Publications of Benoît Mercatoris
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See detailImaging Wheat Canopy Through Stereo Vision: Overcoming the Challenges of the Laboratory to Field Transition for Morphological Features Extraction
Dandrifosse, Sébastien ULiege; Bouvry, Arnaud ULiege; Leemans, Vincent ULiege et al

in Frontiers in Plant Science (2020)

Stereo vision is a 3D imaging method that allows quick measurement of plant architecture. Historically, the method has mainly been developed in controlled conditions. This study identified several ... [more ▼]

Stereo vision is a 3D imaging method that allows quick measurement of plant architecture. Historically, the method has mainly been developed in controlled conditions. This study identified several challenges to adapt the method to natural field conditions and propose solutions. The plant traits studied were leaf area, mean leaf angle, leaf angle distribution, and canopy height. The experiment took place in a winter wheat, Triticum aestivum L., field dedicated to fertilization trials at Gembloux (Belgium). Images were acquired thanks to two nadir cameras. A machine learning algorithm using RGB and HSV color spaces is proposed to perform soil-plant segmentation robust to light conditions. The matching between images of the two cameras and the leaf area computation was improved if the number of pixels in the image of a scene was binned from 2560 × 2048 to 1280 × 1024 pixels, for a distance of 1 m between the cameras and the canopy. Height descriptors such as median or 95th percentile of plant heights were useful to precisely compare the development of different canopies. Mean spike top height was measured with an accuracy of 97.1 %. The measurement of leaf area was affected by overlaps between leaves so that a calibration curve was necessary. The leaf area estimation presented a root mean square error (RMSE) of 0.37. The impact of wind on the variability of leaf area measurement was inferior to 3% except at the stem elongation stage. Mean leaf angles ranging from 53° to 62° were computed for the whole growing season. For each acquisition date during the vegetative stages, the variability of mean angle measurement was inferior to 1.5% which underpins that the method is precise. [less ▲]

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See detailExperimental and Numerical Investigation of the Drying of an Agricultural Soil
Ralaizafisoloarivony, Njaka Andriamanantena ULiege; Tran, Duc Kien ULiege; Degré, Aurore ULiege et al

in E3S Web of Conferences (2020)

Due to climate change, soil desiccating became a serious concern in the agricultural area of Belgium. Knowing soil evaporation kinetic can help to elucidate and predict: the soil moisture regime, soil ... [more ▼]

Due to climate change, soil desiccating became a serious concern in the agricultural area of Belgium. Knowing soil evaporation kinetic can help to elucidate and predict: the soil moisture regime, soil water retention and soil water content. Those parameters are vital for water use efficiency and sustainable agriculture. This research analysed the mechanism of soil evaporation both under laboratory experiment and numerical modelling. Soil samples (Luvisol) were collected from the agricultural field in Gembloux-Belgium, and processed in a small drying chamber. Sensors measured the chamber temperature and humidity, while digital camera monitored the soil surface throughout the experiment. HYPROP device recorded the water change, soil suction, and soil water retention curve. During three evaporation experiments, four periods were observed rather than three according to the common theory. The modelling considered thermo-hydro-mechanical framework for predicting the drying process of Luvisol. The model used the finite element code LAGAMINE created at the University of Liege. The Software aims at assessing the mechanism of water transport between soil and atmosphere. The results of the simulation showed major domination of Darcean flow during desiccating, while some short vapour diffusion occurred only after the soil surface began to de-saturate. [less ▲]

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See detailAssessing soil crack dynamics during dryings from reduced tillage and conventional tillage fields
Ralaizafisoloarivony, Njaka Andriamanantena ULiege; Degré, Aurore ULiege; Mercatoris, Benoît ULiege et al

Poster (2019, September 02)

Crack formation and development has been a general concern in the agricultural science. Cracks contribute to the soil aeration, aggregate formation, and easy root penetration. However, cracks accelerated ... [more ▼]

