Advances in Ground-based Gravity for Hydrologic Studies

Advances in Ground-based Gravity for Hydrologic Studies L. Longuevergne (1,2), C.R. Wilson 1, B.R. Scanlon 2 1 Department of Geological Sciences, UT Austin 2 Bureau of Economic Geology, UT Austin

What is Hydro-geodesy? Study of the continental part of the hydrological cycle Study of the Earth s deformations and the Earth s gravity field aet g Slope Water mass redistribution Gravity variations Load deformation

What is Hydro-geodesy? Conventional units 1 nm.s -2 (i.e. 0.1 Gal) gravity change is the attraction of an infinite 2.4 mm thickness water layer. Significant contribution of hydrology on gravity time series Among others: Lambert and Beaumont, JGR, 1977 Mäkinen and Tattari, IAG, 1990 Bower and Courtier, PEPI, 1998 Hinderer & Crossley, Surv. Geophys, 2000 Kroner et al., J. geodyn, 2004 Van Camp et al., JGR, 2006 Development of geodetic methods for hydrological studies Among others: Pool and Eychaner, Groundwater, 1995 Hasan et al., WRR, 2008 Longuevergne, PhD, 2008 Naujok, PhD, 2008 Krause et al., JoH, 2009 Creutzfeld et al., HESS, 2010

Gravity variations [ nm.s -2 ] Spraying test Spraying test in MOXA observatory (Germany) Spraying Natural rainfall 12 nm.s -2 Drainage Gravi Time From Kroner et al., J. Geodyn., 2004 Measurable effect 4cm fullwaterlayer 12 nms. 2 10 9 g

Another test Comparison to soil moisture measurements From Longuevergne et al., J. Geodyn., 2009

Outline Physical processes and spatial scales Instrument types Observation strategies and examples Application of gravity tools for hydrological studies

Physical processes 1. Newtonian Attraction Horizontal

Physical processes 1. Newtonian Attraction Information on water mass Horizontal Water mass

Physical processes 1. Newtonian Attraction Information on water mass 2. Elastic deformation Horizontal Surface

Physical processes 1. Newtonian Attraction Information on water mass 2. Elastic deformation Information on the pressure applied on the crust (water mass) (Farrell, RGSP, 1977) Water mass Horizontal Surface Phenomenon Eq. water layer Vertical deformation Tide (Britany) 10 m 10 cm (Llubes et al., 2007) Amazon basin water 0.3 m 3 cm (VanDam et al., 2001)

Spatial scales Measurement methods Gravity variations Tilt variations Vertical deformation Main spatial scales of sensitivity Ground gravimeter Tiltmeter GPS Water mass Surface Llubes et al., J. geodyn, 2004 GRACE satellite 100 m 1 km 10km 100 km 1000 km Investigated Spatial scales

Recorded contributions Local mass sensitivity g Surface

Sensitivity Recorded contributions Local mass sensitivity g space

Sensitivity Recorded contributions Local mass sensitivity g Surface h 66% at 6h 70% at 6h Relative height h between gravimeter & target 70% of sensitivity on a 6xh diameter disc space

Sensitivity Recorded contributions Local mass sensitivity g Surface h 66% at 6h 70% at 6h Relative height h between gravimeter & target 70% of sensitivity on a 6xh diameter disc space Sensitivity to global scales, amplitude ~ 30 nm.s -2, seasonal Especially linked to the elastic deformation of the Earth Typically corrected using LSM simulations Boy & Hinderer, J. geodyn, 2006 Wziontek et al., J. geodyn, 2009

Outline Physical processes and spatial scales Instrument types Observation strategies and examples Application of gravity tools for hydrological studies

Hydrology Instrument improvement cm water 25000 2500 250 25 2.5 Relative & absolute gravimeters Year Adapted from Torge, 1989

Hydrology Instrument improvement cm water 25000 2500 250 25 2.5 Relative & absolute gravimeters 2000 Year 0.25 Superconducting gravimeter Adapted from Torge, 1989

