Search

International audience

International audience; An improved version of the MARMIT model is proposed. Wet soil is represented as a dry soil layer surmounted by a layer of a mixture of water and soil particles.

International audience; Surface soil moisture content (SMC) is known to impact soil reflectance at all wavelengths of the solar spectrum. As a consequence, many semi-empirical methods aim at inferring SMC from soil reflectance, but very few rely on physically-based models. This article presents a multilayer radiative transfer model of soil reflectance called MARMIT (multilayer radiative transfer model of soil reflectance) as a function of SMC given on a mass basis and a method called MARMITforSMC to estimate it from soil reflectance spectra. This model depicts a wet soil as a dry soil covered with a thin film of water. It is used to assess SMC over seven independent laboratory datasets gathered from the literature. A learning phase is required to link the thickness of the water film with the SMC. For that purpose, a sigmoid function, the parameters of which are related to soil physical and chemical properties such as porosity, grain size and mineralogy composition, is fitted. SMC can be inferred with good accuracy (RMSE ≈ 3%) if the learning step is applied soil by soil. The link between SMC and water thickness actually depends on soil texture and chemical composition. If the soils are divided into classes and if the learning phase is applied to a class, the RMSE slightly increases up to 5%. Finally, MARMITforSMC provides lower RMSE than any other existing semi-empirical or physically-based method.

International audience; A laboratory experiment is set up to study both surface and in-depth soil moisture content (SMC). For that purpose, an aquarium is filled successively with two soils, a clay loam and a sand. Reflectance spectra are acquired in the solar domain (400–2400 nm) on the soil surface using an ASD FieldSpec 3 HR spectroradiometer and in-depth through the aquarium glass wall using two hyperspectral cameras. Successive amounts of water ranging from low to heavy rainfall in a temperate region are uniformly poured into the aquarium. The MARMITforSMC method based on the MARMIT (MultilAyer Radiative Transfer Model for soIl reflecTance) model is applied to each reflectance spectrum to determine gravimetric SMC. In particular, vertical profiles of SMC are provided with unprecedented spatial accuracy (~0.287 mm). The results are compared with volumetric SMC measured by two time-domain reflectometry (TDR) sensors. The in-depth SMC image produced on the sand shows lower values within the first 2 cm (5%) than below (17%). In contrast, the SMC image produced on the clay loam shows evenly distributed values whatever the position in the aquarium, even 1 h after moistening. The difference in grain size between the soils explains this result.; Une expérience est montée en laboratoire pour étudier l’humidité (SMC) du sol à la fois en surface et en profondeur. Pour cela, un aquarium est successivement rempli avec deux sols, un limon argileux et un sable. Les spectres en réflectance sont acquis dans le domaine solaire (400-2400 nm) en utilisant un spectroradiomètre ASD FieldSpec 3 HR pour la surface et deux caméras hyperspectrales pour le profil derrière la paroi de l’aquarium. Des quantités successives d’eau, allant de la pluie faible à forte pour une région tempérée, sont versées uniformément dans l’aquarium. La méthode MARMITforSMC s’appuyant sur le modèle MARMIT (Modèle de Transfert Radiatif Multicouches pour la réflectance du sol) est appliquée à chaque spectre en réflectance pour déterminer l’humidité gravimétrique correspondante. Ceci permet en particulier d’obtenir des profils verticaux d’humidité avec une résolution spatiale sans précédent (0 ,287 mm). Les résultats sont comparés aux mesures d’humidité volumétrique obtenues à l’aide de sonde de réflectométrie dans le domaine temporel (TDR). Les cartes de profil d’humidité générées pour le sable montrent des valeurs plus faibles pour les 2 premiers centimètres (5 %) qu’au-delà (17 %). Par contre, les cartes de profil d’humidité générées pour le limon argileux montrent des valeurs homogènes quelle que soit la profondeur, même 1 heure après l’humidification. La différence de granulométrie des sols explique ces résultats.