Crack formation and development has been a general concern in the agricultural science. Cracks contribute to the soil aeration, aggregate formation, and easy root penetration. However, cracks accelerated the soil desiccation, allowed deep infiltration of pesticides/pollutants through preferential flow, and polluted the shallow water-table in Belgium. Cracks were mostly studied on pure clay or on high clay content soil (Vertisol). Yet in Wallonia, cracks were present even on silt-loam soil (Luvisol). This study tried to cover this gap by analysing crack dynamics and evaporation process, during drying kinetics of the Luvisol found in Gembloux. Soils were collected right from the agricultural field and processed on a small drying chamber in which evaporation test was taking place. Ceramic-IR-emitter heated the chamber while sensors (DHT22) measured the temperature and relative humidity. Digital camera took photos of the soil surface at 30min interval. Balance and tensiometer were linked to datalogger (CR800) and recorded the soil hydraulic properties (evaporation, water retention, etc.). Cracks were assessed from small samples (~5cm x 1cm thick) and big samples (~20cm size x 1.6 cm thick). Three treatments were considered including: disturbed soil, conventional tillage and reduced tillage. For big samples, results showed higher cracks formation on disturbed soil > reduced-tillage > conventional-tillage due to loose of soil cohesion, soil organic content, soil aggregation, biological activities, and soil porosity. The soil evaporation rate was also greater in disturbed soil > reduced-tillage > conventional tillage. Cracks opening exposed profound-soil-water to the atmosphere without passing through the soil matrix. For small samples, the repetitive drying experiments increased cracks length/width, especially for the dense samples. Future study is needed to assess the presence of pre-(micro)-cracks in soil using X-ray microtomography. [less ▲]

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See detailCereal morphology through proximal stereovision
Dandrifosse, Sébastien ULiege; Bouvry, Arnaud ULiege; Leemans, Vincent ULiege et al

Conference (2019, July 10)

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See detailEstimation of ear density in winter wheat crop by stereoscopic imaging for crop yield prediction
Bouvry, Arnaud ULiege; Dandrifosse, Sébastien ULiege; Dumont, Benjamin ULiege et al

Poster (2019, July)

Presentation exploring the added value of 3D information in winter wheat ear detection compared to RGB image based methods. Image acquisition procedure is presented and a processing pipeline is proposed ... [more ▼]

Presentation exploring the added value of 3D information in winter wheat ear detection compared to RGB image based methods. Image acquisition procedure is presented and a processing pipeline is proposed. The poster focuses on potential of the methods and future improvements. [less ▲]

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See detailStudying the effect of desiccation cracking on the evaporation process of a Luvisol – From a small-scale experimental and numerical approach
Tran, Duc Kien ULiege; Ralaizafisoloarivony, Njaka Andriamanantena ULiege; Charlier, Robert ULiege et al

in Soil and Tillage Research (2019), 193

Cracking formation due to desiccation of the soil surface is a common phenomenon related to the interaction between soil and the atmosphere. Indeed, during dry seasons, high evaporation of pore water near ... [more ▼]

Cracking formation due to desiccation of the soil surface is a common phenomenon related to the interaction between soil and the atmosphere. Indeed, during dry seasons, high evaporation of pore water near the soil surface leads to a more significant soil suction in this region. The suction results in compressive effective stress on the soil structure and produces shrinkage including cracking. As the crack network forms, the initial soil structure is strongly modified, which provides preferential flow pathways for solute-water and influences the soil hydraulic behaviour in general. The work aims to study the formation of cracks during evaporation process of a Cutanic Luvisol and evaluate how cracking affects the soil hydraulic behaviour. Laboratory experiments were performed on undisturbed soil samples. To do that, a small-scale environmental chamber was designed and equipped with sensors for measuring the ambient temperature and relative humidity, and a digital camera for investigating the initiation and propagation of cracks on the soil surface. By combining with a HYPROP device (UMS GmbH, Munich, Germany), the hydraulic properties and the kinetics of evaporation of soil samples were also determined through the tests. Finally, numerical simulations were carried out by using the finite element code LAGAMINE developed at the University of Liege to emphasize the effect of desiccation cracking on the soil hydraulic conductivity and the moisture transport mechanisms in the soil, as well as exchanges with ambient atmosphere. [less ▲]

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See detailDéveloppement d’un système de vision multispectrale pour le phénotypage de culture
Naiken, Alwin ULiege; Mercatoris, Benoît ULiege

Poster (2018, October)

In the frame of research activities regarding the characterization of crop growth dynamics in a context of precision agriculture, we have developed a multispectral proxidetection system covering a wide ... [more ▼]

