Analysis of ERS wind scatterometer time series over Sahel (Mali)

Jarlan, Lionel; Mougin, E.; Frison, Pierre‐Louis; Mazzéga, Pierre; Hiernaux, Pierre

doi:10.1016/s0034-4257(02)00015-9

Cited by 50 publications

(30 citation statements)

References 16 publications

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“…As for previous studies based on ERS scatterometer data [5], soil roughness is assumed constant over the year at the considered spatial scale (3 × 3 km 2 ). Consequently, observed temporal variations of σ 0 are interpreted in terms of SSM and vegetation-cover variations.…”

Section: Temporal Signaturesmentioning

confidence: 99%

“…The potential of spaceborne C-band scatterometers for observing semiarid areas originates from their high sensitivity to the drastic changes of surface dielectric properties between the successive dry and wet seasons, including the amount of green vegetation and the top-soil moisture content [4], [8], [9]. Observations made at large incidence angles were found in to be good agreement with vegetation cover, whereas low incidence angle acquisitions were related to the variation of soil-moisture content [5], [9].…”

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confidence: 98%

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Radar Signatures of Sahelian Surfaces in Mali Using ENVISAT-ASAR Data

Baup

Mougin

Hiernaux

et al. 2007

IEEE Trans. Geosci. Remote Sensing

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Abstract-This paper presents an analysis of ENSIVAT Advanced Synthetic Aperture Radar data acquired over a Sahelian region located in Mali, West Africa. The considered period is 2004-2005 and includes two rainy seasons. Emphasis is put on two ScanSAR modes, namely, the Global Monitoring (GM) and the Wide Swath (WS) modes characterized by spatial resolutions of about 1 km and 150 m, respectively. Results show that the WS mode offers better performance in terms of radiometric resolution, radiometric stability, and speckle reduction than the GM mode. The latter is more appropriate for studies at large scale (> 10 × 10 km). In both modes, pronounced angular and temporal signatures are observed for most soil surfaces, and azimuthal effects are observed on markedly orientated rocky surfaces. In contrast, polarization differences (VV/HH) are small during the dry season except on flat loamy soil surfaces. Finally, a relationship is observed between the normalized WS backscattering signal at HH polarization and the surface soil moisture of sandy soils.

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Section: Temporal Signaturesmentioning

confidence: 99%

mentioning

confidence: 98%

Radar Signatures of Sahelian Surfaces in Mali Using ENVISAT-ASAR Data

Baup

Mougin

Hiernaux

et al. 2007

IEEE Trans. Geosci. Remote Sensing

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“…As the frequency decreases, C-band and Ku-band capture more backscattering from leaf and twigs (leaf biomass) of the upper canopy of tropical forests (Toan et al 1992, Waring et al 1995. Past research has found that for sparse vegetation regions, such as grasslands and savannas, C-band and Ku-band backscattering observations require careful interpretation because soil moisture can modulate the signal (Jarlan et al 2002). By contrast, in tropical evergreen forests, with closed canopies, C-band and Ku-band backscattering are more strongly related to leaf biomass dynamics, with limited contribution from soil moisture (Frolking et al 2011).…”

Section: Introductionmentioning

confidence: 99%

Seasonal coupling of canopy structure and function in African tropical forests and its environmental controls

et al. 2013

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Abstract. Tropical forests provide important ecosystem services in maintaining biodiversity, sequestering carbon and regulating climate regionally and globally. Climate triggers the seasonal transitions of vegetation structure and function in tropical forests. In turn, the seasonal cycles of structure and function in tropical forests feed back to the climate system through the control of land-atmosphere exchange of carbon, water and energy fluxes. Large uncertainties exist in the carbon and water budgets of tropical forests, and environmental controls on phenology are among the least understood factors. Although field studies have identified patterns in the environmental controls on local-scale species-level phenology in the tropics, there is little consensus on large-scale top-down environmental controls on whole-ecosystem seasonality. In this paper, we use both optical and microwave remote sensing to investigate the seasonality of vegetation canopy structure and function in three distinct tropical African forest types, and identify environmental triggers or controls of their variability. For most tropical forests that have a closed canopy and high leaf biomass, optical remote sensing (e.g., vegetation indices) captures canopy photosynthetic capacity (i.e., canopy function), while small-wavelength microwave remote sensing characterizes the leaf biomass and leaf water content of the upper canopy (i.e., canopy structure). Our results reveal a strong coupling of canopy structure with canopy function in the tropical deciduous forests and woody savannas, and their seasonalities are both controlled by precipitation rather than solar radiation. By contrast, tropical evergreen forests in Africa exhibit a decoupling of canopy structure from canopy function revealed by different sensors: canopy photosynthetic capacity shown by the optical remote sensing is linked to the seasonal variation of precipitation, while microwave remote sensing captures semi-annual leaf-flushing that is synchronous with peak insolation intensity at the top of the atmosphere, which is bimodal. The differential coupling of canopy structure and function in tropical forests observed from remote sensing highlights differences inherent in distinct vegetation types within the tropics that may originate in the different life histories of their respective floras. This satellite-based finding encourages more field-based studies to clarify the interpretation of these large scale patterns.

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“…The general trends of the radar response as a function of soil conditions and the sensor characteristics (frequency, incidence, polarisation) are well captured by backscatter models (e.g. Jarlan et al, 2002), but the operational applicability of inversion schemes is still challenging since the parameters required for modelling are difficult to estimate over large areas and since the relative contribution of these parameters on the signal is difficult to decouple.…”

Section: Introductionmentioning

confidence: 99%

Combined use of optical and radar satellite data for the monitoring of irrigation and soil moisture of wheat crops

Fieuzal

Duchemin

Jarlan

et al. 2011

Hydrol. Earth Syst. Sci.

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Abstract. The objective of this study is to get a better understanding of radar signal over irrigated wheat fields and to assess the potentialities of radar observations for the monitoring of soil moisture. Emphasis is put on the use of high spatial and temporal resolution satellite data (Envisat/ASAR and Formosat-2). Time series of images were collected over the Yaqui irrigated area (Mexico) throughout one agricultural season from December 2007 to May 2008, together with measurements of soil and vegetation characteristics and agricultural practices. The comprehensive analysis of these data indicates that the sensitivity of the radar signal to vegetation is masked by the variability of soil conditions. On-going irrigated areas can be detected all over the wheat growing season. The empirical algorithm developed for the retrieval of topsoil moisture from Envisat/ASAR images takes advantage of the Formosat-2 instrument capabilities to monitor the seasonality of wheat canopies. This monitoring is performed using dense time series of images acquired by Formosat-2 to set up the SAFY vegetation model. Topsoil moisture estimates are not reliable at the timing of plant emergence and during plant senescence. Estimates are accurate from tillering to grain filling stages with an absolute error about 9% (0.09 m 3 m −3 , 35% in relative value). This result is attractive since topsoil moisture is estimated at a high spatial resolution (i.e. over subfields of about 5 ha) for a large range of biomass water content (from 5 and 65 t ha −1 ) independently from the viewing angle of ASAR acquisition (incidence angles IS1 to IS6).

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Analysis of ERS wind scatterometer time series over Sahel (Mali)

Cited by 50 publications

References 16 publications

Radar Signatures of Sahelian Surfaces in Mali Using ENVISAT-ASAR Data

Radar Signatures of Sahelian Surfaces in Mali Using ENVISAT-ASAR Data

Seasonal coupling of canopy structure and function in African tropical forests and its environmental controls

Combined use of optical and radar satellite data for the monitoring of irrigation and soil moisture of wheat crops

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