non-academic communities. Although opinions are divergent and a mismatch exists between what science (researchers) currently supplies and what users (practitioners) demand, we found that both communities acknowledged the potential of GRACE data and GRACE-derived products for groundwater management and would be willing to collaborate to develop projects for practical applications. Better communication between researchers and practitioners was recommended as key for greater application of GRACE-derived products into practice. Practitioners noted the need for reliable data for management responsibilities but prefer locally-observed data. The reliability of GRACE at local scales was raised by practitioners as a limitation despite advances in downscaling methods within the academic community. The survey reveals a shared desire for more comparison between GRACE-derived products with local measurements to determine whether GRACE products can be useful for informing local decisions. Using survey outcomes, we propose three recommendations to improve the usefulness of GRACE-derived and other remote sensing products for groundwater management and practices: improving communication strategies; increasing data accuracy, transparency and accessibility to build trust; and conducting solution-oriented research.

Impact of meteorological conditions on water resources in the Upper East Region of Ghana using remotely-sensed and modelled hydrological data

Study regionThe Upper East Region, Ghana, West Africa, lies within the Volta Basin, floods annually, and contributes substantially to Ghana’s food production. Study focusWe assessed precipitation (P), evapotranspiration (ET), and total water storage anomalies from GRACE (TWSA) and GLDAS-Noah (TWCA) to study the influence of the UER’s climate on water availability between 2002 and 2017. We analysed (1) the relative uncertainties of the data sets using the triple-cornered hat method, (2) the terrestrial water budget to validate TWSA/TWCA and (3) cross- and multi-correlation analyses to study the relationship between water storage (or availability) and meteorological variables. New hydrological insightsWe found strong correlations between the different P products (r > 0.96), between the different GRACE products (r > 0.95), but not between the different ET products. The hybrid P, TWSA from the Jet Propulsion Laboratory, and ET from ERA-5 had the smallest relative uncertainties. TWSA increased by 9.8 ± 0.8 mm yr−1 while TWCA decreased. P and ET showed no evidence of a trend and were similarly influenced by the other meteorological variables. However, 93 of 183 months had water surplus and mean net P was positive – indicating the UER received more water than it lost. These agree with the increasing TWSA trend. The water budget validation also confirmed that GRACE can be used for water management; GLDAS-Noah underestimates storage in the UER.

Comparison between Gaussian and decorrelation filters of GRACE-based RL05 temporal gravity solutions over Egypt

This contribution provides a comparison between the Gaussian and decorrelation filters as derived from GRACE products (RL05) estimated by the official GRACE Science Data System centres (GFZ, CSR and JPL) as well as the ITSG-GRACE2016 solutions over Egypt. The outcome of this study will help in finding out which of these centres provides improved temporal gravity solutions as well as the most promising GRACE time series over Egypt. The obtained results regarding Gaussian filters show that the GFZ centre provides the most promising solutions w.r.t. CSR and JPL. Whereas the ITSG-GRACE2016 products provide improvements, especially at Gaussian radius 200 km, of about 56%, 68% and 60% w.r.t. CSR, JPL and GFZ solutions, respectively. Regarding the decorrelation filtering, the ITSG-GRACE2016 provides the least Std. w.r.t. CSR, JPL and GFZ solutions showing for the DDK8 improvement of about 48%, 64% and 68% w.r.t. the three centres GFZ, JPL and CSR, respectively.

