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Impact of land use on the hydraulic properties of the topsoil in a small French catchment

The hydraulic properties of the topsoil control the partition of rainfall into infiltration and runoff at the soil surface. They must be characterized for distributed hydrological modelling. This study presents the results of a field campaign documenting topsoil hydraulic properties in a small French suburban catchment (7 km2) located near Lyon, France. Two types of infiltration tests were performed: single ring infiltration tests under positive head and tension disk infiltration using a mini-disk. Both categories were processed using the BEST Beerkan Estimation of Soil Transfer parameters- method to derive parameters describing the retention and hydraulic conductivity curves. Dry bulk density and particle size data were also sampled. Almost all the topsoils were found to belong to the sandy loam soil class. No significant differences in hydraulic properties were found in terms of pedologic units, but the results showed a high impact of land use on these properties. The lowest dry bulk density values were obtained in forested soils with the highest organic matter content. Permanent pasture soils showed intermediate values, whereas the highest values were encountered in cultivated lands. For saturated hydraulic conductivity, the highest values were found in broad leaved forests and small woods. The complementary use of tension disk and positive head infiltration tests highlighted a sharp increase of hydraulic conductivity between near saturation and saturated conditions, attributed to macroporosity effect. The ratio of median saturated hydraulic conductivity to median hydraulic conductivity at a pressure of -20 mm of water, was about 50. The study suggests that soil texture, such as used in most pedo-transfer functions, might not be sufficient to properly map the variability of soil hydraulic properties. Land use information should be considered in the parameterizations of topsoil within hydrological models to better represent in situ conditions, as illustrated in the paper

Caractérisation hydrodynamique de sols déformables partiellement saturés : étude expérimentale à l'aide de la spectrométrie gamma double-sources

Une méthode est proposée pour determiner la conductivité hydraulique, en fonction de la concentration en eau, de milieux poreux partiellement saturés susceptibles de se déformer. Elle est fondée sur la description eulerienne des transferts d'eau et de particules solides. elle s'appuie sur l'analyse en régime transitoire des profils d'humidité et de masse volumique séche obtenus par spectrométrie gamma double-sources, specialement developpée à cet effet, couplée à des mesures de pression capillaire par tensiométrie. les résultats présentés sont relatifs à des expériences d'infiltration verticale d'eau dans des matériaux poreux compactes et gonflant librement. le bon accord, entre les valeurs déterminées par l'approche lagrangienne, également mise en oeuvre, et celles obtenues dans le cadre de la description eulerienne confirme la pertinence des hypothèses sous-tendant cette dernière. de plus, elle apparait séduisante dans la mesure où elle peut constituer une formulation générale des écoulements d'eau, incluant les milieux déformables et rigides.pas de résum

A Linking Test that investigates the feasibility of inverse modelling: application to a simple rainfall interception model for Mt Gambier, southeast South Australia

International audienceInterception loss has an important influence on the water yield of forested areas. Nevertheless, in most studies stemflow is not measured, therefore the question of how to determine the feasibility of optimizing interception and stemflow parameters simultaneously by matching daily simulated throughfall to fortnightly measurements of cumulative throughfall is an important one. By applying a daily empirical interception model, a goodness fit of 2·2 mm/day is obtained between observed and simulated cumulative throughfall. However, by applying the simple but robust Linking Test, it was shown that the parameters are non-unique and falsely linked, i.e. inter-relationships between different vegetation parameter sets give similar throughfall but non-unique net precipitation. The Linking Test investigates the causes of obtaining falsely linked parameters and shows that objective equifinality is not the source of the problem. Objective equifinality occurs when an inappropriate objective function is used. The Linking Test also shows that falsely linked parameters are not caused by measuring throughfall on a non-daily basis (termed frequency sampling equifinality). By expanding the interception model to the second degree, it was found that the non-uniqueness is due to the inherent nature of interception and stemflow functions that behave similarly and therefore can easily compensate each other (termed similarity equifinality). It is also shown that a simple daily empirical exponential interception model developed for conifers in the uplands of the United Kingdom is suitable to model interception in Pinus radiata plantations in the Mediterranean climate of southern Australia by using only daily gross precipitation data and two parameters

