Vagner G. Ferreira

Monitoring Groundwater Variations from Satellite Gravimetry and Hydrological Models: A Comparison with in-situ Measurements in the Mid-Atlantic Region of the United States

Aimed at mapping time variations in the Earth’s gravity field, the Gravity Recovery and Climate Experiment (GRACE) satellite mission is applicable to access terrestrial water storage (TWS), which mainly includes groundwater, soil moisture (SM), and snow. In this study, SM and accumulated snow water equivalent (SWE) are simulated by the Global Land Data Assimilation System (GLDAS) land surface models (LSMs) and then used to isolate groundwater anomalies from GRACE-derived TWS in Pennsylvania and New York States of the Mid-Atlantic region of the United States. The monitoring well water-level records from the U.S. Geological Survey Ground-Water Climate Response Network from January 2005 to December 2011 are used for validation. The groundwater results from different combinations of GRACE products (from three institutions, CSR, GFZ and JPL) and GLDAS LSMs (CLM, NOAH and VIC) are compared and evaluated with in-situ measurements. The intercomparison analysis shows that the solution obtained through removing averaged simulated SM and SWE of the three LSMs from the averaged GRACE-derived TWS of the three centers would be the most robust to reduce the noises, and increase the confidence consequently. Although discrepancy exists, the GRACE-GLDAS estimated groundwater variations generally agree with in-situ observations. For monthly scales, their correlation coefficient reaches 0.70 at 95% confidence level with the RMSE of the differences of 2.6 cm. Two-tailed Mann-Kendall trend test results show that there is no significant groundwater gain or loss in this region over the study period. The GRACE time-variable field solutions and GLDAS simulations provide precise and reliable data sets in illustrating the regional groundwater storage variations, and the application will be meaningful and invaluable when applied to the data-poor regions.

Estimating Total Discharge in the Yangtze River Basin Using Satellite-Based Observations

The measurement of total basin discharge along coastal regions is necessary for understanding the hydrological and oceanographic issues related to the water and energy cycles. However, only the observed streamflow (gauge-based observation) is used to estimate the total fluxes from the river basin to the ocean, neglecting the portion of discharge that infiltrates to underground and directly discharges into the ocean. Hence, the aim of this study is to assess the total discharge of the Yangtze River (Chang Jiang) basin. In this study, we explore the potential response of total discharge to changes in precipitation (from the Tropical Rainfall Measuring Mission—TRMM), evaporation (from four versions of the Global Land Data Assimilation—GLDAS, namely, CLM, Mosaic, Noah and VIC), and water-storage changes (from the Gravity Recovery and Climate Experiment—GRACE) by using the terrestrial water budget method. This method has been validated by comparison with the observed streamflow, and shows an agreement with a root mean square error (RMSE) of 14.30 mm/month for GRACE-based discharge and 20.98 mm/month for that derived from precipitation minus evaporation (P − E). This improvement of approximately 32% indicates that monthly terrestrial water-storage changes, as estimated by GRACE, cannot be considered negligible over Yangtze basin. The results for the proposed method are more accurate than the results previously reported in the literature.

MONITORING MASS CHANGES IN THE VOLTA RIVER BASIN USING GRACE SATELLITE GRAVITY AND TRMM PRECIPITATION

GRACE satellite gravity data was used to estimate mass changes within the Volta River basin in West African for the period of January, 2005 to December, 2010. We also used the precipitation data from the Tropical Rainfall Measurement Mission (TRMM) to determine relative contributions source to the seasonal hydrological balance within the Volta River basin. We found out that the seasonal mass change tends to be detected by GRACE for periods from 1 month in the south to 4 months in the north of the basin after the rainfall events. The results suggested a significant gain in water storage in the basin at reference epoch 2007.5 and a dominant annual cycle for the period under consideration for both in the mass changes and rainfall time series. However, there was a low correlation between mass changes and rainfall implying that there must be other processes which cause mass changes without rainfall in the upstream of the Volta River basin.

