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High-resolution soil moisture mappingby a proximal ground penetrating radar

A numerical, laboratory and field evaluation

| Minet Julien |PhD public dissertation

19 May 2011

Thse prsente en vue delobtention du grade deDocteur en SciencesAgronomiques et IngnierieBiologique

Facult des BioingnieursEarth and Life Institute

Environmental Sciences

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Earth and Life Institute Environmental Sciences

IntroductionIntroductionIntroductionIntroduction

Soil moisture is a key variable of the water cycle.

http:// www.southtippcoco.ie/water/watercycle.htm

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Soil moisture controls the land water and energy budgets.

NASA

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Soil moisture controls the partitioning of rainfall intorunoff and infiltration.

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Soil moisture controls plant growth and germination.

http://www.lookfordiagnosis.com

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How to measure soil moisture?Reference method: soil sampling

V

W W

w

DRY WET v

.

)(

=

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How to measure soil moisture?Invasive sensors: TDR, capacitance probes, ERT, neutron probes,

Tarrawarra experiments, 1995-1998, Australia

10.5 ha, 10 x 20 m, ~500 point measurements

Western & Grayson, WRR, 1998

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How to measure soil moisture?Remote sensing

SMOS: 50 km resolution,2-3 days revisiting time

Soil moisture estimatedby SMOS, ESA, in April

2010 and April 2011 overFrance

The driest conditions ofthis spring 2011 arerevealed.

www.cesbio.ups-tlse.fr/SMOS_blog

APRIL 2010

APRIL 2011

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Intermediate support scale between remote and invasive sensors: proximalsensing

GPR: many uses in civil engineering, archeology, soil surveys,

How to measure soil moisture?Ground penetrating radar

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What are the needs?Spatial scale gap between remote sensing and invasive

sensors

Limitations of other existing GPR methods


(Vereecken et al. , WRR, 2008)

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ObjectivesObjectivesObjectivesObjectives

1. Develop a new GPR technique for soil moisturemapping

2. Study the spatiotemporal variability of soil moisture atthe field scale and its implications in hydrology

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PlanPlanPlanPlan

Presentation of the GPR method

GPR developments1. Validation of the GPR method for soil moisture mapping

2. Soil moisture sensing by GPR in case of shallow layering

3. Soil moisture profiles determination by GPR

4. A frequency domain reflectometry (FDR) application

Soil moisture studies5. Temporal stability of soil moisture patterns

6. Effect of soil moisture variability on runoff response

Conclusions & Perspectives

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PlanPlanPlanPlan


GPR developments

1. Validation of the GPR method for soil moisture mapping







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GPR transmits a radar wave into the soil and records the backscattered signal

The electromagnetic response is function of the soil electrical properties: , &

water >> soil & air GPR waves are mainly sensitive to soil moisture

GPRGPRGPRGPR methodmethodmethodmethod: basic: basic: basic: basic principleprincipleprincipleprinciple

airairairair

soilsoilsoilsoil , &

antennaantennaantennaantenna

, &

0

20

40

6080

100

Water Soil particle Air

P e r m i t t i v i t y

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GPR setup:

Vector Network Analyzer

Horn antenna

Frequency domain

200-2000 MHz

Off-ground monostatic GPR

GPRGPRGPRGPR methodmethodmethodmethod: setup: setup: setup: setup

Field acquisition:

Motorized platform

DGPS + GPR + laptop

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GPRGPRGPRGPR methodmethodmethodmethod: how: how: how: how itititit worksworksworksworks????

Soil dielectricpermittivity

Soil moisture point data

Antenna filtering

Measured signal

Filtered GPR signal

Petrophysical relationship (e.g., Topp)

GPR inversion

- 3D model of wavepropagation in a multi-layered medium

- Optimization algorithm

Laboratory antennacalibration

Soil moisture map

Interpolation (e.g., kriging)

measuredmodeled

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Surface soil moisture: one-layered soil model inversion

GPRGPRGPRGPR methodmethodmethodmethod: GPR data inversion: GPR data inversion: GPR data inversion: GPR data inversion

airairairairlayer 1layer 1layer 1layer 1

ha

Time-domain inversion focused on soil surface reflection peak

GPR antenna height h a and soil dielectric permittivity are retrieved.

