Méthodologie pour une utilisation conjointe de données eulériennes et langrangiennes de courant pour estimer un coefficient de diffusion horizontale côtières

Le Laboratoire de Sondages Electromagnétiques de l'Environnement Terrestre a une pratique confirmée de la mise en oeuvre des radars Doppler VHF pour la réalisation de mesures eulériennes des courants marins en zone côtière. Avec ce même instrument, on peut réaliser, simultanément aux mesures eulériennes, des mesures lagrangiennes en effectuant le suivi, avec le radar, de flotteurs dérivants de surface équipés de balises.Nous proposons une méthode d'utilisation couplée des données eulériennes et lagrangiennes pour estimer l'importance des phénomènes de diffusion horizontale. L'influence de la résolution spatiale des mesures de courant et du nombre de flotteurs sur la précision des mesures de dispersion a été discutée. Une validation de cette méthode a été effectuée à partir d'une étude numérique sur un exemple de panache fluvial (I'étude de ce dernier est un des axes principaux de recherche du LSEET). Cette étude montre que l'erreur sur l'estimation de ce coefficient pour 5 bouées lâchées et avec une résolution spatiale de (500 x 500) m- peut être de l'ordre de 35%. On obtient ainsi un instrument particulièrement complet de diagnostic dynamique en zone côtière, présentant un grand intérêt pour l'étude d'un rejet ou le transport d'un contaminant.VHF Doppler radars have been used by LSEET for the measurements of Eulerian superficial currents in coastal zones (between 1 to 30 km from the shore). For radar frequencies close to 50 MHz, the velocity measured is integrated over a depth of 25 cm, over an area of a few hundred square metres, and for times of one to ten minutes. The accuracy of the velocity measurements is better than 5 cm/s. The radars are set up on the sea-shore and their maximum range, depending upon sea-state, is 20 to 30 km. Two radars are needed to obtain a mapping of current vectors. These measurements don't account for velocity fluctuations at scales less than the mesh size, which are generally described by a diffusion coefficient (K[inf]h). On the other hand, Lagrangian measurements can be made by tracking surface drifters. The same VHF radars can be used for such tracking by using radio beacons placed on the drifters.This paper proposes a method for using both the Eulerian velocity field (usual product of VHF radars) and the Lagrangian trajectories of several drifters in order to estimate the horizontal diffusion coefficient. This method consists of comparing the statistical characteristics of the observed drifters distribution with those obtained by numerical simulations based on a random walk method. Numerical experiments concerning the influence of the accuracy and the spatial resolution of current measurements and the effect of the number of drifters on the accuracy of dispersion estimate have been carried out. In the particular case of a river plume with an outflow of the order of 1000 m3/s, we have shown that the accuracy of the horizontal diffusion coefficient estimation is of the order of 35 % for the realistic number of 5 drifters, with a spatial resolution of 500 x 500 m2.The system (radars and drifters) provides a powerful tool for a dynamic diagnosis in the coastal zone, and should be of interest for the study of discharges or the evolution of waterborne pollutants

Etude de la précision du satellite lidar GLAS-ICESat pour l'altimétrie des eaux continentales

