Spectral unmixing of vegetation, soil Spectral unmixing of vegetation, soil and dry carbon cover in arid and dry carbon cover in arid regions: comparing multispectral regions: comparing multispectral and hyperspectral observationsand hyperspectral observations

G.P.Asner and G.P.Asner and K.B.HeidebrechtK.B.Heidebrecht

IntroductionIntroduction

Objective – Objective – comparison of multi- and hyper-spectral comparison of multi- and hyper-spectral observations to observations to

decompose remotely sensed datadecompose remotely sensed data Why Important? – Why Important? – Study of impacts ofStudy of impacts of climate variability and landclimate variability and land use on vegetation coveruse on vegetation cover

Difficulties – Difficulties – small individual canopiessmall individual canopies - phenological changes- phenological changes - separation of NPV and bare soil in NDVI- separation of NPV and bare soil in NDVI

ApproachesApproaches – correlation of NDVI – correlation of NDVI - Spectral Mixture Analysis- Spectral Mixture Analysis

SMASMA

Assumes linear combinationAssumes linear combination Two methods of reflectance Two methods of reflectance

coefficient selectioncoefficient selection

Image-basedImage-based reflectances used that are likely to exist in the areareflectances used that are likely to exist in the area

lack of pure pixelslack of pure pixels

Spectral LibrariesSpectral Libraries data readily collecteddata readily collected

lack of generability and scalabilitylack of generability and scalability

Data used – Image Data used – Image basedbased Landsat TM – Landsat TM – commonly availablecommonly available

Terra ASTER – Terra ASTER – dense 5-channel dense 5-channel sampling atsampling at

SWIR2SWIR2

Terra MODIS – Terra MODIS – available daily available daily

15-channel sampling of 15-channel sampling of

visible and NIR visible and NIR

The land under The land under researchresearch Chihuahuan Desert, New Chihuahuan Desert, New

MexicoMexico

- - 210mm ppt per year210mm ppt per year

- Long-term ecological- Long-term ecological

research siteresearch site

- mainly grassland and- mainly grassland and

shrubshrub RequirementsRequirements - low species diversity- low species diversity

- strong differences of PV- strong differences of PV

and NPV between sitesand NPV between sites

- nearly constant soil type- nearly constant soil type

- few soil crusts- few soil crusts

MeasurementsMeasurements

ADC camera for grasslandADC camera for grassland Ikonos camera for shrublandIkonos camera for shrubland Areas 8ha each, with 300m N-S Areas 8ha each, with 300m N-S transect established using GPStransect established using GPS Field SpectroradiometerField Spectroradiometer

- measurements every 5m along transects- measurements every 5m along transects

- all canopies within 5m of sampling pts - all canopies within 5m of sampling pts measuredmeasured

- conversion to reflectance using calibration - conversion to reflectance using calibration panelpanel

AVIRIS sensor – AVIRIS sensor – NASA ER-2 aircraft altitude 20kmNASA ER-2 aircraft altitude 20km - pixels 19m x 19m- pixels 19m x 19m

Model and AnalysisModel and Analysis

Auto MCUAuto MCU - Fully automated Monte Carlo based- Fully automated Monte Carlo based derivation of uncertainty of cover fractionsderivation of uncertainty of cover fractions

- - Code carried out on field spectra andCode carried out on field spectra and sub-sampled to satellite channelssub-sampled to satellite channels

AlgorithmsAlgorithms – – tiedtied SWIR2 PV, NPV, soil spectra ‘tied’ at 2.03SWIR2 PV, NPV, soil spectra ‘tied’ at 2.03μμmm Less dependent on biomass, architecture, Less dependent on biomass, architecture,

biochemistry biochemistry

- - divisiondivision divided spectral reflectance values by reflectance divided spectral reflectance values by reflectance

at first wavelength at first wavelength mathematically inappropriate for linear SMAmathematically inappropriate for linear SMA

ResultsResults

Landsat TM convolved dataLandsat TM convolved data - little difference - little difference between shrubland and grassland sitesbetween shrubland and grassland sites

MODIS and most of AVIRIS -MODIS and most of AVIRIS - spectrally spectrally indistinguishableindistinguishable

ASTER -ASTER - some differences some differences

AVIRIS AVIRIS – finds negative PV fractions– finds negative PV fractions

- bare soil overestimated by ~20% - bare soil overestimated by ~20% - NPV fractions good- NPV fractions good Tied SWIR2 – showed consistent accuracyTied SWIR2 – showed consistent accuracy - corroborated by previous work- corroborated by previous work

FutureFuture

Important to continue thisImportant to continue this

research for ecologicalresearch for ecological

monitoringmonitoring Further research into theFurther research into the

use of instruments such as use of instruments such as

AVIRIS (i.e. high SNRAVIRIS (i.e. high SNR

in SWIR2) in SWIR2)

for use in SMA methodsfor use in SMA methods