Which gravimeter for hydrological studies? Micro g Lacoste Gphone ~ 100 k$ CG-5 ~ 100 k$ http://www.microglacoste.com ZLS Burris ~ 50 k$ http://www.zlscorp.com Instrument Resolution (eq. full water layer) Field gravimeter (relative) 25 mm Drifting instruments Rq Portable instrument, quick measure Ideal for network repetition

Which gravimeter for hydrological studies? Micro g Lacoste FG5 ~ 400 k$ A-10 ~ 400 k$ http://www.microglacoste.com Instrument Resolution (eq. full water layer) Rq Field gravimeter (relative) 25 mm Drifting instruments Absolute gravimeter (free fall) 25 mm Absolute Transportable instrument Provide reference value

Which gravimeter for hydrological studies? GWR OSG ~ 450 k$ igrav ~ 250 k$ http://www.gwrinstruments.com Instrument Resolution (eq. full water layer) Rq Field gravimeter (relative) 25 mm Drifting instruments Absolute gravimeter (free fall) 25 mm Absolute Superconducting (relative) < 2 mm Very low drift Accurate and stable instrument Ideal for continuous monitoring

Outline Physical processes and spatial scales Instrument types Observation strategies and examples Application of gravity tools for hydrological studies

Vertical integration Observation strategy Continuous observation : monitoring vertically integrated mass variations

Vertical integration Observation strategy Continuous observation : monitoring vertically integrated mass variations Unconfined aquifer g geom. Sy. h g h Gravity variation Well level variation Sy ~10 1 geom Sy Geometric factor Porosité de drainage

GRAVITY CHANGE, MICROGAL DEPTH TO WATER, METERS Vertical integration Observation strategy Example in Tucson, Arizona 50 40 30 20 10 0-10 -20-30 -40-50 R = 0.94 S.Y = 0.27 Sep-97 Sep-98 Sep-99 Sep-00 Sep-01 Sep-02 35 40 45 50 Good correlation unconfined aquifer From Pool, Geophysics, 2008

Vertical integration Observation strategy Continuous observation : monitoring vertically integrated mass variations Soil moisture Unconfined aquifer

GRAVITY CHANGE, MICROGAL DEPTH TO WATER, METERS Vertical integration Observation strategy Example in Tucson, Arizona 70 60 50 40 30 20 10 0-10 -20-30 Gravity Depth to Water Sep-97 Sep-98 Sep-99 Sep-00 Sep-01 Sep-02 95 100 105 110 115 Significant vadose zone contribution From Pool, Geophysics, 2008

Vertical integration Observation strategy Continuous observation : monitoring vertically integrated mass variations Soil moisture Unconfined aquifer Confined aquifer

Vertical integration Observation strategy g geom. S. h g h Gravity variation Well level variation S ~10 3 geom S Geometric factor Storativity

GRAVITY CHANGE, MICROGAL DEPTH TO WATER, METERS Vertical integration Observation strategy g geom. S. h g h Gravity variation Well level variation S ~10 3 geom S Geometric factor Storativity Example in Tucson, Arizona 50 40 30 20 10 0-10 -20-30 -40-50 Gravity Depth to Water Sep-97 Sep-98 Sep-99 Sep-00 Sep-01 Sep-02 70 75 80 85 90 From Pool, Geophysics, 2008

Network repetition Observation strategy - No continental effect in spatial differences, focus on local effects - Investigate lateral variations, localize heterogeneities Soil Moisture Groundwater

Network repetition Observation strategy - No continental effect in spatial differences, focus on local effects - Investigate lateral variations, localize heterogeneities Soil Moisture Groundwater Example in MOXA Observatory (Germany) Geometry validation of a spatially-distributed model From Naujok, PhD, 2008, Naujok et al., J.Geod., 2008

Network repetition Observation strategy - No continental effect in spatial differences, focus on local effects - Investigate lateral variations, localize heterogeneities - Vertical gradient: g 2 times water storage in the layer in between Soil Moisture Groundwater Example in Durzon karst (France) Estimation of epikarst storage From Jacob et al., GJI., 2009