In the frame of research activities regarding the characterization of crop growth dynamics in a context of precision agriculture, we have developed a multispectral proxidetection system covering a wide spectrum in the near-infrared and short-wave infrared domains. This part of the spectrum is important for the characterization of vegetation since it includes the reflectance plateau of plants in the near-infrared and the main water absorption bands. Nowadays, the most commonly used tool in this spectral range is the spectrometer, which measures the average reflectance of a scene without any spatial detail. In the infrared range, hyperspectral imaging is ermerging, as a high-potential solution, but remains difficult to implement and process. In this contribution, we propose a solution, based on an optical filter wheel, which provides the spatial dimension in the infrared domain with a simple implementation to study plants at the scale of organs (leaves, stems, ears). The proposed multispectral vision system complements an equivalent tool already developed in the visible and near-infrared field using a monochrome CMOS camera. The particularity of the presented instrument is the use of an InGaAs camera (indium-galllium arsenide sensor) Gold-eye G-007 (Allied Vision) with a bandwidth from 950 nm to 1650 nm. This camera is combined with a wheel composed of 10 optical filters of 100 nm bandwidth and distributed over the camera's bandwidth. The positioning of the filters in front of the camera is automatically controlled by the image acquisition system using a microcontroller (Arduino Uno). The implemented program allows to select successively the filters of interest for the acquisition. This multipectral vision system will allow the selection of wavelength ranges and morphological parameters that explain physiological traits of plants such as water or nitrogen deficiencies. For this purpose, the image characteristics will be correlated with conventional reference measurements. [less ▲]

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See detailEffect of desiccation cracking on the fluid transfer process in agricultural soil
Tran, Duc Kien ULiege; Ralaizafisoloarivony, Njaka; Charlier, Robert ULiege et al

Poster (2018, October)

The natural soil structure can be strongly modified and generate heterogeneities during wetting and drying processes. This significantly affects the transfer of fluids and nutrients between the atmosphere ... [more ▼]

The natural soil structure can be strongly modified and generate heterogeneities during wetting and drying processes. This significantly affects the transfer of fluids and nutrients between the atmosphere, the subsoil, the hydrosphere and the biosphere. Experimental observations on a Cutanic Luvisol from agricultural field in Gembloux, Belgium, by using X-ray microtomography coupled with 3D image analysis have shown the cracking phenomenon occurring and leading to preferential flows in the soil sample during a drainage process. In order to better understand the impact of cracks on the behaviour of this soil type, in this study, we have proposed a numerical modelling of soil evaporation process by using the constitutive models implemented in the finite element code LAGAMINE. Considering that the soil we study is a loamy soil, we have chosen to fit the dual model of Durner (Durner, 1994) for the water retention capacity. The drying kinetics is modelled using the boundary layer model (Gerard et al., 2010), assuming that the vapour and heat transfers take place in a boundary layer at the surface of the porous medium. The embedded fracture model is chosen to represent the development of the fractures in porous medium in which fracture opening is activated by a threshold strain parameter (Olivella et Alonso, 2008). The results obtained have shown that an increase in permeability in the fracture zones makes the permeability tensor anisotropic up to one order and thus strongly modifies the drying kinetics of the soil core (e.g., evaporation rate). The results also have suggested that using a simple concept of cracking development, a continuum model is capable of modelling preferential flows developed in a fractured porous medium such as agricultural soil. [less ▲]

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See detailCharacterization of cereal crop growth dynamics by stereoscopic vision
Bouvry, Arnaud ULiege; Dandrifosse, Sébastien ULiege; Glésener, Michaël et al

Poster (2018, March 20)

The purpose of this study is to characterize plant growth dynamics in cereal crops at the average single-plant level. It aims at monitoring the evolution of morphological and architectural features at the ... [more ▼]

The purpose of this study is to characterize plant growth dynamics in cereal crops at the average single-plant level. It aims at monitoring the evolution of morphological and architectural features at the organ level by means of a customized stereovision system. Relevant features will be evaluated to determine the growing stage on the Zadoks scale (Zadoks et al., 1974). This measurement methodology is being developped to meet the need of innovative phenotyping tools in crop variety evaluation. Furthermore, it will be used in an ecotron facility, a well suited tool to assess the crop growth dynamic response to future climate scenarii. [less ▲]

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See detailEvaluation of leaf area index of winter wheat canopy by means of ground-based stereoscopic vision
Leemans, Vincent ULiege; Glésener, Michaël; Dumont, Benjamin ULiege et al

Poster (2017, November 22)

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See detailUpscaling winter wheat above-ground biomass measurements using multispectral imagery and 3D data from unmanned aerial vehicle
Michez, Adrien ULiege; Bauwens, Sébastien ULiege; Heinesch, Bernard ULiege et al

Poster (2017, October 20)

Field measurements in the ICOS program are spatially limited whereas the monitored gas fluxes may have a large footprint. Aerial remote sensing has the advantage to monitor large areas. The main goal of ... [more ▼]