Time- or frequency-domain inversions

Full-waveform inversions: Maximize the information retrieval capabilities

GPR MODEL

ITERATION

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GPR characteristics:

Support scale of ~1 m

Extent up to 10 ha

Spacing from 1 to several meters> 1000 points/hour

Characterization depth of ~ 5 cm,or more with multi-layered models

low frequency: avoid surfaceroughness and vegetation < 5-10cm

GPRGPRGPRGPR methodmethodmethodmethod: ::: fieldfieldfieldfield conditionsconditionsconditionsconditions

Spacing= 1m

Support=1m

Extent = 10 ha

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PlanPlanPlanPlan


GPR developments

1. Validation of the GPR method for soil moisture mapping







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Is that GPR working in the field?

Inversion uncertainties (What is the precision?)Repeatability (Can the measurements be repeated?)

Ground-truthing (What is the accuracy?)

1. Validation of the GPR method for soil moisture mapping1. Validation of the GPR method for soil moisture mapping1. Validation of the GPR method for soil moisture mapping1. Validation of the GPR method for soil moisture mapping

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aaaaiiiirrrr

The same field wasmapped 3 times within 3hours.

Maps produced bykriging of ~ 1400 points

Similar soil moisture

maps were obtained.

Minet et al., JH, submitted

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Maps of inversion uncertainty

are low: 0.0039 m/m

are spatially and

temporally correlated.

It is precise

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aaaaiiiirrrr

Maps of the difference ininterpolated values ( z)

Average repetition errorof 0.0169 m/m

Measurements can be repeated

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aaaaiiiirrrr

Fit: Ledieu et al. , JH, 1986relationship:

RMSE of the fit is 0.0231m/m in V

20 volumetric measurements of soil moisture VS GPR-derived dielectricpermittivities

It can measure the real soil moisture

ba =

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Vertical soil moisture variations are occurring in natural environments,due to:

evaporation

wetting front infiltration

pedogenetic processes

2. Soil moisture sensing by GPR in case of shallow layering2. Soil moisture sensing by GPR in case of shallow layering2. Soil moisture sensing by GPR in case of shallow layering2. Soil moisture sensing by GPR in case of shallow layering

This causes two problems for soil moisture sensing by radar waves:

Interferences due to shallow layering in backscattered radar data

Shallow soil moisture (e.g., 0-5 cm) is not the soil moisture of interest(e.g., 0-30 cm)

drydrydrydry wetwetwetwet

How to deal with vertical soil moisture variations?

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2. Soil moisture sensing by GPR in case of shallow layering2. Soil moisture sensing by GPR in case of shallow layering2. Soil moisture sensing by GPR in case of shallow layering2. Soil moisture sensing by GPR in case of shallow layering

airairairairlayer 1layer 1layer 1layer 1 1

2 layer 2layer 2layer 2layer 2

ha

h1

airairairairlayer 1layer 1layer 1layer 1

ha

Laboratory experiment: GPR measurements over two-layered media

GPR data inversion with one- and two-layered models

Minet et al., IEEE TGRS, 2010

Soil moisture

D i e l e c t r i c p e r m

i t t i v i

t y

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3. Soil moisture profiles determination by GPR3. Soil moisture profiles determination by GPR3. Soil moisture profiles determination by GPR3. Soil moisture profiles determination by GPR

Two models used to invert field data:

2-layered model

continuous profile model (100 thin layers actually)

(z)

airairairairlayer 1layer 1layer 1layer 1 1

2 layer 2layer 2layer 2layer 2

airairairairsoilsoilsoilsoil

4 parameters: h a, 1, h 1, 2 6 parameters: h a, h 1, 1, 2, , n

0.5 1 1.5 2

x 10-8

-2

0

2

t (s)

G x

x ( t ) ( - )

measured2L modeledvG modeled

ha hah1

h1

1

2

Can vertical soil moisture profiles be retrieved?

