Apport d'un modèle simplifié de simulation du bilan hydrique pour l'analyse de la satisfaction du besoin en eau de prairies irriguées des Alpes internes

Le recours à l'irrigation des prairies permanentes du val d'Aoste (Alpes italiennes) s'explique par la faible pluviosité estivale et la forte demande d'évapotranspiration. Pour étudier la satisfaction des besoins en eau de divers types de prairies irriguées, nous avons utilisé un modèle simplifié de calcul des variations de la teneur en eau du sol. Il s'agit du module bilan hydrique de CREAMS (chemical, runoff, and erosion from agricultural management systems). Dans un premier temps, nous caractérisons la variabilité des différentes variables entrées dans le modèle : propriétés hydriques des sols, fréquences, dates et doses d'irrigation. Nous évaluons par ailleurs la valeur d'un paramètre de calage décrivant la répartition du système racinaire avec la profondeur. Dans un deuxième temps, nous simulons les variations de la teneur en eau du sol sur un réseau de 8 parcelles dont nous suivons l'humidité volumique et la production de matière sèche. Les calculs donnent des résultats comparables aux humidités observées et le rendement en matière sèche est corrélé significativement aux déficits hydriques moyens calculés. Le modèle ainsi validé nous permet de simuler le comportement hydrique de différentes parcelles représentatives de la variabilité régionale des sols et des pratiques d'irrigation.A simplified water-balance model for the analysis of the satisfaction of water requirements in irrigated inner Alps meadows. Irrigation of permanent meadows in Val d'Aosta (inner Italian Alps) is rendered necessary by low summer precipitations and by a high evapotranspiration demand. To study the satisfaction of the water requirement of vegetation, a simple model of water balance calculation was used. This is the 'hydrology' module of CREAMS (chemical, runoff, and erosion from agricultural management systems). We first characterized the variability of the different input variables: hydric properties of the soils, dates and frequencies of irrigation, and doses of water supplied. We also assessed the value of a parameter that relates root distribution with depth. We noticed a great variability of soil properties and irrigation practices. We then ran a simulation of water balance on 8 plots where soil moisture and dry-matter yield were monitored. The results of the calculations were comparable to the observed soil moisture values. Moreover, the dry-matter yields were correlated with the calculated mean water deficit. Simulations run with the model provide a framework adapted to the regional variability of soil properties and agricultural practices, which can be used to analyse interaction of the factors involved in the water balance

The effect of grass buffer strips on phosphorus dynamics - A critical review and synthesis as a basis for application in agricultural landscapes in France. Agriculture, Ecosystems and Environment

Abstract Recommending the use of ''grass buffer strips'' to control diffuse P transfer has become well accepted among extension advisors, agricultural consultants, planners, and other practitioners that influence the structure of the agricultural landscape. These grassed areas are put in place to capture the P contained in runoff from source fields. They are designed to function as a filter and a sediment trap although it is often unclear what the long-term disposition of the accumulated P may be. The objective of this work was to determine if the available scientific literature justifies the continued recommendation of this approach in the prevention of phosphorus movement from agricultural soils to surface waters. We employed a theoretical analysis of the mechanisms of the buffering effect and the specific behaviour of phosphorus in typical grass buffer strips to establish the critical set of literature applicable to this question. An adequate body of literature exists describing many aspects of P dynamics and the short-term functioning of grass buffer strips over their seasonal cycles. Despite variable results in a diversity of landscape contexts, overall, the use of grass buffer strips appears to provide useful short-term functions in the reduction of P transport to surface waters. Long-term benefits remain questionable given the relatively short-term use of this approach in P reduction and the lack of long-term experimental results, but this current lack of data is not sufficient to deter the continued incorporation of grass buffer strips in the landscape of French agricultural. Additionally, a more comprehensive conceptual model integrating the shortterm functioning of grass buffer strips with seasonal cycles and the long-term consequences of cumulative storage emerged from our synthesis.