Influence of the irrigation technique and strategies on the nitrogen cycle and budget: A review

The objective of this review is to remedy the lack of knowledge about the expected relationship between the irrigation techniques (flood irrigation, sprinkler irrigation, surface and subsurface drip irrigation) and the nitrogen transformations (fixation, mineralization, immobilization and nitrification) or fluxes (denitrification, runoff, volatilization and lixiviation) in agricultural contexts. This study investigates thus the various controls on the nitrogen cycle and budget, either site-specific (soil C/N ratio, pH, salinity, texture and temperature) or corresponding to deliberate strategies in water and nitrogen management. The comprehensive view gained from the gathered literature elements identifies the local variations (in space and time) of soil water content profile as both the strongest control and the control most directly related to the irrigation technique and to the practitioner’s decisions. In the overall picture, flood irrigation and sprinkler irrigation are the techniques in which most transformations or fluxes may be enhanced or reduced, also with the risks associated with non-optimal practices. By contrast, subsurface drip irrigation seems the technique with the least unwanted impacts. Besides the academic aspects (bringing together scattered literature elements) an outcome of this review is thus to facilitate decision-making regarding the choice and/or use of irrigation-fertilization techniques and strategies (doses and scheduling) for given agro-pedoclimatic contexts, also for combined agricultural production, economic and site preservation objectives

Estimating preferential water flow parameters using a binary genetic algorithm inverse method

International audienceKDW-GA is a framework for the simulation of preferential water flow through unsaturated soils. Preferential flow can be described by a Kinematic Dispersive Wave (KDW) equation which depends on three transport parameters. Transport parameters are estimated with the binary genetic algorithm (GA) inverse method by reducing the errors (cost function) between estimated and observed water flux values. Different GA components are discussed in order to find the best strategy that fits our problem. Recommendations concerning the mutation rate, the elite number, and the pairing technique are deduced with regard to the algorithm performance

Two-scale modeling of unsaturated water flow in a double-porosity medium under axisymmetric conditions

International audienceIn this paper the development and experimental validation of a numerical model of two-dimensional unsaturated flow in a double-porosity medium is presented. The model is based on the coupled formulation for flow in macro- and micropores obtained by homogenization. It was applied to simulate the axisymmetrical tension disk infiltration experiments that were carried out in a double-porosity medium. The physical model was a three-dimensional periodic structure, composed of porous spheres made of sintered clay and embedded in Hostun fine sand HN38. The hydraulic parameters of both porous materials were determined by inverse analysis of independent infiltration experiments performed on sand and sintered clay. The effective parameters of the double-porosity medium were calculated from the solution of the local boundary value problem, obtained from the homogenization procedure. The cumulative infiltration curve and the global dimensions of the humidified zone obtained from the numerical solution are in good agreement with the observations. Moreover, numerical simulations showed the existence of a narrow zone of local nonequilibrium that moves with the infiltration front. Upstream of this zone, the infiltration bulb is in the local equilibrium conditions

Reactivity of vadose-zone solids to S-metolachlor and its two main metabolites: case of a glaciofluvial aquifer

