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Annual time scale Rapid time scale Data and processing 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, [email protected] ; 2 IRD-HSM UM2, Montpellier, France, [email protected] ; 3 IGN Madrid, Spain 4 IRD, Cotonou, Bénin, [email protected] ; 5 LTHE, Grenoble, France; 6 Dipartimento di Scienze della Terra Università “Federico II” di Napoli, Italy • 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 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 SG location in northern Benin and other features: - Absolute gravity measurements with FG5 - Hydro: Observation Well (OW) and Neutron probe (NP) SG data processing: - Removal of earth tides and pressure effect (Δg P ) (Δg P =Δg NonLocal + Δg Local = 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 Δg NonLocal is 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) SG data & processing: A) Raw data B) Residuals (gaps, offsets, spikes, tides, pressure) and drift estimates C) Final residuals A B C Hydrological data: D) Water table depth E) WSC from NP measurements (0- 7m) F) Daily and cumulative rainfall Hydrological data - Water table fluctuations (daily) - WSC from Neutron Probe (NP) measurements (0-7m, weekly) - Rainfall (5minutes) Mask effect: gravity effect of a 1mm thick layer of water at different depths, for different instrumental heights (z) Mask effect due to the shelter around the SG Rainfall admittance : the supposed (empirical) linear relationship between gravity increase and rainfall amount Rainfall admittance: Regression of gravity increase vs rainfall amount Theoreti cal Empirical 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. Rainfall admittance is 0.22 ± 0.02 nm/s²/mm Distribution of gravity slopes after >15mm rainfall Exemple of gravity slopes following rainfall events. Blue: SG residuals. Red: linear regression. Green: day-night variations for ET. Hydrological processes at rapid time scales: - ET: decreases gravity signal - Lateral losses: decreases gravity signal - Infiltration below the shelter: increases gravity signal Analysis of Evapotranspiration: Gravity decrease is 1.5 ± 1.8 nm/s²/day -Admittance 0.22 nm/s²/day => 6.8mm/day -Admittance 0.44 nm/s²/day => 3.4mm/day 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 Introduction • Evapotranspiration estimation with SG data is complex and requires proper understanding of the shelter’s mask effect. SGs should be as high as possible with respect to the shelter area. Infiltration may need some basic modeling. • Annual WSC are clearly dominating the SG residuals. Misfits with hydrological data are still to be investigated but may enlight the advantages of integrative gravity measurements with respect to non-representative hydrological point measurements. Acknowledgements: La Direction Générale de l'Eau du Bénin, l'ANR Ghyraf, l'ORE AMMA-Catch, Maxime Wubda,Théo Ouani, Simon Afouda, Conclusion OBJECTIVE: What hydrological information can we derive from SG time series? Piezo NP D E F SG vs NP: SG vs NP, water table, and 2D model for Specific Yield (Sy): 27/08/08 30/12/09 05/05/11 07/09/12 -4 -2 0 2 4 6 8 10 12 14 µgal NP NP w shelter SG FG 5 corrected 27/01/09 15/08/09 03/03/10 19/09/10 07/04/11 24/10/11 11/05/12 27/11/12 -10 -5 0 5 10 15 µgal SG residuals = 0.56 = 0.60 (sheltereffect) 0 1 2 3 4 0 0.2 0.4 0.6 0.8 Frequency (in cpd) M agnitude R esponse ofP ressure fields Strasbourg D jougou Pressure spectrum: strong S1 and S2 components -2 0 2 4 -20 -10 0 10 rain:78 m m days (0=2010-8-13) nm/s² -2 0 2 4 -20 -10 0 10 rain:44 m m days (0=2010-8-17) nm/s² -2 0 2 4 -20 -10 0 10 rain:65 m m days (0=2010-9-20) nm/s² -2 0 2 4 -20 -10 0 10 rain:66 m m days (0=2010-10-7) nm/s² -2 0 2 4 -20 -10 0 10 rain:40 m m days (0=2011-9-27) nm/s² -2 0 2 4 -20 -10 0 10 rain:19 m m days (0=2011-10-7) nm/s² -2 0 2 4 -20 -10 0 10 rain:23 m m days (0=2011-10-13) nm/s² -2 0 2 4 -20 -10 0 10 rain:70 m m days (0=2012-2-21) nm/s² -2 0 2 4 -20 -10 0 10 rain:28 m m days (0=2012-4-24) nm/s² 0 1 2 3 4 5 6 7 0 0.1 0.2 0.3 0.4 0.5 depths (m ) rainfalladm ittance (nm /s²/m m ) gravity effectofa 1m m layerofw ater unm asked z=0.20 m z=0.30 m z=0.40 m z=0.60 m 0 20 40 60 80 0 5 10 15 20 rainfalleffectR ²=0.78 rainfallam ount(m m ) gravity increase (nm /s²) data reg slope = 0.22 nm /s²/m m Higher than expected, because: • Neglected rainfall falling from rooftop • Further infiltration during the event • Runoff • Residuals in the pressure correction -5 0 5 10 0 2 4 6 8 M ass losses distribution m ass loss m =1.5 nm /s²/d std=1.8 nm /s²/d count 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. SG residuals and WSC derived from NP measurements in the vadoze zone and water table variations in the saturated zone. Apparent resistivity map and MR soundings (θ MRS ) used to derive a 2D model for Sy • 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. To account for spatial variability, a 2D model for Sy is build from resistivity mapping and Magnetic Resonance Soundings (MRS). • The best-fitting value is ranging from 0.56 to 0.6 • Comparison shows a fair agreement except for 2011 WSC=Δh..θ MRS (x,y)+WSC NP(vadoze zone) , where Δh are water table levels, and WSC NP(vadoze zone) are WSC in the vadoze zone measured by NP at the SG site Need for further investigation of spatial variability of processes and state variables. (Micro-gravimetry) Because MRS is known to overestimate Sy, its value is scaled with = Sy/θ MRS (θ MRS is the MRS- derived water content). Then: Water table Vadoze zone OW
description
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 - PowerPoint PPT Presentation
Transcript of Annual time scale
Annual time scaleRapid time scale
Data and processing
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, [email protected]; 2 IRD-HSM UM2, Montpellier, France, [email protected]; 3IGN Madrid, Spain4IRD, Cotonou, Bénin, [email protected]; 5LTHE, Grenoble, France; 6 Dipartimento di Scienze della Terra Università “Federico II” di Napoli, Italy
• 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
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
SG location in northern Benin and other features:- Absolute gravity measurements with FG5- Hydro: Observation Well (OW) and Neutron probe (NP)
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 ΔgNonLocal is 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)
SG data & processing: A) Raw dataB) Residuals (gaps, offsets, spikes, tides, pressure) and drift estimates C) Final residuals
A
B
C
Hydrological data: D) Water table depthE) WSC from NP measurements (0-7m) F) Daily and cumulative rainfall
Hydrological data- Water table fluctuations (daily)- WSC from Neutron Probe (NP) measurements (0-7m, weekly)- Rainfall (5minutes)
Mask effect: gravity effect of a 1mm thick layer of water at different depths, for different instrumental heights (z)
Mask effect due to the shelter around the SGRainfall admittance: the supposed (empirical) linear relationship between gravity increase and rainfall amount
Rainfall admittance: Regression of gravity increase vs rainfall amount
Theoretical
Empirical
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.
Rainfall admittance is 0.22 ± 0.02 nm/s²/mm
Distribution of gravity slopes after >15mm rainfall
Exemple of gravity slopes following rainfall events. Blue: SG residuals. Red: linear regression. Green: day-night variations for ET.