Field measurements in the ICOS program are spatially limited whereas the monitored gas fluxes may have a large footprint. Aerial remote sensing has the advantage to monitor large areas. The main goal of our research was to test the potential of unmanned aerial vehicles (UAV) to upscale parameters monitored through the ICOS program. In this study, we specifically focus on above-ground biomass (AGB) monitoring in a winter wheat crop. We used a octocopter drone (X frame type) to acquire a time series over the crop growing season (8 flights from the 14th of February 2017 to the 7th of July 2017) of multispectral imagery covering the ICOS candidate station of Lonzée (Wallonia, Belgium) and the surrounding field crop areas (ca. 0.25 km² per flight). The multispectral camera provides spectral information on the green (550 +/- 50 nm), red (660 +/- 50 nm), near infrared (735 +/- 50 nm) and red-edge (790 +/- 10 nm) wavelengths bands. The UAV also brought an off-the-shelf high resolution (20 Mpx) RGB camera to derive accurate 3D data. We performed a photogrammetric 3D reconstruction of the acquired imagery for every flight survey. The images provided by the RGB sensor (Sony RX100) were used to produce a high spatial resolution Digital Surface Model (0.05 m) and the images acquired by the multispectral sensor were used to derive reflectance maps (0.1 m) in the four wavelengths bands. The four reflectance layers were combined to produce two straight-forward vegetation indices (Normalized Difference Vegetation Index and Green NDVI). The photogrammetric DSM’s were combined to a LiDAR Digital Terrain Model (public database, survey in winter 2013) to produce Crop Height Models (CHM) of the study area. We used multiple linear regressions modelling in order to predict the AGB of the field crop monitored by the ICOS station of Lonzée with UAV imagery. AGB=a+b*GNDVI+c*NDVI+d *CHM The field crop data were provided by the ICOS program and by field research conducted in experimental field crops close to the flux tower. The field sampling consisted in destructive samples of the crop which were weighted after drying. For each field sample, an associated area was computed based on the outdistance sowing and the number of sampled crops in order to compute an AGB per area unit (t / Ha). Each AGB field estimation was associated to the closest flight date to build a multi date model presenting good performances (r² = 0.85, RMSE = 2.3 t/Ha). We used the same modelling approach to adjust a single date model to derive a predicted AGB map for the 7th of July. The performance of the single date model is lower but still highlights the biomass variation within the crop (r² = 0.71, RMSE = 1.9 t/Ha). The predicted AGB map displays a high spatial heterogeneity with some spatial patterns. Locally low AGB values are found along two old pedestrian whereas higher AGB values can be associated to areas which were sprayed twice (in-between two tractor tracks). Our results highlight the potential of UAV multispectral imagery to monitor the AGB variation within the footprint of the flux tower and highlight the need for repeated field sampling with a precise geolocation to improve the matching between the flight and the field surveys. [less ▲]

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See detailLa terre ferme
Favre, Juliette; Beckers, Yves ULiege; Bindelle, Jérôme ULiege et al

Article for general public (2017)

Drones, GPS, robots, QR codes et autres lampes LED gagnent du terrain dans les fermes belges. Tandis que certains agriculteurs se réjouissent de cette vague technologique et voient déjà pointer une ... [more ▼]

Drones, GPS, robots, QR codes et autres lampes LED gagnent du terrain dans les fermes belges. Tandis que certains agriculteurs se réjouissent de cette vague technologique et voient déjà pointer une troisième révolution agricole, d’autres craignent de se transformer en de simples « presse-boutons ». Smartphone dans une main, joystick dans l’autre… Les agriculteurs touchent-ils encore seulement la terre ? [less ▲]

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See detailEstimation of leaf nitrogen concentration on winter wheat by multispectral imaging
Leemans, Vincent ULiege; Marlier, Guillaume; Destain, Marie-France ULiege et al

in Hyperspectral Imaging Sensors: Innovative Applications and Sensor Standards 2017 (2017)

Precision agriculture can be considered as one of the solutions to optimize agricultural practice such as nitrogen fertilization. Nitrogen deficiency is a major limitation to crop production worldwide ... [more ▼]

