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On the use of terrestrial On the use of terrestrial data (gravity, GPS, data (gravity, GPS,

hydrology) to validate hydrology) to validate GRACEGRACE

Caroline de Linage, Jacques Hinderer, Caroline de Linage, Jacques Hinderer, Jean-Paul Boy & Pascal GégoutJean-Paul Boy & Pascal Gégout

EOST-Institut de Physique du Globe de StrasbourgEOST-Institut de Physique du Globe de Strasbourg

October 15-17, 2007October 15-17, 2007 GRACE Science Team MeetingGRACE Science Team Meeting

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IntroductionIntroduction

Comparison GRACE/global hydrology Comparison GRACE/global hydrology modelsmodels large variability between modelslarge variability between models lack of well sampled hydrology datalack of well sampled hydrology data

Need to have other independent data sets Need to have other independent data sets for comparison to GRACEfor comparison to GRACE

Comparison GRACE/GGP in Europe Comparison GRACE/GGP in Europe (Neumeyer, Crossley)(Neumeyer, Crossley) small amplitude of the signal (a few µGal)small amplitude of the signal (a few µGal)

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OutlineOutline

Main issues on the validation of Main issues on the validation of satellite-derived gravity observations satellite-derived gravity observations with ground gravity measurementswith ground gravity measurements

A new validation experiment in A new validation experiment in western Africawestern Africa

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Comparison between ground Comparison between ground and satellite measurementsand satellite measurements

GRACE Absolute gravimeter

Gravity resolution

a few µGal 1 µGal

Spatial resolution

(harmonic degree)

400 - 660 km

(n<30-50)

point measureme

nt

Temporal resolution

10 days(1 month)

24h

need to have GRACE solutions with the best spatial and temporal resolution (mascons)

calibration is not possible

rather intercomparison between different data sets

October 15-17, 2007October 15-17, 2007 GRACE Science Team MeetingGRACE Science Team Meeting

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5 10 15 20 25 30

0,5

0,6

0,7

0,8

0,9

1,0

Ra

tio

Harmonic degree

satellite/ground

Satellite/ground gravity due to hydrology

Degree-power spectra (µGal) of gravity predictions from the global

hydrology model LaDWorld for April 2002

Satellite gravity

Ground gravity

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)

90%

90%

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Validation experiment in AfricaValidation experiment in Africa

Average annual precipitations in western Africa (1951–1989) from L’Hôte and Mahé (1996)

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)

absolute gravity measurements (FG5) every 2 absolute gravity measurements (FG5) every 2 months at 4(5) stations (2008-2010)months at 4(5) stations (2008-2010)

+ complementary measurements (A10)+ complementary measurements (A10)+ relative gravity measurements (field SG) at + relative gravity measurements (field SG) at

DjougouDjougou

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Monthly predictions from the GLDAS Monthly predictions from the GLDAS model (2002-2005)model (2002-2005)

Equivalent water height(mm)

Ground gravity (μGal)

Vertical displacement (mm)

-2.3 2.3Free-air gravity effect

µGal

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Predictions at each station from Predictions at each station from GLDAS 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

October 15-17, 2007October 15-17, 2007 GRACE Science Team MeetingGRACE Science Team Meeting

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GPS station arrayGPS station array

From Olivier Bock

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

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Estimation of the local Estimation of the local hydrological gravity changeshydrological gravity changes

Niamey, Diffa and Djougou have been studied since the Niamey, Diffa and Djougou have been studied since the 90’s by hydrologists and hydrogeologists90’s by hydrologists and hydrogeologists

dense network of hydrological, geophysical and dense network of hydrological, geophysical and meteorological meteorological in situ in situ observations at Djougou and Niamey observations at Djougou and Niamey (AMMA-CATCH 2003-2010)(AMMA-CATCH 2003-2010)

local ground water budget estimatelocal ground water budget estimate topographytopography precise modelling of the local Newtonian attraction term in precise modelling of the local Newtonian attraction term in

gravitygravity

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ConclusionConclusion

Need to have independent in-situ Need to have independent in-situ measurements in addition to GRACE measurements in addition to GRACE observations in order to improve the observations in order to improve the quality of hydrology modelsquality of hydrology models

Continuous GPS measurements to assess Continuous GPS measurements to assess the free-air effect not seen by GRACEthe free-air effect not seen by GRACE

In-situ measurements of hydrological In-situ measurements of hydrological parameters at each station to assist us in parameters at each station to assist us in modelling the local gravity effectmodelling the local gravity effect