Hydrological processes at rapid time scales:- ET: decreases gravity signal- Lateral losses: decreases gravity signal- Infiltration below the shelter: increases gravity signal
Analysis of Evapotranspiration:
Gravity decrease is 1.5 ± 1.8 nm/s²/day
-Admittance 0.22 nm/s²/day => 6.8mm/day-Admittance 0.44 nm/s²/day => 3.4mm/day
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
Introduction
• Evapotranspiration estimation with SG data is complex and requires proper understanding of the shelter’s mask effect. SGs should be as high as possible with respect to the shelter area. Infiltration may need some basic modeling. • Annual WSC are clearly dominating the SG residuals. Misfits with hydrological data are still to be investigated but may enlight the advantages of integrative gravity measurements with respect to non-representative hydrological point measurements.Acknowledgements: La Direction Générale de l'Eau du Bénin, l'ANR Ghyraf, l'ORE AMMA-Catch, Maxime Wubda,Théo Ouani, Simon Afouda, Sarah Soubeyran, Idrissou Imaourou, Emile Pagou, Saré et les villageois de Nalohou. Julia Pfeffer, J.P. Vandervaere .
Conclusion
OBJECTIVE:What hydrological information can we derive from SG time series?
Piezo
NP
D
E
F
SG vs NP:
SG vs NP, water table, and 2D model for Specific Yield (Sy):27/08/08 30/12/09 05/05/11 07/09/12
-4-202468
101214
µgal
NPNP w shelterSGFG5 corrected
27/01/09 15/08/09 03/03/10 19/09/10 07/04/11 24/10/11 11/05/12 27/11/12-10
-5
0
5
10
15
µgal
SG residuals = 0.56 = 0.60 (shelter effect)
0 1 2 3 40
0.2
0.4
0.6
0.8
Frequency (in cpd)
Magnitude Response of Pressure fields
StrasbourgDjougou
Pressure spectrum: strong S1 and S2 components
-2 0 2 4-20-10
010
rain: 78 mm
days (0=2010-8-13)
nm/s
²
-2 0 2 4-20-10
010
rain: 44 mm
days (0=2010-8-17)
nm/s
²
-2 0 2 4-20-10
010
rain: 65 mm
days (0=2010-9-20)
nm/s
²
-2 0 2 4-20-10
010
rain: 66 mm
days (0=2010-10-7)
nm/s
²
-2 0 2 4-20-10
010
rain: 40 mm
days (0=2011-9-27)
nm/s
²
-2 0 2 4-20-10
010
rain: 19 mm
days (0=2011-10-7)
nm/s
²
-2 0 2 4-20-10
010
rain: 23 mm
days (0=2011-10-13)
nm/s
²
-2 0 2 4-20-10
010
rain: 70 mm
days (0=2012-2-21)
nm/s
²
-2 0 2 4-20-10
010
rain: 28 mm
days (0=2012-4-24)
nm/s
²
0 1 2 3 4 5 6 70
0.1
0.2
0.3
0.4
0.5
depths (m)
rain
fall
adm
ittan
ce (n
m/s
²/mm
)
gravity effect of a 1mm layer of water
unmaskedz=0.20 mz=0.30 mz=0.40 mz=0.60 m
0 20 40 60 800
5
10
15
20rainfall effect R²=0.78
rainfall amount (mm)
grav
ity in
crea
se (n
m/s
²)
datareg slope = 0.22 nm/s²/mm
Higher than expected, because:
• Neglected rainfall falling from rooftop• Further infiltration during the event• Runoff• Residuals in the pressure correction
-5 0 5 100
2
4
6
8
Mass losses distribution
mass loss m=1.5 nm/s²/d std=1.8 nm/s²/d
coun
t
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.
SG residuals and WSC derived from NP measurements in the vadoze zone and water table variations in the saturated zone.
Apparent resistivity map and MR soundings (θMRS ) used to derive a 2D model for Sy
• 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.
To account for spatial variability, a 2D model for Sy is build from resistivity mapping and Magnetic Resonance Soundings (MRS).
• The best-fitting value is ranging from 0.56 to 0.6
• Comparison shows a fair agreement except for 2011
WSC=Δh..θMRS(x,y)+WSCNP(vadoze zone), where Δh are water table levels, and WSCNP(vadoze zone) are WSC in the vadoze zone measured by NP at the SG site
Need for further investigation of spatial variability of processes and state variables. (Micro-gravimetry)
Because MRS is known to overestimate Sy, its value is scaled with = Sy/θMRS (θMRS is the MRS-derived water content). Then:Water table Vadoze zone
OW