Multi-instrumental investigation Observation strategy 1. Precise local measurements 2. Inference at larger scale 3. Localization of water mass 4. Investigation of lateral & vertical heterogeneity Soil Moisture Groundwater Ongoing projects (among others) International project gathering several teams operating superconducting gravimeters GGP network (Global Geodynamic Project) Crossley et Hinderer, J. Geodyn, 2009

Multi-instrumental investigation Observation strategy 1. Precise local measurements 2. Inference at larger scale 3. Localization of water mass 4. Investigation of lateral & vertical heterogeneity Soil Moisture Groundwater Ongoing projects (among others) Ground and space gravity In AMMA / CATCH area GHYRAF project (Gravity and Hydrology in Africa) Hinderer et al. J. Geodyn, 2009 GGP network (Global Geodynamic Project) Crossley et Hinderer, J. Geodyn, 2009

Outline Physical processes and spatial scales Instrument types Observation strategies and examples Application of gravity tools for hydrological studies

Pumping test Hydrological applications Simulation of a homogeneous unconfined aquifer Q=5 m 3 /day over 7 days From Damiata et al., JoH; 2006

Pumping test Hydrological applications Simulation of a homogeneous unconfined aquifer Q=5 m 3 /day over 7 days T = 5.10-2 m 2 /s Sy=0.25 T = 5.10-3 m 2 /s Sy=0.15 T = 5.10-3 m 2 /s Sy=0.25 T = 5.10-3 m 2 /s Sy=0.35 T = 5.10-4 m 2 /s Sy=0.25 From Damiata et al., JoH; 2006

Pumping test Hydrological applications T = 5.10-3 m 2 /s Sy=0.15 T = 5.10-3 m 2 /s Sy=0.25 T = 5.10-3 m 2 /s Sy=0.35 Simulation of a homogeneous unconfined aquifer Q=5 m 3 /day over 7 days T = 5.10-2 m 2 /s Sy=0.25 T = 5.10-3 m 2 /s Sy=0.15 T = 5.10-3 m 2 /s Sy=0.25 T = 5.10-3 m 2 /s Sy=0.35 - Complementary information - Heterogeneity investigation from surface measurements - Artificial recharge monitoring T = 5.10-4 m 2 /s Sy=0.25 From Damiata et al., JoH; 2006

Model validation Hydrological applications Mass balance equation S Total water storage (catchment) d ( S) dt P aet Q s Q sub P aet Q Precipitations Actual evapotranspiration Surface runoff Q sub Subsurface runoff After time integration g ( t) S P Q s dt aet Q sub dt Measurable fluxes Fluxes to be modeled Any systematic error generate a drift-like signal Gravity brings a additional constrain on fluxes realism

Model validation Hydrological applications Example in Strasbourg observatory (France) SG CLSM model Residuals

Model validation Hydrological applications Example in Strasbourg observatory (France) SG CLSM model Residuals Main discrepancy linked to 2003 European heat wave Gravity data highlight a systematic error in ET CLSM does not stress vegetation enough in dry conditions

Adapting SGs for field Hydrological applications Idea: Keep SG characteristics for field studies Example in Austin, Texas From Wilson et al. IAG, 2009

Adapting SGs for field Hydrological applications Idea: Keep SG characteristics for field studies Example in Austin, Texas Austin, TX Worst drought in 50 years Results: original sensitivity for periods > 1 day Importance of pillar From Wilson et al. IAG, 2009

Adapting SGs for field Hydrological applications Idea: Keep SG characteristics for field studies Next generation of SGs igrav, GWR ~ 50 kg Low energy consumption ( ~1.5 kw )

Conclusions Interest of gravity instruments Sensitive to water storage variations complementary information Integrative information (~ 100 m) Non-destructive & well calibrated information Multi-instrument study Precise local measurements (~ few mm) Inference at larger scale with network repetition Water mass localization in active hydrological systems Lateral & vertical heterogeneity investigation

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