A new method for assessing satellite-based hydrological data products using water budget closure

Remote sensing products have been widely used in water resources assessment and management. However, the accuracy varies in different products. Water budget closure provides an important breakthrough for better assessing the uncertainties and quantifying error sources of satellite-based data products using water budget imbalance, especially in ungauged basins where conventional verification methods are not applicable. However, the commonly used assessment methods mainly refer to closure residual of water budget ΔRes or its proportion to corresponding mean precipitation (P) ΔRes/P, which have apparent limitations due to the dimensional effects in the former method and the high dependence on precipitation in the latter one. More critically, these two methods do not consider measured values (referring to in-situ values in gauged basins or average values of multiple satellite products in ungauged basins as in this study) as benchmarks of water budget components in the assessment of satellite-based hydrological data products, resulting in a wrong choice because the closure result is subject to the mutual cancellation in the errors of water budget components from different satellite-based products. This study proposes a novel error-based method to assess the accuracy of satellite-based hydrological data products for both gauged and ungauged basins based on water budget equation, measured values, and the omission error (OE) which is generally overlooked in existing studies. The method is applied to a typical closed basin, the Tarim River Basin (TRB) of China, to verify the applicability and reliability of the proposed method. Sixty combinations of water budget components from various satellite sources can be designed based on five precipitation (P) products, four evapotranspiration (ET) products and three terrestrial water storage change (TWSC) products. First order reliability method (FORM) is employed to assess the reliability of water budget closure results caused by the errors of satellite sources. The results indicated that different combinations produced different annual ΔRes and ΔRes/P, indicating great water budget closure errors for different combinations due to errors in different satellite sources. The OE error from satellite-based hydrological data products of water budget components clearly showed a greater impact on water budget imbalance than water budget components, indicating that OE error is important for understanding quality of satellite products. By comparing the results of ΔRes and ΔRes/P, it can be seen that our proposed method had great advantages in assessment of satellite products with a low dependence on P, and a better comparability between different combinations. The value of our assessment criterion ranges from 0 to 1 which is easier to understand in practice. The combinations of water budget components from TRMM_3B43 (P), GPM_3IMERGHH (P), GLDAS_NOAH025 (ET), and GRACE (TWSC) products show satisfactory water budget closure results in TRB using our proposed method. The proposed method provides a new aspect for assessing the accuracy of satellite-based hydrological data products especially in ungauged basins, which provides insights into water resources management in basins.

Monitoring Droughts From GRACE

With ongoing climate change, we are staring at possibly longer and more severe droughts in the future. Therefore, monitoring and understanding duration and intensity of droughts, and how are they evolving in space and time is imperative for global socio-economic security. Satellite remote sensing has helped us a lot in this endeavor, but most of the satellite missions observe only near-surface properties of the Earth. A recent geodetic satellite mission, GRACE, measured the water storage change both on and beneath the surface, which makes it unique and valuable for drought research. This novel dataset comes with unique problems and characteristics that we should acknowledge before using it. In this perspective article, I elucidate important characteristics of various available GRACE products that are important for drought research. I also discuss limitations of GRACE mission that one should be aware of, and finally I shed some light on latest developments in GRACE data processing that may open numerous possibilities in near future.

Evaluation of GRACE data for water resource management in Iberia: a case study of groundwater storage monitoring in the Algarve region

Study regionIberia, Algarve basin, South Portugal. Study focusThis study evaluates the performance of several GRACE products in Iberia using the closure of the water budget. Then, it focusses on the Algarve region and explores the potential of GRACE as a tool of quantitative groundwater monitoring capable of bridging gaps in the existing ground-based network. Monthly data from GRACE, ancillary datasets from E-OBS, GLEAM, GRUN and ERA5, and groundwater level measurements from 12 karst-porous aquifers in the Algarve basin (5000 km2) are analyzed from 2004 to 2014. New Hydrological Insights for the RegionWhen considering the closure of the water budget at the Iberian scale, GRACE Mascon solutions perform remarkably well and better than the products based on spherical harmonics. When considering only the Algarve region, the results are similar to the ones obtained for Iberia, but the GRACE solution that performs the best is the average of the CSR and JPL Mascon products. In spite of the Algarve’s extremely small area when compared to the GRACE footprint, the satellite is capable of capturing the regionally averaged seasonal and de- seasonalized variations in observed groundwater storage (correlation between GRACE-derived and regionally averaged ground-based measurements is 0.82). For the first time ever at the regional Algarve scale, bounds are placed on the aquifer’s storage properties which vary from 3.65 × 10−3 to 4.92 × 10−2.