For the coming century, the control of water resources will be certainly the key of all the stakes for billions of human beings. Unfortunately a reduction in the number of stations is observed with a decline of measurements quality. Remote sensing, which saw the development of numerous satellite radar altimeters and more recently the launch of the satellite lidar ICESat, could be an interesting alternative for the study of the hydrological networks. The objective of this study is to estimate the potential of ICESat for monitoring continental water missions through the cases of the Lake Geneva (Switzerland and France) and rivers of Metropolitan France. Our first axis of study concerned the satellite-based assessment of ICESat on the Lake Geneva by comparing laser data to hydrological gauge water levels. Two hydrological stations (Chillon and Saint Prex) were used to evaluate the accuracy of ICESat elevations. First it was necessary that all data was in the same datum to conduct a consistent comparison. ICESat elevations, which are referenced in the Topex ellipsoid, were converted into orthometric elevations by a translation between Topex ellipsoid and WGS84 and then into the vertical reference IGN69 (RGF93) with the grid RAF98. The shots of water alone were then extracted track by track and the mean elevation calculated for each track was used for the comparison with reference elevations (hydrological gauges). The error RMS is 33 cm (-0.20 cm ± 0.21 cm) without any saturation correction. When the saturation correction is supplied and different from -999.000, the quality of water elevation data is improved : the error RMS is 14 cm (0.01 cm ± 0.10 cm). However GLAS temporal profiles show a slow progressive adaptation of GLAS sensor before proposing correct elevations. On the passage of ICESat from the land to water, the first spots elevations are higher than reference elevation and the following spots from 30 cm to 50 cm. The progressive return to the normal can last 0.2 s. It corresponds to 8 measurements and an adaptation distance of 1.360 km. When the transition footprints are excluded, the accuracy for the ICESat elevation measurements is 5 cm. Besides hydrological objects with a small size (small lakes, small rivers), which can not apply a margin of 1.5 km to remove transition footprints, could not be monitoring using ICESat with a good accuracy. Next the accuracy of ICESat was investigated on French rivers with a width larger than the size of ICESat footprint (about 55 m for the laser 3). The error RMS is 1.15 m (0.03 m ± 1.17 m) due to the time of ICESat adaptation on the passage from land to water. ICESat is not adapted for the monitoring of the continental water resource

Use of TerraSAR-X data to retrieve soil moisture over bare soil agricultural fields

The retrieval of the bare soil moisture content from TerraSAR-X data is discussed using empirical approaches. Two cases were evaluated: 1) one image at low or high incidence angle and 2) two images, one at low incidence and one at high incidence. This study shows by using three databases collected between 2008 and 2010 over two study sites in France (Orgeval and Villamblain) that TerraSAR-X is a good remote sensing tool for the retrieving of surface soilmoisture with accuracy of about 3% (rmse).Moreover, the accuracy of the soil moisture estimate does not improve when two incidence angles (26◦–28◦ or 50◦–52◦) are used instead of only one. When compared with the result obtained with a high incidence angle (50◦–52◦), the use of low incidence angle (26◦–28◦) does not enable a significant improvement in estimating soil moisture (about 1%)

Remote Sensing of Soil

Remote sensing has shown a high potential in soil characteristics retrieving in the last three decades. Different methodologies have been proposed for the estimation of soil parameters, based on different remote sensing sensors and techniques (passive and active)

Multitemporal observations of sugarcane by TerraSAR-X sensor

International audienceLe potentiel du capteur radar TerraSAR-X (bande X) pour le suivi de la croissance de la canne à sucre a été étudié sur l'ile de la Réunion. Des données multitemporelles du capteur TerraSAR-X acquises à différentes angles d'incidence (17°, 31°, 37°, 58°) et polarisations (HH, HV, VV) ont été analysées dans le but d'étudier le comportement du signal radar en fonction de la hauteur de la canne à sucre. Le potentiel de TerraSAR pour la cartographie de la coupe a également été étudié.Le signal radar augmente rapidement avec la hauteur de la canne jusqu'à une hauteur seuil qui dépend de la polarisation et de l'angle d'incidence. Au delà de ce seuil, le signal croit légèrement ou reste constant. Les résultats montrent également que le signal TerraSAR augmente après des fortes pluies suite à une augmentation de la contribution du sol pour des hauteurs de canne inférieures à 50 cm (2 mois d'age). Finalement, les données TerraSAR à haute résolution spatiale sont très utiles pour le suivi de la coupe quand les champs de canne à sucre sont de petites tailles ou quand la coupe est échelonnée dans le temps. L'incidence radar de 37° est la plus convenable pour le suivi et la cartographie de la coupe de canne. / The potential of TerraSAR-X (X-band) in monitoring sugarcane growth was investigated on Reunion Island. Multi-temporal TerraSAR data acquired at various incidence angles (17°, 31°, 37°, 47°, 58°) and polarizations (HH, HV, VV) were analyzed in order to study the behaviour of SAR (synthetic aperture radar) signal as a function of sugarcane height. The potential of TerraSAR for mapping the sugarcane harvest was also studied. Radar signal increased quickly with crop height until a threshold height, which depended on polarization and incidence angle. Beyond this threshold, the signal increased only slightly, remained constant, or even decreased. TerraSAR data showed that after strong rains the soil contribution for the backscattering of sugarcane fields can be consequent for canes with heights of terminal visible dewlap (htvd) less than 50cm (total cane heights around 155cm). Finally, TerraSAR data at high spatial resolution were shown to be useful for monitoring sugarcane harvest when the fields are of small size or when the cut is spread out in time. The radar incidence of 37° is more suitable to monitor the sugarcane harvest