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3. Soil moisture profiles determination by GPR3. Soil moisture profiles determination by GPR3. Soil moisture profiles determination by GPR3. Soil moisture profiles determination by GPR

Maps of surface andsubsurface soil moistureare obtained.

3D-image of soilmoisture

2 3 4 5 6

0

0.01

0.02

0.03

0.04

D e p

t h [ m ]

r

van Genuchten

2-layered1-layered

SURFACE SUBSURFACE

2 - L a y e r e

d

P r o

f i l e

Minet et al., Geoderma, 2011

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4. A frequency domain4. A frequency domain4. A frequency domain4. A frequency domain reflectometryreflectometryreflectometryreflectometry (FDR) application(FDR) application(FDR) application(FDR) application

A simple FDR probe calibration was developed.

FDR system could be modeled.

Can the electromagnetic modeling approach be applied to FDR?Minet et al., VZJ, 2010

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PlanPlanPlanPlan


GPR developments

1. Validation of the GPR method for soil moisture mapping2. Soil moisture sensing by GPR in case of shallow layering






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5. Temporal stability of soil moisture patterns5. Temporal stability of soil moisture patterns5. Temporal stability of soil moisture patterns5. Temporal stability of soil moisture patterns

Knowing temporal stability of soilmoisture can help for:

installing permanent sensors;

disaggregating coarse-scale soilmoisture data;

understanding hydrological

processes (infiltration, water uprisingand redistribution).

Soil moisture patterns aredetermined by:

topography;

soil type;

solar exposition;

Do soil moisture patterns show temporal stability?Minet et al., to be submitted

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5. Temporal stability of soil moisture patterns5. Temporal stability of soil moisture patterns5. Temporal stability of soil moisture patterns5. Temporal stability of soil moisture patterns

Some zones are persistently drier, and other wetter.

Temporal stability index: Mean difference to the field-average

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6. Effect of soil moisture variability on runoff response6. Effect of soil moisture variability on runoff response6. Effect of soil moisture variability on runoff response6. Effect of soil moisture variability on runoff response

Soil moisture determines the partitioning of rainfall into infiltration and runoffand is a key variable in hydrological modeling.

In hydrological modeling, spatial variability of soil moisture is often unknown athigh resolution.

How to disaggregate coarse-scale soil moisture into fine-scale soil moisturepatterns?

How spatial variability of soil moisture impacts on the simulation ofrunoff at the field scale?

Minet et al., HESS, 2011

REMOTE SENSING

HYDROLOGICALMODELING

DISAGGREGATION

DOWNSCALING

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7 scenarios of antecedent soil moisture

10 field campaigns

Hydrological simulations with a single rainfall event

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Large differences between soil moisture scenariosThe Constant scenario gives a smaller runoff than the average of Random.

The best scenario among 10 fields is the Structured.

Time

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PlanPlanPlanPlan


GPR developments

1. Validation of the GPR method for soil moisture mapping2. Soil moisture sensing by GPR in case of shallow layering






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Validation in field conditions:Accurate and precise method for soil moisture sensing (1.5% -2.5% soil

moisture error)

Faster and more practicable than other existing GPR methods

Limitations:

Effect of large surface roughness not well known

Measurements in presence of vegetation

What is exactly the depth of characterization?

Conclusions & PerspectivesConclusions & PerspectivesConclusions & PerspectivesConclusions & Perspectives

1. GPR developments1. GPR developments1. GPR developments1. GPR developments

Is that GPR working in the field?

