Particle Image Velocimetry (PIV) measurements in a water film, application to a tire rolling through a puddle

International audienceA measurement method based on Particle Image Velocimetry with refraction of the laser sheet at a win-dow/water interface is proposed for the measurement of the velocity field of a water flow formed by a tire moving inside a water puddle. This study focuses of the feasibility and repeatability of this optical measurement method. The characterization of the optical properties of the measurement technique defines the integration effect in height of the measurement method. The analysis of the overall features of the flow is focused on two main zones in front and around the tire. The flow inside the first zone is defined by a characteristic velocity of the water displaced in an area located in front of the tire ; in the second zone a characteristic velocity representative of the flow in the vicinity of the shoulder of the tire is also defined. Correlations of both characteristic velocities with the car speed and water film height are established. New and worn tires were tested in this work

Caractérisation hydrodynamique d'un sol encroûté en zone sahélienne. 2. Modélisation du processus d'infiltration

Dans une première partie, nous avons présenté des résultats expérimentaux de détermination et de suivi de la résistance hydraulique des croûtes de surface d'un sol sahélien au cours d'averses successives. Les valeurs mesurées sont utilisées comme données d'entrée dans un modèle numérique déterministe de simulation du phénomène. La comparaison entre les lames infiltrées et ruisselées mesurées et simulées montre que les valeurs expérimentales de la résistance hydraulique de la croûte sont déterminées avec une précision satisfaisante. En revanche, la mesure de la relation conductivité hydraulique - teneur en eau devrait être affinée. (Résumé d'auteur

Caractérisation hydrodynamique d'un sol encroûté en zone sahélienne. 1. Approche expérimentale in situ

La présence d'une croûte superficielle a une influence prépondérante sur les capacités d'infiltration de la plupart des sols en zone sahélienne. La caractérisation, in situ, de la résistance hydraulique développée au sein de cette croûte a longtemps été limitée par des contraintes métrologiques. Le protocole expérimental présenté nous a permis de suivre l'évolution du comportement hydrodynamique d'un sol ferrugineux du Sénégal soumis à deux pluies artificielles. On observe une certaine cohérence entre les résultats des mesures tensio-capacitives effectuées durant les averses et ceux fournis par l'infiltrométrie à succion contrôlée en fin d'expérimentation. Cependant, seule la première technique permet d'accéder à la conductivité hydraulique propre de la croûte. L'analyse d'échantillons de sol non remaniés par porosimétrie au mercure confirme la modification structurale qui intervient à la surface du sol. (Résumé d'auteur

Toward a satellite-based system of sugarcane yield estimation and forecasting in smallholder farming conditions : a case study on Reunion Island

Estimating sugarcane biomass is difficult to achieve when working with highly variable spatial distributions of growing conditions, like on Reunion Island. We used a dataset of in-farm fields with contrasted climatic conditions and farming practices to compare three methods of yield estimation based on remote sensing: (1) an empirical relationship method with a growing season-integrated Normalized Difference Vegetation Index NDVI, (2) the Kumar-Monteith efficiency model, and (3) a forced-coupling method with a sugarcane crop model (MOSICAS) and satellite-derived fraction of absorbed photosynthetically active radiation. These models were compared with the crop model alone and discussed to provide recommendations for a satellite-based system for the estimation of yield at the field scale. Results showed that the linear empirical model produced the best results (RMSE = 10.4 t.ha(-1)). Because this method is also the simplest to set up and requires less input data, it appears that it is the most suitable for performing operational estimations and forecasts of sugarcane yield at the field scale. The main limitation is the acquisition of a minimum of five satellite images. The upcoming open-access Sentinel-2 Earth observation system should overcome this limitation because it will provide 10-m resolution satellite images with a 5-day frequency

Coupling a sugarcane crop model with the remotely sensed time series of fIPAR to optimise the yield estimation

The objective of this study was to assess the efficiency of the assimilation of the fraction of intercepted photosynthetically active radiation (fIPAR) data derived from Satellite Pour l'Observation de la Terre SPOT images into the MOSICAS sugarcane crop growth model for estimating the yield at field scale on Reunion Island. Over 3 years, time series of SPOT satellite imagery were used to estimate the daily evolution of NDVI for 60 plots located on two climatically contrasted farms. Ground measurements of the fIPAR were performed on 5 reference fields and used to calibrate a relationship with the corresponding NDVI values. Forced and not forced simulations were run and compared with respect to their ability to predict the final observed yield. Forcing MOSICAS with fIPAR values derived from SPOT images improved the accuracy of the model for the yield estimation (RMSE = 12.2 against 14.8 t ha(-1)) closer to the 1:1 line. However, underestimations of the yield by the forced model suggest that some of the model parameters were not optimal. The maximal radiation use efficiency parameter (RUEm) was optimised for each field, and an analysis of variance showed the significant effect of the ratoon number of the field, of its cultivar and of the farm where it is planted. Accordingly, the RUEm was recalibrated for each cultivar for the number of ratoons and farms. New RUEm values ranged from 3.09 to 3.77 gMJ(-1), and new computations were run using the optimised values of RUEm The results indicate that recalibrating the maximal radiation use efficiency according to the number of ratoons improved the yield estimation accuracy by as much as 10.5 t ha-1 RMSE. This study highlights the potential of time series of satellite images to enhance the estimation of the yield by a forced ecophysiological model and to obtain better knowledge about the ecophysiological processes that are involved in crop dynamics with the recalibration method