POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

On the retrieval of soil moisture content over agricultural sites using

L band SAR data

Francesco Mattia(1), Giuseppe Satalino(1), Anna Balenzano(1), Malcolm Davidson(2)

(1) CNR-ISSIA, Bari, Italy

(2) ESA ESTEC, Noordwijk, The Netherlands

This work was partially funded by ESA ESTEC under contracts 19558/06/NL/HE and by MI-PAF under contract 209/7393/05.

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

develop and assess a retrieval algorithm estimating soil moisture over agricultural sites from L band SAR data

Objective

Application context: agriculture and/or hydrology

Outline

Experimental data: Terra SAR SIM 03 campaign

The retrieval algorithm: direct and inverse approach

Results

Conclusions and future work

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

TerraSARSIM campaign: 2-25 April, 2003(ESA project n. 16736/02/NL/FF)

• Barrax, La Mancha, Spain• Mediterranean climate• Average yearly rainfall: 400 mm• Flat morphology• Agricultural area• 65% dry land, 35% irrigated land

SAR data (5 Flights)

Sensor: E-SAR onboard DLR Dornier Frequency: C-, L- and X-bandIncidence angle: 26° - 56°Doy: 92, 92, 94, 94, 115

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

In situ dataN° of monitored fields: 28Land use: 7 irr. wheat, 4 irr. barley, 5 irr. alfalfa, 12 not-irr. barleyData:

gravimetric and volumetric soil moisture at 0-5 cm (some problems!)qualitative information on surface roughnessfresh and dry biomass & crop height

Selected 12 cereal fields: 8 non irr. barley; 2 irr. barely; 2 irr. wheat

0,0

0,5

1,0

1,5

2,0

2,5

90 95 100 105 110 115 120

Dates of SAR acquisitions (Doy)

Fres

h bi

omas

s (K

g/m

2)

0,0

5,0

10,0

15,0

20,0

25,0

30,0

90 95 100 105 110 115 120

Dates of SAR acquisitions (Doy)

Vol.

moi

stur

e co

nten

t (%

)

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

a priori estimates of surface and vegetation parametersˆ jp

direct model

( ) ( )0 1,...ˆ , , j M ii pσ θ λ ε== +iF total model & measurement error

backscattering

Maximum likelihood solution for parameters

( ) ( )( ) ( )

2 20 1..

2 20

, , ˆ1 1i j M j jii j

i j

p p pC

N M p

σ θ λ

σ=− −

= +∆ ∆

∑ ∑F

error on a priori estimates

total rms error

( ) 20 iiσ ε∆ =

jp

Retrieval algorithm

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

( ) ( ) 2 sec( )0 0ˆ ˆ i hSi i eτ θσ σ −=

h

, ,..i HH VV=

( )0 , , , .,...w v dg m f strε ρ′′ = ( )vv vb W≈ vW [Kg/m3]2 2

VV VV s VV sh b W b Fπ πτλ λ

≈ ∝ sW [Kg/m2]

sF [Kg/m2]

(Veget. Water cont.)/m2

(Fresh biomass )

( )0ˆ S iσ Integral Equation Model (IEM)2

VV oπτ ελ

′′∝

Direct model

rε

An anisotropic uniaxialdielectric slab with an effective permittivity

θz

x

0 00 00 0

e er

e e

o o

jj

j

ε εε ε ε

ε ε

′ ′′+⎛ ⎞⎜ ⎟′ ′′= +⎜ ⎟⎜ ⎟′ ′′+⎝ ⎠

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

Free parameterssoil:

surface height rms (s), surface corr. length (l) and soil moisture (mv)

vegetation:

fresh biomass (F), structural parameters bhh & bvv

Multiple regression: ∆σ0s & ∆F Updating guess values

Temporal updating: 2 SAR acquisitions

σ0t =σ0s e(-g ·F)∆σ0t/ σ0t=∆σ0s/ σ0s – g ∆F

1st SAR date: s, l, mv, F, bhh, bvv

measured estimated on the 1st date

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

SAR data used in the study

L band HH & VV SAR data acquired on:

- April 2, 2003 (flight n.1, in the morning)

- April 4, 2003 (flight n.3, in the morning)

- April 25, 2003 (flight n.5, in the morning)

The same guess values obtained as averages of in situ measurements

guess values obtained by temporal updating

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

0

5

10

15

20

25

30

0,0 0,5 1,0 1,5 2,0

Fresh biomass (Kg/m2)

Vol.

moi

stur

e co

nt.

(%)

measured

retrieved: no attenuation

retrieved: with attenuation

Retrieved soil moisture: April 2 & 4

: mean =17 %

: mean=9.4 %, rms=9%

: mean =18.7 %, rms=4.2 %

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

-0,5

0,0

0,5

1,0

1,5

2,0

2,5

10 12 14 16 18 20 22 24

vol. moisture content (%)

Fres

h bi

omas

s (K

g/m

2)

measured

retrieved

Retrieved biomass: April 2 & 4

: mean = 0.8 Kg/m2

: mean = 1 Kg/m2, rms=0.4 Kg/m2

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

Biomass updating: April 4 & 25

0,0

0,5

1,0

1,5

2,0

2,5

1 2 3 4 5 6 7 8 9 10 11 12Selected Fields

Incr

emen

ts o

f Fre

sh b

iom

. (K

g/m

2)

true increments of Fresh biomass

estimated " "

: mean = 0.9 Kg/m2

: mean = 1.3 Kg/m2, rms= 0.64 Kg/m2

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

10

15

20

25

30

1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4

Fresh biomass (kg/m2)

Vol.

soil

moi

stur

e (%

)

measured

retrieved (updated guess values)

Retrieved soil moisture: April 25

: mean =16.4 %

: mean=19.6 %, rms=4.6%

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

1,0

1,5

2,0

2,5

3,0

10 12 14 16 18 20 22 24

Vol. soil moisture (%)

Fres

h bi

omas

s (k

g/m

2)

measured

retrieved (updated guess values)

Retrieved biomass: April 25

: mean =1.7 Kg/m2

: mean=1.8 Kg/m2, rms=0.4 Kg/m2

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

Vol. Soil moist. Content (%)3 3015

Soil moisture map: April 25

POLinSAR 2007, 22-26 January 2007, ESRIN Frascati, Italy

Conclusions

Future worksystematic assessment of the algorithm accuracy through

simulation studies;

application of the algorithm to other data sets and extend the approach to other crops

soil moisture and vegetation biomass maps of cereal fieldshave been retrieved, with accuracies of approximately 5 % and 0.5 Kg/m2, respectively, by inverting L band SAR data and using a priori information;

a technique to update a priori information on surface parameters using multi-temporal SAR data has been proposed and tested on the Barrax test site;