Uncertainties of the Gravity Recovery and Climate Experiment time-variable gravity-field solutions based on three-cornered hat method

Abstract. Currently, various satellite processing centers produce extensive data, with different solutions of the same field being available. For instance, the Gravity Recovery and Climate Experiment (GRACE) has been monitoring terrestrial water storage (TWS) since April 2002, while the Center for Space Research (CSR), the Jet Propulsion Laboratory (JPL), the GeoForschungsZentrum (GFZ), and the Groupe de Recherche de Géodésie Spatiale (GRGS) provide individual monthly solutions in the form of Stokes coefficients. The inverted TWS maps (or the regionally averaged values) from these coefficients are being used in many applications; however, as no ground truth data exist, the uncertainties are unknown. Consequently, the purpose of this work is to assess the quality of each processing center by estimating their uncertainties using a generalized formulation of the three-cornered hat (TCH) method. Overall, the TCH results for the study period of August 2002 to June 2014 indicate that at a global scale, the CSR, GFZ, GRGS, and JPL presented uncertainties of 9.4, 13.7, 14.8, and 13.2 mm, respectively. At a basin scale, the overall good performance of the CSR was observed at 91 river basins. The TCH-based results were confirmed by a comparison with an ensemble solution from the four GRACE processing centers.

Validation of GOCE gravity field models using GPS-leveling data and EGM08: a case study in Brazil

Abstract Validation of geopotential models derived from Gravity field and steady-state Ocean Circulation Explorer (GOCE) observations is a challenging task in regions with less advanced geodetic infrastructure such as Brazil. In order to assess the current performance of these models, 262 GPS-leveling sites, Earth Gravitational Model 2008 (EGM08) and Residual Terrain Model (RTM) are employed. The validation is based on the differences between GPS-leveling and GOCE-derived models. For the former, the spectral content beyond the GOCE-derived models’ maximum degree is removed by using EGM08 and RTM. The results indicate that the GOCE-based models: DGM-1S, SPW (Releases 1 and 2), TIM (Releases 1, 2, 3 and 4), and DIR (Releases 2, 3 and 4), at their maximum degrees have a worse performance than EGM08 while DIR-R1 shows an improvement of 11%. Furthermore, from the steepness of the slopes of the root mean square error (RMSE), it is observed that the optimal combination between DIR-R1 and EGM08 occurs at degree 230 (RMSE of 0.201 m). For the satellite-only models, DIR-R3 reduces the RMSE by ~1.4% compared to TIM-R4 at degree 190. These results are important for Brazil where the accuracy of the current geoid model is approximately 0.28 m.

An Investigation on the Closure of the Water Budget Methods Over Volta Basin Using Multi-Satellite Data

The terrestrial water budget over a particular hydrological domain (e.g., a drainage basin) plays an important role in understanding the interactions among the energy and water cycles. In this work, we assess the closure of the water budget methods [i.e., terrestrial water budget (TWB) and the coupled atmospheric-terrestrial water budget (ATWB)] as well as the control of total water storage gain over Volta Basin of West Africa. To achieve this, we explore the available satellite and data products: GRACE-derived terrestrial water storage changes (TWSC), satellite altimetry, TRMM-measured rainfall, MODIS-estimated evaporation, atmospheric moisture storage and divergence (ERA-Interim reanalysis data), and in situ discharge. The closure of water budgets are assessed by comparison with GRACE-derived TWSC. The results indicate that the closure of water budgets over the Basin, considering the agreement with GRACE-derived TWSC, present an RMSE of 33.72 and 48.22 mm/month for TWB and ATWB, respectively. This implies that the net precipitation (precipitation minus evaporation) computed from ERA-Interim reanalysis data presents high uncertainties over the Basin. Further, the significant accumulated water gain of 162.84 mm for the period of January 2003–December 2012, is 48% controlled by Lake Volta.

