On the use of terrestrial data (gravity, GPS, hydrology) to validate GRACE

1. On the use of terrestrial data (gravity, GPS, hydrology) to validate GRACE Caroline de Linage, Jacques Hinderer, Jean-Paul Boy & Pascal Gégout EOST-Institut de Physique du Globe de Strasbourg

2. Introduction • Comparison GRACE/global hydrology models • large variability between models • lack of well sampled hydrology data • Need to have other independent data sets for comparison to GRACE • Comparison GRACE/GGP in Europe (Neumeyer, Crossley) • small amplitude of the signal (a few µGal) GRACE Science Team Meeting Potsdam

3. Outline • Main issues on the validation of satellite-derived gravity observations with ground gravity measurements • A new validation experiment in western Africa GRACE Science Team Meeting Potsdam

4. Comparison between ground and satellite measurements • need to have GRACE solutions with the best spatial and temporal resolution (mascons) • calibration is not possible • rather intercomparison between different data sets GRACE Science Team Meeting Potsdam

5. low-degree effect in ground gravity due to the free-air effect (elastic part of the loading) • estimating the hydrology signal in the GPS time series of vertical displacement and removing the free-air effect from the ground gravity residual signal (after the same corrections as for the GRACE observations) Degree-power spectra (µGal) of gravity predictions from the global hydrology model LaDWorld for April 2002 Ground gravity Satellite gravity 90% 90% GRACE Science Team Meeting Potsdam

6. Average annual precipitations in western Africa (1951–1989) from L’Hôte and Mahé (1996) Validation experiment in Africa absolute gravity measurements (FG5) every 2 months at 4(5) stations (2008-2010) + complementary measurements (A10) + relative gravity measurements (field SG) at Djougou • Tamanrasset: 20 mm/yr (Sahara) -> null test • Agadez: 100 mm/yr (Sahara-Sahel transition) • Diffa: 300 mm/yr (Sahara-Sahel transition, lake Chad) • Niamey: 560 mm/yr (Sahel, Niger basin) • Djougou: 1200 mm/yr (monsoon) GRACE Science Team Meeting Potsdam

7. Equivalent water height (mm) Ground gravity (μGal) Vertical displacement (mm) Monthly predictions from the GLDAS model (2002-2005) Free-air gravity effect µGal -2.3 2.3 GRACE Science Team Meeting Potsdam

8. Predictions at each station from GLDAS Gravity changes (µGal)Vertical displacement (mm) Peak-to-peak amplitudes: • Djougou: 15 µGal 9.5 mm • Niamey: 10 µGal 5 mm • Diffa: 10 µGal 4.5 mm • Tamanrasset: 1-2 µGal 3.5 mm GRACE Science Team Meeting Potsdam

9. GPS station array • 6 stations from the AMMA experiment; maintained by EOST from 2009 • TOMB, OUAG, TAMA, DJOU, NIAM, GAO • 2 EOST stations • Diffa, Agadez • 3 IGS stations • Dakar, Libreville, Franceville • 1 CRAAG (Algeria) station • Tamanrasset Agadez Diffa From Olivier Bock GRACE Science Team Meeting Potsdam

10. Estimation of the local hydrological gravity changes • Niamey, Diffa and Djougou have been studied since the 90’s by hydrologists and hydrogeologists • dense network of hydrological, geophysical and meteorological in situ observations at Djougou and Niamey (AMMA-CATCH 2003-2010) • local ground water budget estimate • topography • precise modelling of the local Newtonian attraction term in gravity GRACE Science Team Meeting Potsdam

11. GRACE Science Team Meeting Potsdam

12. Conclusion • Need to have independent in-situ measurements in addition to GRACE observations in order to improve the quality of hydrology models • Continuous GPS measurements to assess the free-air effect not seen by GRACE • In-situ measurements of hydrological parameters at each station to assist us in modelling the local gravity effect GRACE Science Team Meeting Potsdam