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Vertical soil moisture measurements:Multi-layered models can deal with vertical soil

moisture variations.

A better assessment of depth of

characterizationLimitations:

Difficulties to perform ground-truthmeasurements along a soil profile

Uncertainties in subsurface soil moisture

Limitation of the depth of penetration of theradar waves


1. GPR developments1. GPR developments1. GPR developments1. GPR developments

How to deal with vertical soil moisture variations?

2 3 4 5 6

0

0.01

0.02

0.03

0.04

D e p

t h [ m ]

r

van Genuchten2-layered1-layered

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Spatio-temporal soil moisture patterns:

These patterns inform about hydrological processes at the field scale.

Time-stable soil moisture areas can be revealed.

Limitations:

Limited time frame in which soil moisture was measured

Lack of soil data to explain soil moisture patterns

Lack of topographic contrast in the field


2. Soil moisture studies2. Soil moisture studies2. Soil moisture studies2. Soil moisture studies

Do soil moisture patterns show temporal stability?

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Effect of antecedent soil moisture on runoff:

Large effect of soil moisture spatial variabilityon simulated runoff

Best modeling scenario is to arrange soilmoisture according to the topography, testedon 10 different fields

Limitations:

Lack of measured runoff data

Other spatial attributes could explain soilmoisture (e.g., soil data)


2. Soil moisture studies2. Soil moisture studies2. Soil moisture studies2. Soil moisture studies

How spatial variability of soil moisture impacts on the simulation of runoff at the field scale?

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Perspectives in soil moisture sensingPerspectives in soil moisture sensingPerspectives in soil moisture sensingPerspectives in soil moisture sensing

Digital soil mapping and precision agriculture

Time-lapse soil moisture data e.g., soil hydraulic properties

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Perspectives in soil moisture sensingPerspectives in soil moisture sensingPerspectives in soil moisture sensingPerspectives in soil moisture sensing

Validation of remote sensing data

e.g., Lievens et al., submitted

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ThanksThanksThanksThanks

Research project:

funded by:

with:

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Li ffLi ffLi ffLi ff

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Line effectLine effectLine effectLine effect

D th f h t i tiD th f h t i tiD th f h t i tiD th f h t i ti

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Depth of characterizationDepth of characterizationDepth of characterizationDepth of characterization

air

layer 1

layer 2

air

layer 1

layer 2

P t t i tiP t t i tiP t t i tiP t t i ti

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Parameters uncertaintiesParameters uncertaintiesParameters uncertaintiesParameters uncertainties

Optimized parameter uncertainties: a probabilistic distribution is obtainedinstead of a unique value.

Soil moisture mapsSoil moisture mapsSoil moisture mapsSoil moisture maps

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Soil moisture mapsSoil moisture mapsSoil moisture mapsSoil moisture maps

sites.uclouvain.be/MappingResearch

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How to measure soil moisture?Remote sensing

Passive sensors (radiometers): > 5 km resolution

Active sensors (scatterometers, SAR): >5 m resolution

ERS/MetOp Soil Moisture, July 1999, TUWien

50 km resolution

ENVISAT-2-VV, 20/03/2009, Luxembourg,

from Lievens et al., submitted

12.5 m resolution

Validation of the GPR method for soil moisture mappingValidation of the GPR method for soil moisture mappingValidation of the GPR method for soil moisture mappingValidation of the GPR method for soil moisture mapping

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Validation of the GPR method for soil moisture mappingValidation of the GPR method for soil moisture mappingValidation of the GPR method for soil moisture mappingValidation of the GPR method for soil moisture mapping

Three independent uncertainties assessment were performed.

Average uncertainties agree.

0.0231 m/m0.0142 m/m0.0169 m/m0.0039 m/m

Comparison withsoil sampling

Deviation of thedifference

Repetitionsuncertainties

Inversionuncertainties

GPR measurementerror

Interpolation error Unknown/residual

error

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