Geopotential numbers from GPS satellite surveying and disturbing potential model: a case study of Parana, Brazil

Abstract This article deals with the approach to determining the geopotential numbers from Global Positioning System (GPS) satellite surveying and disturbing potential at a point on the Earths surface. The disturbing potential was solved by the Brovar-type solution of Molodenskiis boundary value problem but in the context of the fixed problem. The solution is compatible with the techniques for smoothing of external field: removerestore techniques, residual terrain model, and use of high-resolution global geopotential models. This method provides an absolute geopotential numbers related to the so-called world height system. As an example, we calculate the disturbing potential model in Southern Brazil. The general condition of the sparse and absent gravity values is the main point to be solved. The absolute and relative accuracies of the quasigeoid model were tested vs. GPS/leveling data. Based on 99 GPS/leveling points, the mean value of fitting for the quasigeoid model was estimated near 0.032 m in the absolute view and 0.2 ppm in the relative view.

Analysis of the Discrepancies Between the Vertical Reference Frames of Argentina and Brazil

The vertical reference frames for Argentina and Brazil present discrepancies due to their different datums and realizations. Thus, since 2008, we have started a series of activities with the aim of unifying the Argentine and Brazilian national vertical networks (NVNs). To achieve this goal, we have connected the two NVNs at three border points by using the geodetic levelling approach. Additionally, the gravity field approach was also applied, based on a suitable representation of the geoid by considering the Earth Gravitational Model (EGM2008) in its full resolution. In this regard, 1266 co-located Global Positioning System (GPS) and levelling benchmarks regularly distributed over Argentina (612) and Brazil (654) were considered. The geodetic levelling approach shows an offset value of 54 cm, which implies that the Argentine vertical reference frame is above that of the Brazilian vertical reference frame. However, the result of the gravimetric approach shows an offset of 57 cm, which implies a difference of approximately 3 cm between both methods. Hence, since Brazil and Argentina represent a significant part of South America, the solution to the datum problem between both countries could point towards a common vertical reference frame for the Atlantic side.

Analysis of the Discrepancies Between the Brazilian Vertical Reference Frame and GOCE-Based Geopotential Model

In this study we estimate the discrepancies between the Brazilian vertical network, realized in the system of normal-orthometric heights, and a global quasi-geoid model. We consider the GNSS-leveling and a global quasi-geoid model derived from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission enhanced by high degree components of the Earth Gravitational Model 2008 (EGM2008). The results indicate that the enhanced geopotential model fits the GNSS-leveling with a root mean square error of 20.2 cm. The estimated bias of − 0. 4 ± 0.6 cm (w.r.t. the centroid of the network) implies that any future changes to the geopotential value W0 (62636856.0 m2 s−2) should be minor for the Brazilian Height System (BHS). However, since the GNSS-leveling based “height anomalies” refer to the Brazilian Vertical Datum at Imbituba harbor (BVD-I), Southern Brazil, the results of the comparisons may be an indicator of the mean bias of the national leveling network due to the effect of Mean Dynamic Topography (MDT) at the BVD-I.

An attempt to link the Brazilian Height System to a World Height System

This paper deals with the geopotential approach to investigate the present Brazilian Height System (BHS). Geopotential numbers are derived from Global Positioning System (GPS) satellite surveying and disturbing potential on selected benchmarks. A model for the disturbing potential can be obtained by an existing set of spherical harmonic coefficients such as the Earth Gravity Model 2008 (EGM08). The approach provides absolute evaluation of local normal geopotential numbers (aka spheropotential numbers) related to a so-called World Height System (WHS). To test the validity of the proposed methodology, a numerical experiment was carried out related to a test region in Southern Brazil. The accuracy of the derived geopotential numbers was tested versus local normal geopotential numbers based on 262 GPS/leveling points. The root mean square error (RMSE) value for metric offset of BHS derived from geopotential numbers and the disturbing potential modeling in the test area was estimated to be near 0.224 meters in the absolute view. Therefore, since these spheropotential numbers are referred to a local datum, these results of comparisons may be an indicator of the mean bias of local network due to the effect of local Sea Surface Topography (SSTop) and possible offset between the unknown reference for the BHS and the quasigeoid model in the region.