Guillaume Ramillien

Time variations of the regional evapotranspiration rate from Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry

Since its launch in March 2002, the GRACE mission is measuring the global time variations of the Earths gravity field with a current resolution of ~500 km. Especially over the continents, these measurements represent the integrated land water mass including surface waters (lakes, wetlands and rivers), soil moisture, groundwater and snow cover. In this study, we use the GRACE land water solutions computed by Ramillien et al. (2005a) through an iterative inversion of monthly geoids from April 2002 to May 2004, to estimate time-series of basin-scale regional evapotranspiration rate -and associated uncertainties-. Evapotranspiration is determined by integrating and solving the water mass balance equation, which relates land water storage (from GRACE), precipitation data (from the Global Precipitation Climatology Centre), runoff (from a global land surface model) and evapotranspiration (the unknown). We further examine the sensibility of the computation when using different model runoff. Evapotranspiration results are compared to outputs of four different global land surface models. The overall satisfactory agreement between GRACE-derived and model-based evapotranspiration prove the ability of GRACE to provide realistic estimates of this parameter.

Evolution of high‐latitude snow mass derived from the GRACE gravimetry mission (2002–2004)

Abstract: Since March 2002, the GRACE mission provi des monthly global maps of geoid time-variations. These new data carry informatio n on the continental water storage, including snow mass variations, with a ground resolution of ~600-700 km . We have computed monthly snow mass solutions from the inversion of the 22 GRACE geoids (04/2002 - 05/2004). The inverse approach developed here allows to sepa rate the soil waters from snow signal. These snow mass solutions are further compared to predictions from three global land surface models and snow depths derived from satellite microwave data. We find that the GRACE solutions correlate well with the high-latitude zones of strong accumulation of snow. Regional means computed for four large boreal basins (Yenisey, Ob, Mac Kenzie and Yukon) show a good agreement at seasonal scale between the snow mass solutions and model predictions (global rms ~30-40 mm of equivalent-water height and ~10-20 mm regionally). Index terms: GRACE Gravimetry Mission, Hydrolology, Snow cover. hal-00280239, version 1 - 5 Nov 2009

Surface freshwater storage and dynamics in the Amazon basin during the 2005 exceptional drought

The Amazon river basin has been recently affected by extreme climatic events, such as the exceptional drought of 2005, with significant impacts on human activities and ecosystems. In spite of the importance of monitoring freshwater stored and moving in such large river basins, only scarce measurements of river stages and discharges are available and the signatures of extreme drought conditions on surface freshwater dynamics at the basin scale are still poorly known. Here we use continuous multisatellite observations of inundation extent and water levels between 2003 and 2007 to monitor monthly variations of surface water storage at the basin scale. During the 2005 drought, the amount of water stored in the river and floodplains of the Amazon basin was 130 km 3 ( 70%) below its 2003‐7 average. This represents almost a half of the anomaly of minimum terrestrial water stored in the basin as estimated using the Gravity Recovery and Climate Experiment (GRACE) data.

Modeling deformation induced by seasonal variations of continental water in the Himalaya region: Sensitivity to Earth elastic structure

Strong seasonal variations of horizontal and vertical positions are observed on GPS time series from stations located in Nepal, India, and Tibet (China). We show that this geodetic deformation can be explained by seasonal variations of continental water storage driven by the monsoon. For this purpose, we use satellite data from the Gravity Recovery and Climate Experiment to determine the time evolution of surface loading. We compute the expected geodetic deformation assuming a perfectly elastic Earth model. We consider Greens functions, describing the surface deformation response to a point load, for an elastic homogeneous half-space model and for a layered nonrotating spherical Earth model based on the Preliminary Reference Earth Model and a local seismic velocity model. The amplitude and phase of the seasonal variation of the vertical and horizontal geodetic positions can be jointly adjusted only with the layered Earth model, while an elastic half-space model fails, emphasizing the importance of using a realistic Earth elastic structure to model surface displacements induced by surface loading. We demonstrate, based on a formal inversion, that the fit to the geodetic data can be improved by adjusting the layered Earth model. Therefore, the study also shows that the modeling of geodetic seasonal variations provides a way to probe the elastic structure of the Earth, even in the absence of direct measurements of surface load variations.

Global bathymetry derived from altimeter data of the ERS-1 geodetic mission

Abstract A global 2-D spectral inversion of dense geoid data of the ERS-1 Geodetic Mission is presented to compute the ocean-floor topography in the waveband 15–500 km. The model accounts for flexural compensation with spatially varying elastic thickness of the lithosphere. The inverted 2-D bathymetric signal is then completed by wavelengths longer than 500 km using either the ETOPO-5 dataset or a bilinear interpolated grid from direct shiptrack measurements (NGDC data). For the inversion a global marine geoid grid computed from the ERS-1 data using the collocation method was used. An analytical transfer function beween 2-D spectra of geoid and bathymetry is obtained from a simple lithospheric flexure model, which depends upon several parameters such as mean ocean depth, crustal thickness, elastic thickness, density distribution of sea water, crust and mantle. An analysis of sensibility of the prediction filter with respect to these parameters provides a hierarchy between them, the elastic thickness being the most critical. The elastic thickness vs age variations is taken into account using the half-space cooling model. It is assumed that the elastic thickness follows an isotherm. Two cases are considered: 450 °C and 600 °C isotherms. Validation of the predicted bathymetry is made through comparison with long profiles of NGDC original soundings and ETOPO-5 data in two selected areas: Central Pacific and North Atlantic oceans. Results show that the predicted bathymetry is generally closer to NGDC data than ETOPO-5 data, which are smoother, and that the 600 °C isotherm case gives lower root-mean square values.

