The evaluation of unmanned aerial systems-based photogrammetry and terrestrial laser scanning to generate DEMs of agricultural watersheds

Agricultural watersheds tend to be places of intensive farming activities that permanently modify their microtopography. The surface characteristics of the soil vary depending on the crops that are cultivated in these areas. Agricultural soil microtopography plays an important role in the quantification of runoff and sediment transport because the presence of crops, crop residues, furrows and ridges may impact the direction of water flow. To better assess such phenomena, 3-D reconstructions of high-resolution agricultural watershed topography is essential. Fine-resolution topographic data collection technologies can be used to discern highly detailed elevation variability in these areas. Knowledge of the strengths and weaknesses of existing technologies used for data collection on agricultural watersheds may be helpful in choosing an appropriate technology. This study assesses the suitability of terrestrial laser scanning (TLS) and unmanned aerial system (UAS) photogrammetry for collecting the fine-resolution topographic data required to generate accurate, high-resolution digital elevation models (DEMs) in a small watershed area (12 ha). Because of farming activity, 14 TLS scans (≈ 25 points m− 2) were collected without using high-definition surveying (HDS) targets, which are generally used to mesh adjacent scans. To evaluate the accuracy of the DEMs created from the TLS scan data, 1,098 ground control points (GCPs) were surveyed using a real time kinematic global positioning system (RTK-GPS). Linear regressions were then applied to each DEM to remove vertical errors from the TLS point elevations, errors caused by the non-perpendicularity of the scanner’s vertical axis to the local horizontal plane, and errors correlated with the distance to the scanner’s position. The scans were then meshed to generate a DEMTLS with a 1 × 1 m spatial resolution. The Agisoft PhotoScan and MicMac software packages were used to process the aerial photographs and generate a DEMPSC (Agisoft PhotoScan) and DEMMCM (MicMac), respectively, with spatial resolutions of 1 × 1 m. Comparing the DEMs with the 1,098 GCPs showed that the DEMTLS was the most accurate data product, with a root mean square error (RMSE) of 4.5 cm, followed by the DEMMCM and the DEMPSC, which had RMSE values of 9.0 and 13.9 cm, respectively. The DEMPSC had absolute errors along the border of the study area that ranged from 15.0 to 52.0 cm, indicating the presence of systematic errors. Although the derived DEMMCM was accurate, an error analysis along a transect showed that the errors in the DEMMCM data tended to increase in areas of lower elevation. Compared with TLS, UAS is a promising tool for data collection because of its flexibility and low operational cost. However, improvements are needed in the photogrammetric processing of the aerial photographs to remove non-linear distortions

Monitoring the temporal evolution of soil structure of three innovative production systems in the field

editorial reviewedAlternative agricultural practices emerge to provide more sustainable productions systems and to meet tomorrow's diets. These practices and varying climatic conditions will have impacts on soil structure and thus on soil hydraulic properties. However, most models do not consider the temporal variability of soil hydraulic properties, which can lead to poor decision making. Thus, quantifying the temporal evolution of hydraulic properties is essential to better understand the impact of emerging agricultural practices on soil structure (Chandrasekhar et al., 2018). In most studies, temporal variation of soil hydraulic properties is investigated using punctual measurements in the field or in the laboratory (Alskaf et al., 2021; Geris et al., 2021). Results are often inconsistent between studies due to the timing and type of measurement performed (Chandrasekhar et al., 2018; Strudley et al., 2008). In addition, most research focuses on the topsoil layers and does not consider the longer term effects on the deeper layers of the soil (Wahren et al., 2009). In this research, temporal evolution of the hydraulic properties of three innovative production systems is continuously monitored up to 90 cm depth. The three systems are designed to disrupt current agronomic trials and aim to produce the ingredients of tomorrow’s diets. They are pesticide-free and have long-term rotations of 8 years with intercrops. These systems are implemented on 8 parcels of the University of Gembloux Agro-Bio Tech on a typical loamy soil in Belgium. The innovative systems were instrumented with 24 Teros 12 water content and 24 Teros 21 water potential sensors from MeterGroup. Both types of sensor are robust and highly accurate. The Teros 12 probes also measure soil temperature and salinity. Potential probes can measure potential over a wide range of values from -9 to -2000 kPa. All probes are connected to MeterGroup's ZL6 data loggers which allow real-time data collection. The water content and potential probes are placed in parallel in the first three soil layers at 30, 60 and 90 cm depth in 8 plots. Intact soil cores are also taken every two months to determine bulk density and total soil porosity. The simultaneous determination of both water content and water potential over time under natural conditions allows the temporal evolution of the hydrodynamic properties to be captured at the level of the first three horizons. This monitoring will make it possible to quantify the temporal evolution of the structure of a loamy soil under the effect of alternative agricultural practices and varying climate conditions. The firsttwo years were contrasted in climatic conditions with a wet and a dry year. In addition, a diverse range of agricultural practices with different crops such as beet, camelina, corn, rapeseed and winter wheat were grown in both years. The results of these first two years of monitoring will be presented at the EGU 2023 General Assembly and compared to theoretical properties that would be obtained using classical PTF.AIL4WaterQualit