Sustainable management scenarios for northern Africa’s fossil aquifer systems

Achieving sustainability for the groundwater resources is more crucial and challenging in northern Africa than in any other part of the globe. We integrated temporal (April 2002–September 2019) terrestrial water storage (TWS) data derived from the Gravity Recovery and Climate Experiment (GRACE; April 2002–June 2017) and GRACE-Follow On (GRACE-FO; June 2018–present) missions with soil moisture, altimetry, and rainfall remote-sensing products to assess the sustainability of groundwater resources in northern Africa’s largest fossil aquifer systems: the Nubian Sandstone Aquifer System (NSAS) and the Northwestern Saharan Aquifer System (NWSAS). Results indicate the following: (1) in Algeria, Libya, and Tunisia, the NWSAS is currently being depleted due to excessive groundwater extraction; sustainable groundwater utilization could be achieved there if the average annual groundwater withdrawal is reduced by 2.70 km3; (2)the NSAS in Egypt (Dakhla sub-basin) is experiencing groundwater depletion, but it could achieve sustainable groundwater utilization if average annual groundwater withdrawal is reduced by 1.50 km3; and (3) the entire NSAS, in Egypt, Libya, Chad, and Sudan; the northern Sudan sub-basin in northern Sudan; and the Kufra sub-basin in Libya, northeastern Chad, and northwestern Sudan are healthy, and all three of them can be sustained at current levels of utilization. This study promises to improve our understanding of northern Africa’s aquifer dynamics and sustainability, signals an urgent need for integrated responses to develop sustainable utilization plans for groundwater resources, and promotes expanded and improved usage of GRACE and GRACE-FO products by the scientific community, end-users, and decision makers around the globe.

Seasonal and Decadal Groundwater Changes in African Sedimentary Aquifers Estimated Using GRACE Products and LSMs

Increased groundwater abstraction is important to the economic development of Africa and to achieving many of the Sustainable Development Goals. However, there is little information on long-term or seasonal groundwater trends due to a lack of in situ monitoring. Here, we used GRACE data from three products (the Centre for Space Research land solution (CSR), the Jet Propulsion Laboratory’s Global Mascon solution (JPL-MSCN), and the Centre National D’etudes Spatiales / Groupe de Recherches de Géodésie Spatiale solution (GRGS)), to examine terrestrial water storage (TWS) changes in 12 African sedimentary aquifers, to examine relationships between TWS and rainfall , and estimate groundwater storage (GWS) changes using four Land Surface Models (LSMs) (Community Land Model (CLM2.0), the Variable Infiltration Capacity model (VIC), the Mosaic model (MOSAIC) and the Noah model (NOAH)). We find that there are no substantial continuous long-term decreasing trends in groundwater storage from 2002 to 2016 in any of the African basins, however, consistent rising groundwater trends amounting to ~1 km3/year and 1.5 km3/year are identified in the Iullemmeden and Senegal basins, respectively, and longer term variations in ΔTWS in several basins associated with rainfall patterns. Discrete seasonal ΔTWS responses of ±1–5 cm/year are indicated by GRACE for each of the basins, with the exception of the Congo, North Kalahari, and Senegal basins, which display larger seasonal ΔTWS equivalent to approx. ±11–20 cm/year. The different seasonal responses in ΔTWS provide useful information about groundwater, including the identification of 5 to 9 month accumulation periods of rainfall in many semi-arid and arid basins as well as differences in ΔTWS responses between Sahelian and southern African aquifers to similar rainfall, likely reflecting differences in landcover. Seasonal ΔGWS estimated by combining GRACE ΔTWS with LSM outputs compare inconsistently to available in situ measurements of groundwater recharge from different basins, highlighting the need to further develop the representation of the recharge process in LSMs and the need for more in situ observations from piezometry.