Comparison of in situ bottom pressure data with GRACE gravimetry in the Crozet-Kerguelen region

Two time series of deep ocean bottom pressure records (BPRs) in between the Crozet Islands and Kerguelen are compared with GRACE (Gravity Recovery And Climate Experiment) equivalent water heights. An analysis of the correlation is performed for four time series: 1) monthly averages of the equivalent water height at the Crozet Islands, 2) the same near the Kerguelen Islands, 3) the mean of the two preceding series and 4) the difference between the two locations expressed in terms of geostrophic transport. We find that smoothed GRACE solutions are strongly correlated with the BPR data with correlation coefficients in the order of 0.7–0.8. Consequently GRACE measures real oceanic mass variations in this region.

Glacial isostatic adjustment and nonstationary signals observed by GRACE

[1] Changes in hydrologic surface loads, glacier mass balance, and glacial isostatic adjustment (GIA) have been observed using data from the Gravity Recovery and Climate Experiment (GRACE) mission. In some cases, the estimates have been made by calculating a combination of the linear rate of change of the time series and periodic seasonal variations of GRACE estimates, yet the geophysical phenomena are often not stationary in nature or are dominated by other nonstationary signals. We investigate the variation in linear rate estimates that arise when selecting different time intervals of GRACE solutions and show that more accurate estimates of stationary signals such as GIA can be obtained after the removal of model-based hydrologic effects. We focus on North America, where numerical hydrological models exist, and East Antarctica, where such models are not readily available. The root mean square of vertical velocities in North America are reduced by ~20% in a comparison of GRACE- and GPS-derived uplift rates when the GRACE products are corrected for hydrological effects using the GLDAS model. The correlation between the rate estimates of the two techniques increases from 0.58 to 0.73. While acknowledging that the GLDAS model does not model all aspects of the hydrological cycle, it is sufficiently accurate to demonstrate the importance of accounting for hydrological effects before estimating linear trends from GRACE signals. We also show from a comparison of predicted GIA models and observed GPS uplift rates that the positive anomaly seen in Enderby Land, East Antarctica, is not a stationary signal related to GIA.

Denoising Satellite Gravity Signals by Independent Component Analysis

Independent component analysis (ICA) is a blind separation method based on simple assumptions of the independence of sources and the non-Gaussianity of observations. An approach based on ICA is used here to extract hydrological signals over land and oceans from the polluting striping noise due to orbit repetitiveness and present in the gravity anomalies detected by the Gravity Recovery and Climate Experiment (GRACE) satellites. We took advantage of the availability of monthly level-2 GRACE solutions from three official providers (i.e., CSR, JPL, and GFZ) that can be considered as different observations of the same phenomenon. The efficiency of the methodology is demonstrated on a synthetic case. Applied to one month of GRACE solutions, it allows for clearly separating the total water storage change from the meridional-oriented spurious gravity signals on the continents but not on the oceans. This technique gives results equivalent to the destriping method for continental water storage.

Application of the Regional Water Mass Variations from GRACE Satellite Gravimetry to Large-Scale Water Management in Africa

Time series of regional 2° × 2° Gravity Recovery and Climate Experiment (GRACE) solutions of surface water mass change have been computed over Africa from 2003 to 2012 with a 10-day resolution by using a new regional approach. These regional maps are used to describe and quantify water mass change. The contribution of African hydrology to actual sea level rise is negative and small in magnitude (i.e., −0.1 mm/y of equivalent sea level (ESL)) mainly explained by the water retained in the Zambezi River basin. Analysis of the regional water mass maps is used to distinguish different zones of important water mass variations, with the exception of the dominant seasonal cycle of the African monsoon in the Sahel and Central Africa. The analysis of the regional solutions reveals the accumulation in the Okavango swamp and South Niger. It confirms the continuous depletion of water in the North Sahara aquifer at the rate of −2.3 km3/y, with a decrease in early 2008. Synergistic use of altimetry-based lake water volume with total water storage (TWS) from GRACE permits a continuous monitoring of sub-surface water storage for large lake drainage areas. These different applications demonstrate the potential of the GRACE mission for the management of water resources at the regional scale.

Water balance of the Arctic drainage system using GRACE gravimetry products

Land water and snow mass anomalies versus time were computed from the inversion of 50 Gravity Recovery and Climate Experiment (GRACE) geoids (August 2002 to February 2007) from the RL04 GeoForschungZentrum (GFZ) release and used to characterize the hydrology of the Arctic drainage system. GRACE-based time series have been compared to snow water equivalent and snow depth climatologies, and snowfall for validation purpose. Time series of regional averages of water volume were estimated for the 11 largest Peri-Arctic basins. Strong correlations were found between the snow estimates and river discharges in the Arctic basins (0.49–0.8). Then changes in land water storage were compared to precipitation minus evapotranspiration fluxes to determine which flux of the hydrological budget controls the Arctic hydrology. Results are very contrasted according to the basin. Trends of snow and land water masses were also computed over the 2003–2006 period. Eurasian basins lose snow mass whereas North American basins are gaining mass.