International audienceThe vulnerability of groundwater to pesticides is governed in part by sorption mechanisms in the vadose zone, commonly studied in soil but less well-known in the geological solids. To alleviate this lack of knowledge, adsorption of the herbicide S-metolachlor (SMOC) and of two of its metabolites—metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA)—was studied with batch equilibrium method on seventeen surface soils and three geological solids of the vadose zone overlying a glaciofluvial aquifer. In grainsize terms, the latter three were sand for the first two samples and gravel for the third. Adsorption is ordered as follows: SMOC > > MESA > MOXA, except for one of the geological solids for which MESA adsorption was slightly higher than that of SMOC (Kd = 0.73 vs. 0.44 L kg−1). The low MOXA adsorption could only be quantified for the gravel sample (Kd = 0.74 L kg−1), which was also more reactive than all the other samples to MESA and SMOC (Kd = 2.08 and 28.8 L kg−1, respectively). Statistical multivariate tests related the highest Kd values for SMOC with the soils and geological solids with the highest organic-carbon and clay-fraction contents. The highest Kd values for MESA were found in the samples containing high oxide concentrations. Our results shed a new light on the adsorption of SMOC, MESA and MOXA suggesting that during their transfer to groundwater, pesticides and metabolites can be adsorbed in the vadose zone on both soils and geological solids

Effects of wetting and drying cycles on in situ soil particle mobilization

International audienceUnderstanding particle mobilization and transport in soils is a major concern for environmental protection and water resource management as they can act as vectors for sorbing pollutants. In natural soils, the existence of a finite size and renewable pool of dispersible particles has been hypothesized. Even though freeze-thaw and wetting-drying cycles have been identified as possible mechanisms of pool replenishment between rainfall events, to date the underlying phenomena ruling the renewal of particle pools are still largely unexplored. We carried out a series of infiltration-drainage experiments to study systematically the effects of periods without rain (pauses) on in situ particle mobilization in undisturbed soil columns. We found that, for a given column, pause duration between two rainfall events has a major influence on subsequent particle mobilization: the mass of leached particles increases with pause duration until it reaches a maximum (mass for a 200-hours pause is 15 time greater than for a 1-hour pause), and then it decreases for even longer pauses. This behaviour was correlated with soil water content, and can be explained by soil matrix weakening due to differential capillary stresses during drying. The consequences of this finding are important because the 15-fold increase in mass of leached particles, when pause duration is changed from 1 hour to 4 days, might overwhelm variations caused by changes in other parameters such as the ionic strength of the incoming solution or the rainfall intensity

Un test de liaison explarant la non unicité des paramètres de sol et de végétation d'un modèle de transfert en zone non saturée

Groundwater recharge can be accurately computed by using models that solves the Richards' equation. A drawback of such models is that they require a considerable number of parameters that need to be determined: Mualem-van Genuchten, interception and crop factor. It is preferable to estimate these parameters indirectly from in situ measurements since values retrieved from sample analysis poorly characterize field conditions. Groundwater recharge can be modelled once the hydraulic and the vegetation parameters have been optimised simultaneously. The optimisation can be achieved by matching solely observed and simulated time series of soil moisture profiles. A good match between simulated and measured soil moisture can be obtained but the parameters are non-unique, leading to the problem of equifinality. We found that a unique groundwater recharge could be obtained only if the vegetation parameters were known. We investigated why the optimum hydraulic parameters were not unique by using the Linking Test. The Linking Test is a simple inverse method developed by the authors that takes into account the non uniqueness of the parameters. The Linking Test shows that very different soils could give similar groundwater recharge. This test also provides all the different combinations of the hydraulic parameters that give similar groundwater recharge. The Linking Test showed that the hydraulic parameters were linked due to "range" and "compensation" equifinalities. We showed that only 2 functional parameters are required to get a satisfactory simulation. It should notably simplify the determination of the hydraulic parameters in a region where various soil types are encountered and thus help in designing upscaling methods for these parameters. Other foreseen applications of knowing how the hydraulic parameters are linked are an improvement (1) of the sensitivity analysis to the hydraulic parameters, (2) the uncertainty analysis of the groundwater recharge by using soil water flow models, (3) the interpretation of the physical meaning of the hydraulic parameters determined by inverse modelling. The Linking Test could be applied to any inverse problem prior to modelling to determine its feasibility and to establish if the non-uniqueness of the parameters is of concern for the problem to be solved