What Is the Spatial Resolution of grace Satellite Products for Hydrology?

The mass change information from the Gravity Recovery And Climate Experiment (grace) satellite mission is available in terms of noisy spherical harmonic coefficients truncated at a maximum degree (band-limited). Therefore, filtering is an inevitable step in post-processing of grace fields to extract meaningful information about mass redistribution in the Earth-system. It is well known from previous studies that a number can be allotted to the spatial resolution of a band-limited spherical harmonic spectrum and also to a filtered field. Furthermore, it is now a common practice to correct the filtered grace data for signal damage due to filtering (or convolution in the spatial domain). These correction methods resemble deconvolution, and, therefore, the spatial resolution of the corrected grace data have to be reconsidered. Therefore, the effective spatial resolution at which we can obtain mass changes from grace products is an area of debate. In this contribution, we assess the spatial resolution both theoretically and practically. We confirm that, theoretically, the smallest resolvable catchment is directly related to the band-limit of the spherical harmonic spectrum of the grace data. However, due to the approximate nature of the correction schemes and the noise present in grace data, practically, the complete band-limited signal cannot be retrieved. In this context, we perform a closed-loop simulation comparing four popular correction schemes over 255 catchments to demarcate the minimum size of the catchment whose signal can be efficiently recovered by the correction schemes. We show that the amount of closure error is inversely related to the size of the catchment area. We use this trade-off between the error and the catchment size for defining the potential spatial resolution of the grace product obtained from a correction method. The magnitude of the error and hence the spatial resolution are both dependent on the correction scheme. Currently, a catchment of the size ≈63,000 km 2 can be resolved at an error level of 2 cm in terms of equivalent water height.

A Data‐Driven Approach for Repairing the Hydrological Catchment Signal Damage Due to Filtering of GRACE Products

AbstractOne of the major sources of uncertainty in mass change estimates from level 02 grace products comes from the signal degradation due to filtering of noisy gravity field products. Filtering suppresses noise but also changes the signal via attenuation and leakage. Therefore, many methods have been devised to tackle the unavoidable signal loss due to filtering. However, most of these methods lack mathematical analysis that is essential for understanding the cause and effect of filtering. Furthermore, they use hydrological models to compute correction terms, such as leakage, bias, or scale factor, for repairing the damage due to filtering. Recently, a data‐driven method was proposed for improving the filtered grace products, which was shown to be superior to three widely used model‐based methods. However, the method works efficiently only for catchments above a minimum size. This limitation is due to the usage of a uniform layer approximation for deriving a scale factor, which is used to counter the attenuation of the catchment‐confined signal. In this contribution, we avoid this approximation, and therefore the usage of scale factor, which lifts the limitation and provides a better mathematical relation. The new data‐driven method is able to restore the signal loss due to filtering independent of the catchment size. We validate the method in a realistic grace‐type closed‐loop simulation environment and compare it with other popular approaches. We show that for 22 out of 32 catchments (small to large size and located in different climatic zones) the improved data‐driven method outperforms other methods.

Validation of GRACE based groundwater storage anomaly using in-situ groundwater level measurements in India