Characterization by X‐ray <scp>μCT</scp> of the air‐filled porosity of an agricultural soil at different matric potentials

peer reviewedTo describe various important soil processes like the release of greenhouse gases or the proliferation of microorganisms, it is necessary to assess quantitatively how the geometry and in particular the connectivity of the air-filled pore space of a soil evolves as it is progressively dried. The availability of X-ray computed microtomography (μCT) images of soil samples now allows this information to be obtained directly, without having to rely on the interpretation of macroscopic measurements using capillary theory, as used to be the case. In this general context, we present different methods to describe quantitatively the configuration of the air-filled pore space in 3D μCT images of 20 separate samples of a loamy soil equilibrated at different matric potentials. Even though measures using μCT on such multi-scale materials strongly depends on image resolution, our results show that in general, soil samples most often behave as expected, e.g., connectivity increases with higher negative matric potential, while tortuosity decreases. However, simple correlations could not be found between the evolution of quantitative descriptors of the pore space at the different matric potentials and routinely measured macroscopic soil parameters. A statistical analysis of all soil samples concurrently confirmed this lack of correspondence

Soil structure changes over time, and it matters!

The emergence of alternative agricultural practices aims to create sustainable production systems to meet future dietary needs. These practices and climate changes (Linnerooth-Bayer et al., 2015) will affect soil structure and hydraulic properties (Chandrasekhar et al., 2018). However, most models do not consider changes in hydraulic properties over time, leading to incorrect decisions. Therefore, understanding these changes is crucial. This study aimed to monitor the temporal evolution of hydraulic properties in three innovative production systems up to 90 cm depth. The project focuses on the value and resilience of innovative rotation systems (vegan, agro-ecological, off-soil). For this purpose, different theoretical water retention curves (WRCs) such as pedotransfer functions (PTFs) (HYPRES and ROSETTA 1,2,3 and EUHYDI) were evaluated and compared. The EU-HYDI WRC were then compared with i) experimental WRC determined by an evaporation method (Schindler et al., 2006); ii) continuous measurements taken in situ. Results showed that theoretical EU-HYDI WRCs were overestimated, and there were technical limitations in visualizing soil dynamics below the sensor threshold. Continuous measurements were analysed for each plot's three horizons (30, 60 and 90 cm) at different time scales, highlighting the impact of annual rainfall on the soil retention profile and the influence of agronomic itineraries. A comparative analysis of WRC and yield was performed. The communication will present the first results

Soft Skills: how to make the young engineers aware of their new talents?

peer reviewedThe competency framework attached to the Life science engineering Master at Gembloux Agro-Bio Tech, University of Liege (Belgium) is composed of technical and scientific skills but also soft skills which are not connected to academic courses. For the training of these skills, university needs the collaboration of the professional world. Therefore, the role of our teachers evolves towards a guiding or mentoring role. They will help students to analyse their professional experiences in order to shape their professional identity, to bring to light their acquired skills. This article describes how the portfolio, used as internship’s report, will help students to gain the self-confidence about their abilities and how professors can use these reflexive analyses to evaluate the acquisition of these soft skills

