Annual time scale

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Rainfall impact on gravity at annual and rapid time scales from a superconducting gravimeter record in Benin, West-Africa Hector B.1, Hinderer J.1, Séguis L.2, Boy JP.1, Calvo M.1,3, Descloitres M.4,5, Rosat, S.1, Riccardi U.6,Littel, F. 1 1 IPGS-EOST, CNRS/UdS, Strasbourg, France, basile.hector@unistra.fr; 2 IRD-HSM UM2, Montpellier, France, luc.seguis@ird.fr; 3IGN Madrid, Spain 4IRD, Cotonou, Bénin, marc.descloitres@ird.fr; 5LTHE, Grenoble, France; 6 Dipartimento di Scienze della Terra Università “Federico II” di Napoli, Italy • SG data processing: • Removal of earth tides and pressure effect (ΔgP)(ΔgP=ΔgNonLocal+ ΔgLocal= loading(ECMWF(0.25°-180°))-2.79*P) • Where P is the local pressure, -2.79 is the analytical pressure admittance in the 0°-0.25° cylinder, and ΔgNonLocalis the result of the loading calculations with ECMWF pressure fields. • Corrections of gaps, spikes and offsets • Tide model upgrade, removal of non local hydrological load (derived from GLDAS model) • Drift estimation with FG5 and NP data • Re-correction of offsets with gap-filling using rain admittance (see rapid time-scale below) • Water Storage Changes (WSC) and evapotranspiration (ET) are the less known terms of the hydrological budget equation • Δ(stock)  Δ(gravity) GRAVIMETRY • Superconducting Gravimeter (SG) records = continuous measurements • Hydrological data • Water table fluctuations (daily) • WSC from Neutron Probe (NP) measurements (0-7m, weekly) • Rainfall (5minutes) Conclusion D Annual time scale Rapid time scale Data and processing Introduction • 2 time scales • Rapid (2-3days): rainfall-triggered processes (ET and redistribution) • Annual: time-integrated processes -> state variable: WSC •  Need for proper processing of SG data to retrieve WSC and ET A E NP B OW Piezo F • SG location in northern Benin and other features: • Absolute gravity measurements with FG5 • Hydro: Observation Well (OW) and Neutron probe (NP) C SG data & processing: A) Raw data B) Residuals (gaps, offsets, spikes, tides, pressure) and drift estimates C) Final residuals Hydrological data: D) Water table depth E) WSC from NP measurements (0-7m) F) Daily and cumulative rainfall OBJECTIVE: What hydrological information can we derive from SG time series? Pressure spectrum: strong S1 and S2 components Rainfall admittance: the supposed (empirical) linear relationship between gravity increase and rainfall amount SG vs NP: Mask effect due to the shelter around the SG At the surface, rainfall admittance should be 0.05 to 0.20 nm/s²/mm. This is lower than the 0.44 value of the topographic effect alone (close to the Bouguer plate). Instrumental height is also critical. • WSC (NP monitoring) at the SG site  distributed on the topography •  Calculation of the gravity effect at the SG sensor location. • Results show a fair agreement in phase and behavior but not in amplitude:  due to poor calibration of the NP, to strong spatial heterogeneities, to poor corrections in the SG time series (offsets, drift), or non-expected processes. • Mask effect insignificant at the annual time-scale, as water redistributes below the shelter. Theoretical Rainfall admittance is 0.22 ± 0.02 nm/s²/mm Empirical Higher than expected, because: SG vs NP, water table, and 2D model for Specific Yield (Sy): Mask effect: gravity effect of a 1mm thick layer of water at different depths, for different instrumental heights (z) SG residuals and WSC derived from NP measurements and distributed on the topography with/without the shelter’s mask effect. In red are FG5 data. To account for spatial variability, a 2D model for Sy is build from resistivity mapping and Magnetic Resonance Soundings (MRS). • Neglected rainfall falling from rooftop • Further infiltration during the event • Runoff • Residuals in the pressure correction Rainfall admittance: Regression of gravity increase vs rainfall amount Because MRS is known to overestimate Sy, its value is scaled with =Sy/θMRS (θMRSis the MRS-derived water content). Then: Analysis of Evapotranspiration: Water table Vadoze zone WSC=Δh..θMRS(x,y)+WSCNP(vadozezone), whereΔh are water table levels, and WSCNP(vadozezone) are WSC in the vadoze zone measured by NP at the SG site • The best-fitting  value is ranging from 0.56 to 0.6 • Comparison shows a fair agreement except for 2011 • Need for further investigation of spatial variability of processes and state variables. (Micro-gravimetry) Distribution of gravity slopes after >15mm rainfall Apparent resistivity map and MR soundings (θMRS) used to derive a 2D model for Sy SG residuals and WSC derived from NP measurements in the vadoze zone and water table variations in the saturated zone. Exemple of gravity slopes following rainfall events. Blue: SG residuals. Red: linear regression. Green: day-night variations for ET. Close to known values of Potential Evapotranspiration (4-5mm/day). But several processes are acting, with different admittances.  Need for modeling of the infiltration below the shelter Gravity decrease is 1.5 ± 1.8 nm/s²/day • Evapotranspiration estimation with SG data iscomplex and requiresproperunderstanding of the shelter’smaskeffect. SGsshouldbe as high as possible with respect to the shelter area. Infiltration mayneedsome basic modeling. • Annual WSC are clearlydominating the SG residuals. Misfitswithhydrological data are still to beinvestigated but mayenlight the advantages of integrativegravitymeasurementswith respect to non-representativehydrological point measurements. • Acknowledgements:La Direction Générale de l'Eau du Bénin, l'ANRGhyraf, l'ORE AMMA-Catch, MaximeWubda,ThéoOuani, Simon Afouda, Sarah Soubeyran, IdrissouImaourou, Emile Pagou, Saré et les villageois de Nalohou. Julia Pfeffer, J.P. Vandervaere . • Hydrological processes at rapid time scales: • ET: decreases gravity signal • Lateral losses: decreases gravity signal • Infiltration below the shelter: increases gravity signal -Admittance 0.22 nm/s²/day => 6.8mm/day -Admittance 0.44 nm/s²/day => 3.4mm/day