In this study, we tried to validate groundwater storage (GWS) anomaly obtained from a combination of GRACE and land-surface model based estimates, for the first time, with GWS anomaly obtained from a dense network of in-situ groundwater observation wells within 12 major river basins in India. We used seasonal data from >15,000 groundwater observation wells between 2005 and 2013, distributed all over the country. Two recently released GRACE products, RL05 spherical harmonics (SH) and RL05 mascon (MS) products are used for comparison with in-situ data. To our knowledge, this is the first study of comparing the performance of two independent GRACE products at a sub-continental scale. Also for the first time, we have created a high resolution (0.10×0.10) map of specific yield for the entire country that was used for calculating GWS. Observed GWS anomalies have been computed using water level anomalies and specific yield information for the locale of individual observation wells that are up-scaled to basin-scale in order to compare with GRACE-based estimates. In general GRACE-based estimates match well (on the basis of the statistical analyses performed in the study) with observed estimates in most of the river basins. On comparing with observed GWS anomaly, GRACE-SH estimates match well in terms of RMSE, while GRACE-MS estimates show better association in terms of correlation, while the output of skewness, kurtosis, coefficient of variation (CV) and scatter analyses remain inconclusive for inter-comparison between two GRACE estimates. We used a non-parametric trend estimation approach, the Hodrick-Prescott (HP) filter, to further assess the performance of the two GRACE estimates. GRACE-MS estimates clearly outperform GRACE-SH estimates for reproducing observed GWS anomaly trends with significantly (>95% confidence level) strong association in 10 out of 12 basins for GRACE-MS estimates, on the other hand, GRACE-SH estimates show significantly (>95% confidence level) strong association in 6 out of 12 basins. On the basis of the study output, we recommend using GRACE-MS estimates for groundwater studies over the region and other regions of the globe with similar climatic, hydrogeologic or groundwater withdrawal conditions.

Monthly and sub-monthly hydrological variability: in-orbit validation by GRACE level 1B observations

In this study, we present an approach to validate hydrological model output directly on the level of GRACE level 1B observations by analyzing K-band range-rate residuals. Modeled water mass variations are converted to simulated satellite observations and subtracted from the original measurements. This procedure bypasses the downward continuation and filtering steps generally required for water cycle analysis on the basis of gravity field maps. The goal of the study is twofold: (1) we demonstrate the feasibility of using residuals analysis for hydrological model validation in general and (2) we focus on the potential of the approach to investigate the signal content of temporally high-frequent (daily) modeled hydrological mass variations. In addition to the output of three different hydrological process models, we study mass changes computed from two different daily GRACE products. GRACE here serves as a reference, but its spatial resolution is limited and the daily models are not computed independently. Regarding aspect (1), our results show that the agreement of each of the models with GRACE varies strongly depending on geographical location. Aspect (2) is not only interesting for model validation, but it is also important in the context of improving the GRACE de-aliasing concept. We demonstrate that not only the daily GRACE models, but also the daily hydrological model output contains information on time scales smaller than 1 month. Realistically modeled or observed short-term hydrological mass changes may serve as additional de-aliasing product for GRACE and thus contribute to increasing the accuracy and resolution of future GRACE products.

A method for reconstructing global ocean-induced surface displacements from land-based in-situ stations

We present a method for reconstructing a global, time-variable field of displacements of the Earth's crust caused by non-tidal ocean loading (NTOL) from a small number of local displacement measurements. This is achieved by the decomposition of the NTOL field into empirical orthogonal functions (EOFs), which we obtain from GRACE satellite data. With this method, we achieve the reproduction of a highly correlated displacement signal across 95% of the land surface and the reduction of residual continental displacement from up to 1mm to mostly below 0.5mm on monthly GRACE data. The extrapolation of the displacement signal into the time for which no global EOF patterns can be recovered is exemplarily shown. Further, the possibilities and limitations of applying the method on submonthly GRACE products are investigated. We conclude that the described method can be a useful complement to existing data sets of ocean mass-induced surface displacement, especially when in-situ measurement accuracy has improved.

Validation of GRACE-derived terrestrial water storage from a regional approach over South America

We propose to validate regional solutions consisting of 2° surface tiles of surface mass concentration over South America (90°W–30°W; 60°S–20°N) computed using accurate Level-1 GRACE measurements, by confronting them to other GRACE products (i.e., global GRGS and ICA-400km GFZ/CSR/JPL combined solutions) and independent in situ river level and discharge datasets. For this purpose, Principal Component Analysis (PCA) was applied to all of these types of GRACE-based solutions to extract the corresponding main orthogonal spatial and temporal modes of variability to be compared for 2003–2010. For the first three spatial modes, regional solutions provide a better geographical localization of hydrological structures, especially indicating major surface and groundwater systems of South America. Over hydrological patterns, records of river level versus time are particularly consistent with the GRACE temporal modes, especially for our regional solutions (i.e., correlations generally greater than 0.7). Interannual variations of GRACE-based Terrestrial Water Storage (TWS) clearly exhibit the signatures of extreme climatic events as the recent droughts and floods that affected South America. Very significant agreement is also found at interannual time-scale between TWS and discharges in drainage basins dominated by the surface reservoir (more than 0.9 of correlation in the Amazon basin).