TEMPORAL VARIABILITY OF N2O FLUXES FROM A FERTILIZED GRASSLAND: PRELIMINARY RESULTS FROM DYNAMIC CLOSED CHAMBERS

This work presents preliminary results of nitrous oxide (N2O) fluxes measured by dynamic closed chambers from a fertilized grassland grazed by the Belgian Blue breed of cattle. It is part of a project funded by the public service of Wallonia (SPW-DGARNE), whose objectives are to make a carbon/CO2 balance of the grassland (Jérôme et al., 2013) and to quantify CH4 (Dumortier et al., 2013) and N2O fluxes. The site is located in Dorinne (Dorinne Terrestrial Observatory), Belgium (50° 18’ 44” N; 4° 58’ 07” E; 248 m al.). It is a permanent grassland of ca. 4.2 ha with a moderate slope of 1 to 2 %. Mineral fertilisation took place in March and May 2012. Two cylindrical chambers of 19,2 cm diameter and 11,5 cm height were placed inside a protected area around a micrometeorological station. An infrared gas analyser (Thermofischer 46i) was used in order to measure the N2O concentrations inside of the chambers, closed by automatically controlled lids and ventilated by a constant air flow of 1liter/min. These devices were completed by adjacent soil humidity and temperature sensors. The first measurement campaign took place during June and July 2012. The chambers were installed in the field and N2O fluxes were followed without manipulation. N2O fluxes were characterised by a background emission (between 2 and 10 ngN.m2s1) on which intense but time limited peaks (between 50 and 300 ngN.m2s1) superimposed. Peaks were found to be mainly linked to fertilisation and driven by precipitation. Background fluxes were found to correlate positively with soil temperature. Secondly, a manipulation experiment took place in November 2012: two different fertilizer treatments were applied to the chambers. Doses of respectively 100 and 200 kg N/ha of ammonium nitrate were sprayed in the chambers (equivalent to a 8mmprecipitation). N2O fluxes peaked shortly after fertiliser application (respectively 300 and 550 ngN.m2s1), as well as after a posterior rain event (respectively 800 and 1500 ngN.m2s1). The peak dynamics suggests a complex interaction between soil humidity and nitrogen availability, which is under study. Dumortier et al., Geophysical Research Abstracts, Vol. 15, EGU2013-2083-1, 2013 Jérôme et al., Geophysical Research Abstracts Vol. 15, EGU2013-6989, 201

Les choix méthodologiques influencent-ils les résultats ? Application aux essais de lixiviation en colonne de sol