Precision agriculture can be considered as one of the solutions to optimize agricultural practice such as nitrogen fertilization. Nitrogen deficiency is a major limitation to crop production worldwide whereas excess leads to environmental pollution. In this context, some devices were developed as reflectance spot sensors for on-the-go applications to detect leaves nitrogen concentration deduced from chlorophyll concentration. However, such measurements suffer from interferences with the crop growth stage and the water content of plants. The aim of this contribution is to evaluate the nitrogen status in winter wheat by using multispectral imaging. The proposed system is composed of a CMOS camera and a set of filters ranged from 450 nm to 950 nm and mounted on a wheel which moves due to a stepper motor. To avoid the natural irradiance variability, a white reference is used to adjust the integration time. The segmentation of Photosynthetically Active Leaves is performed by using Bayes theorem to extract their mean reflectance. In order to introduce information related to the canopy architecture, i.e. the crop growth stage, textural attributes are also extracted from raw images at different wavelength ranges. Nc was estimated by partial least squares regression (R² = 0.94). The best attribute was homogeneity extracted from the gray level co-occurrence matrix (R² = 0.91). In order to select in limited number of filters, best subset selection was performed. Nc could be estimated by four filters (450 ± 40 nm, 500 ± 20 nm, 650 ± 40 nm, 800 ± 50 nm) (R² = 0.91). [less ▲]

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See detailDetection of nitrogen stress on winter wheat by multispectral machine vision
Marlier, Guillaume ULiege; Gritten, Fanny; Leemans, Vincent ULiege et al

Conference (2016, August 01)

Hand-held sensors (SPAD meter, N-Tester, …) used for detecting the leaves nitrogen concentration (Nc) present several drawbacks. The nitrogen concentration is gained by an indirect way through the ... [more ▼]

Hand-held sensors (SPAD meter, N-Tester, …) used for detecting the leaves nitrogen concentration (Nc) present several drawbacks. The nitrogen concentration is gained by an indirect way through the chlorophyll concentration and the leaves have to be fixed in a defined position for the measurements. These drawbacks could be overcome by an imaging device that measures the canopy reflectance. Hence, the objective of the paper is to analyse the potential of multispectral imaging for detecting nitrogen concentration. The tests were carried out on parcels submitted to nitrogen inputs varying from 0 to 180 kg N.ha-1. Reference Nc measurements were obtained by the Kjeldahl method and a Hydro N-Tester (Yara). The developed imaging system comprised a CMOS camera and a set of 22 interference filters ranging from 450 to 950 nm mounted on a wheel steered by a stepper motor. The image acquisition and the motor rotation were controlled by a program written in C++. The crop was imaged vertically at one meter height. The raw images presented 1280×1024 pixels covering an area of approximately 0.25 m² and were recorded with a 12-bit luminance resolution. To deal with the natural irradiance variability of the scene, a white reference was used and the integration time was automatically adjusted for each image. The image treatment included the segmentation of Photosynthetically Active Leaves (PAL) by using Bayes theorem and the computation of the mean PAL reflectance after correction of background and illumination fluctuations. Nc was estimated on the basis of the 22 filters by the Partial Least Square (PLS) method and by four filters selected by the Best Subset Selection (BSS) method. In comparison with the Kjeldahl method, the estimation of Nc by means of the Hydro N-Tester, the PLS method and the BSS method (filters 600-80, 950-100, 650-40 and 450-80 nm) gave determination coefficients equal to 0.53, 0.63, and 0.62, respectively. This indicated that the full multi-spectral approach gave significantly better Nc estimation than a portable device and suggested that a camera equipped with four filters would give similar results. [less ▲]

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See detailDynamics of a thin radial liquid flow
De Cock, Nicolas ULiege; Massinon, Mathieu ULiege; Ouled Taleb Salah, Sofiene ULiege et al

in Fire Safety Journal (2016), 83

The present work proposes an extension of the existing analytical development on the radial spread of a liquid jet over a horizontal surface to the case of a thin radial flow. When the gap, H, between the ... [more ▼]

The present work proposes an extension of the existing analytical development on the radial spread of a liquid jet over a horizontal surface to the case of a thin radial flow. When the gap, H, between the jet nozzle and the plate is reduced the discharging area may be smaller than the inlet area leading to an increase of the main flow velocity downstream of the thin cylindrical opening. This increase of velocity, de fined here as 1/alpha , can be related to the relative gap of the nozzle H/R with R the nozzle pipe radius. Numerical computations with a volume of fluid method were realised with for H/R ranging from 0.2 to 3 and with flow rates Q of 3 and 6 l/min1. The results of these computations allowed to express alpha in respect of H/R. Taking in account the flow acceleration allowed to extend the set of equation from the jet impacting flow to the thin cylindrical opening flow. The liquid layer thickness and the surface velocity diff er with a maximum error of 4% between the flow predicted by the model and computations. Main discrepancies appear in the region close to the nozzle where the analytical model assumption of a constant velocity outside the boundary layer is not valid. However, further downstream the model and the computations are in good agreement. [less ▲]

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