Semi-empirical calibration of the Integral Equation Model for SAR data in C-band and cross polarization using radar images and field measurements

The estimation of surface soil parameters (moisture and roughness) from Synthetic Aperture Radar (SAR) images requires the use of well-calibrated backscattering models. The objective of this paper is to extend the semi-empirical calibration of the backscattering Integral Equation Model (IEM) initially proposed by Baghdadi et al. (2004 and 2006) for HH and VV polarizations to HV polarization. The approach consisted in replacing the measured correlation length by a fitting/calibration parameter so that model simulations would closely agree with radar measurements. This calibration in C-band covers radar configurations with incidence angles between 24° and 45.8°. Good agreement was found between the backscattering coefficient provided by the SAR and that simulated by the calibrated version of the IEM

Distribution and status of Posidonia oceanica meadows on the North-Western coast of Algeria

Posidonia oceanica is an endemic species of the Mediterranean Sea that in pristine coastal waters forms extensive meadows, which are, actually, exposed to natural and anthropogenic pressures, causing their regression throughout the basin. The aim of our study is to summarise and evaluate current knowledge about the characterisation of Posidonia oceanica along the Western Algerian coast, in order to provide new evidence about the distribution and health status this species. To realize our objectives, we combined extracted data from published articles, theses, results of project reports, in situ observations and laboratory analysis. The results show the presence of the meadows in fifteen coastal zones of Western Algeria, mainly located in shallow surface waters from 0.5 to 18 metres depth.The mean leaf length across all sites ranged from 153 to 667 mm. The mean number of leaves/shoot ranged between 4.7 ± 0.9 and 6.0 ± 0.5 leaves/shoot. Leaf length showed a highly significant difference between the cold and hot season. The data analysis shows that the leaf biometric values and the shoot densities of the P. oceanica are greater than those of Mediterranean samples although these measures tend to decrease deeply. The analyses carried out on structural features of P. oceanica show that the Western Algerian meadows are, globally, in good health status according to standardised scale. The available data on P. oceanica summarised in this article represent an important starting point to build effective plans for understanding levels of environmental threats and for supporting conservation strategies for these important ecosystems. Conversely, the limited information available on this seagrass along the Western Algerian costs only allows the description of some structural features, and permits us to draw overall conclusions on its general health status

Analysis of Sentinel-1 radiometric stability and quality for land surface applications

International audienceLand monitoring using temporal series of Synthetic Aperture Radar (SAR) images requires radiometrically well calibrated sensors. In this paper, the radiometric stability of the new SAR Sentinel-1A 'S-1A' sensor was first assessed by analyzing temporal variations of the backscattering coefficient (sigma°) returned from invariant targets. Second, the radiometric level of invariant targets was compared from S-1A and Radarsat-2 "RS-2" data. The results show three stable sub-time series of S-1A data. The first (between 1 October 2014 and 19 March 2015) and third (between 25 November 2015 and 1 February 2016) sub-time series have almost the same mean sigma°-values (a difference lower than 0.3 dB). The mean sigma°-value of the second sub-time series (between 19 March 2015 and 25 November 2015) is higher than that of the first and the third sub-time series by roughly 0.9 dB. Moreover, our results show that the stability of each sub-time series is better than 0.48 dB. In addition, the results show that S-1A images of the first and third sub-time series appear to be well calibrated in comparison to RS-2 data, with a difference between S-1A and RS-2 lower than 0.3 dB. However, the S-1A images of the second sub-time series have sigma°-values that are higher than those from RS-2 by roughly 1 dB