L’intensification de l’agriculture avec l’utilisation importante de pesticides a eu des répercussions néfastes sur l’environnement avec des pollutions au niveau des sols, de l’air et des masses d’eau. A ce jour, l’agriculture est devenue une des pressions majeures qui pèsent sur la qualité des eaux. En effet, une grande partie des pesticides appliquée au sol n’atteint pas sa cible et est volatilisée, adsorbée, dégradée en métabolites ou lixiviée à travers le profil de sol. Pour cette raison, la recherche scientifique sur le devenir de ces composés dans l’environnement s’est grandement développée. En effet, les connaissances en terme du transfert de pesticides au sein d’un profil de sol sont encore très limitées. Or, une meilleure compréhension du devenir de ces derniers est indispensable afin d’en améliorer la gestion et d’assurer la protection de l’environnement. Afin d’étudier le devenir des pesticides au sein d’un profil de sol, plusieurs types d’expériences sont retrouvés dans la littérature. Les expériences les plus proches de la réalité sont les longues expériences de terrain avec l’application de pesticides aux sols agricoles. Cependant, ces expériences sont généralement très onéreuses et ne peuvent être réalisées partout. Il est alors utile d’acquérir des informations grâce à des expériences en laboratoire. Les expériences en laboratoire sont généralement des expériences en batch, de chromatographie en couche mince ou en colonne de sol. Afin d’obtenir des informations pertinentes sur la mobilité potentielle des pesticides dans les sols et de bien simuler les flux d’eau, les expériences en colonnes de sol sont massivement rencontrées dans la littérature (USEPA, 2008). Elles permettent de comparer la mobilité des pesticides entre eux ainsi que d’étudier leurs comportements de lixiviation dans différents sols. Les expériences en colonne servent également à analyser les mécanismes qui sous-tendent les mouvements de ces pesticides en colonne (Katagi, 2013). La courbe d’élution des pesticides ainsi que la distribution des pesticides adsorbés au sein de la colonne peuvent alors être étudiées et permettent d’obtenir les paramètres de sorption et de dégradation des pesticides. Cependant, au sein de la littérature traitant des expériences de lixiviation en colonne de sol, les modalités des colonnes utilisées sont très variables, empêchant les résultats obtenus d’être comparés ou transposés à d’autres cas. Une première modalité variant fortement dans la littérature est la structure du sol mise dans les colonnes. Les expériences de mobilité des pesticides sont généralement réalisées avec du sol remanié, à savoir préalablement séché, tamisé puis tassé uniformément dans la colonne. Ces colonnes remaniées ont l’avantage d’être plus reproductibles que des colonnes non remaniées (Isensee et al., 1992). Les colonnes de sol non perturbées, prélevées directement sur le terrain, offrent quant à elles l’avantage de se rapprocher des conditions réelles et d’investiguer les flux préférentiels, l’effet de l’amendement, du travail du sol ou encore des cultures. Une deuxième modalité très variable est la taille de la colonne. Le diamètre de la colonne ainsi que la longueur de cette dernière varient fortement d’une expérience à l’autre sans justifications. Ainsi, l’objectif de cette étude est d’analyser l’impact des choix méthodologiques sur le comportement de lixiviation au sein d’une colonne de sol. Cette recherche vise à comparer l’effet de la structure du sol, du diamètre et de la hauteur des colonnes sur le comportement de lixiviation d’un soluté au sein d’une colonne de sol. Un pulse de CaCl2 a été appliqué à 15 colonnes de sol limoneux agricole prélevé à Gembloux. Des colonnes de sol remanié et non remanié, de 8,4 et de 24 cm de diamètre ainsi que de 20 et de 35 cm de hauteur ont été réalisées. Une hauteur d’eau de 2,21 cm a été appliquée régulièrement à la surface des colonnes. Le pH, la conductivité électrique et le volume de l’eau percolée ont été mesurés. Les résultats montrent une grande influence de la structure du sol sur la lixiviation du CaCl2. La courbe d’élution des colonnes remaniées montre un pic plus haut et plus fin que la courbe d’élution des colonnes non remaniées (figure 1). Le CaCl2 est rapidement lixivié des colonnes remaniées avec 65,9% de la masse de CaCl2 qui ressort après 10 cm d’eau percolé alors qu’il est plus longtemps retenu dans les colonnes en structure conservée où seulement 47,7% ressort après 10 cm d’eau percolé. De plus, après 16 cm d’eau percolé, 100% du CalCl2 est lixivié à travers la colonne contre 80,1 % pour les colonnes non remaniées. De plus, l’avantage des colonnes remaniées mis en avant dans la littérature est la plus grande reproductivité de ces colonnes. Or, les écarts-types des colonnes non remaniées sont en moyenne plus faible que les écarts-types des colonnes remaniées (40,5 mg L-1 pour les colonnes non remaniées contre 67,8 mg L-1 pour les colonnes remaniées). Le diamètre de la colonne a une influence moindre sur la lixiviation des solutés. La courbe d’élution des colonnes de 24 cm de diamètre et des colonnes de 8,4 cm de diamètre se suivent globalement (figure 1). Cependant, un plus grand effet de dispersion est observé dans les colonnes de 24 cm de diamètre avec un pic légèrement plus aplati et plus large. Ainsi, après 12 cm d’eau percolé, 91,5% de CaCl2 est ressorti pour les colonnes de 8,4 cm de diamètre contre 84,4% pour les colonnes de 24 cm de diamètre. La variation de la hauteur de la colonne de sol n’a pas une grande influence sur la courbe d’élution du CaCl2 excepté au niveau du pic, plus important pour les colonnes de 20 cm. En effet, le CaCl2 sort plus rapidement des colonnes de 20 cm que des colonnes de 35 cm. Après 10 cm d’eau percolé, 69,2 % du CaCl2 est ressorti des colonnes de 20 cm de haut contre 63,7% pour les colonnes de 35 cm de haut. La hauteur de la colonne de sol ainsi qu’une potentielle semelle de labour dans les colonnes de 35 cm pourraient également expliquer ces résultats. Afin de déterminer la dispersion du CaCl2 au sein des colonnes et l’influence de la semelle de labour, une modélisation des colonnes sur le logiciel Hydrus sera nécessaire.AIL4WaterQualit