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

Calibration of soil moisture sensors for a long-term field experiment

In the framework of the ICOS RI network, a field site in Lonzée, Belgium, is equipped to provide long-term data on greenhouse gas emissions from an agricultural field and the associated environmental variables. Soil moisture is one of the state variables which are monitored with high temporal resolution and with several repetitions in the field to take into account soil heterogeneity. In order to facilitate field installation in combination with agricultural practices, Sentek Enviroscan sensors, a collection of FDR sensors at different depths on a stick, were chosen to measure soil moisture. In this contribution, we will discuss the results of a detailed calibration experiment we performed for this sensor type and compare it to the results we got from a different FDR sensor: the ML3 Thetaprobe. We calibrated the probes for the different soil horizons at 3 different locations in the field using big reconstructed soil columns which were brought to defined soil moisture levels in the lab. The results showed that the universal calibration relationship of the sensors gave quite similar results as the soil-specific calibration up till a moisture content of 40%. We also observed that the higher the soil moisture content becomes, the more difficult it is to obtain a homogeneous distribution of the water in the calibration column which might have an impact on the sensor readings