Terrestrial waters and sea level variations on interannual time scale

On decadal to multidecadal time scales, thermal expansion of sea waters and land ice loss are the main contributors to sea level variations. However, modification of the terrestrial water cycle due to climate variability and direct anthropogenic forcing may also affect sea level. For the past decades, variations in land water storage and corresponding effects on sea level cannot be directly estimated from observations because these are almost unexistent at global continental scale. However, global hydrological models developed for atmospheric and climatic studies can be used for estimating total water storage. For the recent years (since mid-2002), terrestrial water storage change can be directly estimated from observations of the GRACE space gravimetry mission. In this study, we analyse the interannual variability of total land water storage, and investigate its contribution to mean sea level variability at interannual time scale. We consider three different periods that, each, depend on data availability: (1) GRACE era (2003–2009), (2) 1993–2003 and (3) 1955–1995. For the GRACE era (period 1), change in land water storage is estimated using different GRACE products over the 33 largest river basins worldwide. For periods 2 and 3, we use outputs from the ISBA-TRIP (Interactions between Soil, Biosphere, and Atmosphere—Total Runoff Integrating Pathways) global hydrological model. For each time span, we compare change in land water storage (expressed in sea level equivalent) to observed mean sea level, either from satellite altimetry (periods 1 and 2) or tide gauge records (period 3). For each data set and each time span, a trend has been removed as we focus on the interannual variability. We show that whatever the period considered, interannual variability of the mean sea level is essentially explained by interannual fluctuations in land water storage, with the largest contributions arising from tropical river basins.

Quantifying GRACE data contamination effects on hydrological analysis in the Murray–Darling Basin, southeast Australia

This paper investigates and quantifies the near-field and far-field contamination effects from Groupe de Recherche en Géodesie Spatiale GRACE products to assess whether or not they influence the accuracy with which hydrological signals in the Murray-Darling Basin, southeast Australia can be estimated. Far-field contamination was assessed by simulating some of the world's largest geophysical processes which generate major gravitational signals (e.g. melting of the Greenland icesheet, hydrology in the Amazon Basin) and measuring the proportion of the simulated signal detected in the Murray-Darling Basin. Near-field contamination from the Australian continent (excluding the Murray-Darling Basin) was also assessed. The sum of the near-field and far-field effects revealed a maximum of ∼10 mm (equivalent water height) of spurious signal within the Murray-Darling Basin. This equates to only one quarter of the formal uncertainty of the basin-scale estimates of changes in total water storage. Thus, GRACE products can be used to monitor broad-scale hydrological trends and variability in the Murray-Darling Basin without the need to account for contamination from external geophysical sources.

Errors of regional mass variations inferred from GRACE monthly solutions

Error correlations in current GRACE monthly gravity field models strongly affect the errors of inferred regionally averaged mass variations (RMV). To demonstrate this, we develop an empirical method which roughly estimates scaling factors to convert error bars computed by ignoring error correlations into more realistic error bars for RMV. For current solutions by CSR and GFZ, we find factors in the order of 2 or more for near‐polar regions and below one for near‐equatorial regions. A comparison of RMV computed from the CSR and the GFZ models for Greenland, Antarctica and the Amazon basin supports our findings. We discuss consequences on error assessment and error separation. For example, previously published GRACE‐derived RMV of Greenland may be still strongly affected by errors in the GRACE products.