Desert roughness retrieval using CYGNSS GNSS-R data

The aim of this paper is to assess the potential use of data recorded by the Global Navigation Satellite System Reflectometry (GNSS-R) Cyclone Global Navigation Satellite System (CYGNSS) constellation to characterize desert surface roughness. The study is applied over the Sahara, the largest non-polar desert in the world. This is based on a spatio-temporal analysis of variations in Cyclone Global Navigation Satellite System (CYGNSS) data, expressed as changes in reflectivity (G). In general, the reflectivity of each type of land surface (reliefs, dunes, etc.) encountered at the studied site is found to have a high temporal stability. A grid of CYGNSS G measurements has been developed, at the relatively fine resolution of 0.03° x 0.03°, and the resulting map of average reflectivity, computed over a 2.5-year period, illustrates the potential of CYGNSS data for the characterization of the main types of desert land surface (dunes, reliefs, etc.). A discussion of the relationship between aerodynamic or geometric roughness and CYGNSS reflectivity is proposed. A high correlation is observed between these roughness parameters and reflectivity. The behaviors of the GNSS-R reflectivity and the Advanced Land Observing Satellite-2 (ALOS-2) Synthetic Aperture Radar (SAR) backscattering coeffcient are compared and found to be strongly correlated. An aerodynamic roughness (Z0) map of the Sahara is proposed, using four distinct classes of terrain roughness

Coupling SAR C-band and optical data for soil moisture and leaf area index retrieval over irrigated grasslands

International audienceThe objective of this study was to develop an approach for estimating soil moisture and vegetation parameters in irrigated grasslands by coupling C-band polarimetric Synthetic Aperture Radar (SAR) and optical data. A huge dataset of satellite images acquired from RADARSAT-2 and LANDSAT-7/8, and in situ measurements were used to assess the relevance of several inversion configurations. A neural network (NN) inversion technique was used. The approach for this study was to use RADARSAT-2 and LANDSAT-7/8 images to investigate the potential for the combined use of new data from the new SAR sensor SENTINEL-1 and the new optical sensors LANDSAT-8 and SENTINEL-2. First, the impact of SAR polarization (mono-, dual- and full-polarizations configurations) and the Normalized Difference Vegetation Index (NDVI) calculated from optical data for the estimation error of soil moisture and vegetation parameters was studied. Next, the effect of some polarimetric parameters (Shannon entropy and Pauli components) on the inversion technique was also analyzed. Finally, configurations using in situ measurements of the fraction of absorbed photosynthetically active radiation (FAPAR) and the fraction of green vegetation cover (FCover) were also tested.The results showed that HH polarization is the SAR polarization most relevant to soil moisture estimates. An RMSE for soil moisture estimates of approximately 6 vol.% was obtained even for dense grassland cover. The use of in situ FAPAR and FCover only improved the estimate of the leaf area index (LAI) with an RMSE of approximately 0.37 m²/m². The use of polarimetric parameters did not improve the estimate of soil moisture and vegetation parameters. Good results were obtained for the biomass (BIO) and vegetation water content (VWC) estimates for BIO and VWC values lower than 2 and 1.5 kg/m², respectively (RMSE is of 0.38 kg/m² for BIO and 0.32 kg/m² for VWC). In addition, a high under-estimate was observed for BIO and VWC higher than 2 and 1.5 kg/m², respectively (a bias of -0.65 kg/m² on BIO estimates and -0.49 kg/m² on VWC estimates). Finally, the estimation of vegetation height (VEH) was carried out with an RMSE of 13.45 cm