Conditions d'apparition du ruisellement dans les cultures annuelles de la région lémanique. Relation avec le fonctionnement des exploitations agricoles

Afin de déterminer les facteurs qui contrôlent le ruissellement des terres agricoles et de mettre en évidence les situations et périodes à risque, nous avons observé ses symptômes pendant une année, sur 200 parcelles de cultures annuelles représentatives des conditions de milieu et de pratiques culturales de la rive française du lac Léman (400 km2 ). Les résultats obtenus mettent en évidence que le ruissellement affecte la moitié des parcelles et présente une grande diversité de formes et d’origines. Les formes liées à la dégradation structurale superficielle des sols prédominent. Leur précocité et leur fréquence dépendent surtout des pratiques culturales. Ainsi, les chantiers de récolte laissant le sol nu, certaines pratiques comme l’implantation des cultures réalisées par de nombreux passages d’outils traînés ou les successions de culture dans lesquelles le maïs revient souvent, favorisent la battance ou le tassement des sols et constituent des facteurs de risque de ruissellement. Sur ces bases, nous construisons une échelle de risque de ruissellement en fonction des pratiques et des rotations. Elle permet une analyse comparée des risques potentiels de ruissellement des divers systèmes d’exploitation de la région étudiée. Les fonctionnements d’exploitation traditionnels, présentent les plus fortes proportions de surface cultivée à risque de ruissellement, du fait de leur équipement et de leurs rotations. Cependant, à l’échelle de la région, le risque global lié à ce type d’exploitation reste limité en raison de la faible extension de leurs cultures annuelles. Les exploitations en début d’intensification appliquent les mêmes pratiques à risque, mais sur de vastes surfaces. Elles sont de ce fait, à l’échelle de la région, à l’origine d’un risque de ruissellement important. Les exploitations les plus intensives présentent des proportions de surface à risque beaucoup plus limitées, compte tenu d’un équipement et de rotations qui limitent les problèmes de dégradation structurale des sols. Néanmoins, ce type d’exploitation contribue de façon importante au risque global, en relation avec la forte surface des cultures annuelles qu’il gère. Le travail réalisé permet de proposer des axes d’action différenciés selon les types d’exploitation, pour favoriser des pratiques agricoles limitant le risque de ruissellement et, au-delà, le transfert de polluants vers les eaux de surface.Agricultural runoff from the Chablais region of France, is believed to contribute to the pollution of the waters of Lac Leman (Lake Geneva). This study aimed at determining which factors influence the frequency of runoff from agricultural lands. Two hundred agricultural fields under a wide diversity of agricultural practices were monitored during a winter and a summer period. For each field the kinetics of formation of superficial soil crusting that can prevent infiltration of rainfall into deeper soil layers was evaluated and the presence of runoff was observed. Additionally, the farmers were surveyed to determine the specific agricultural practices that were used on each field. The observations showed that both Hortonian and non-Hortonian runoff occurred. Sheet runoff was observed as well as runoff following tracks of cultivation equipment. The frequency of runoff varied among agricultural practices. Fields cultivated with nonrotative implements had a higher frequency of runoff than fields cultivated with rotative implements. Fields in long periods of rotation of corn or other crops without mulching showed high frequencies of runoff. A scale of runoff risk was developed to compare the frequency with which the different farming systems in Chablais produced runoff. For each farming pratice this scale takes into account, at the field level, the kinetics of the development of impermeable soil crusting and the duration of the time the field is at risk to produce runoff. Fields on which the mechanization and rotational practices of traditional systems are used have the highest risk of producing runoff per unit area. Intensive farmiing systems have a lower risk of producing runoff per unit area. Intensive farming systems have a lower risk of producing runoff per unit area. However, because of the relatively small land area presently devoted to the traditional farming systems, both intensive and traditional systems contribute equally to the pollution of Lake Leman. As a result of this work, changes in traditional and intensive practices are suggested that are believed to enhance agricultural sustainability on the French shores of Lake Leman and reduce environmental impacts

Coupling indicators and lumped-parameter modeling to assess suspended matter and soluble phosphorus losses

International audienceFor ecological and economic issues, evaluating the environmental fate of dissolved and suspended matter in catchments and river ecosystems still remains a challenge for the preservation and management of natural resources. Models are useful tools and may help to cope with this challenge, and especially to define the relationships between the state of natural systems and land and river management/uses. As it is difficult -even impossible- to carry out experiments on natural systems such as catchments, models are also useful to test hypotheses about the underlying processes acting on dissolved and suspended losses. We propose a innovative approach to achieve these objectives. By coupling environmental indicators and lumped modeling, this study aims to develop a conceptual and general framework to evaluate and test the functions that drive particulate and dissolved matter flows at the catchment and landscape scales, while respecting the constraint of parsimony for the number of model parameters. Calculated suspended matter (SM) and soluble reactive phosphorus (SRP) losses agreed well with field data. 210Pbex (excess Pb) activities in core sediments were also compared to those of 210Pbex calculated from the filling of the reservoir. Our models are parsimonious and this does not impair their accuracy in reproducing recorded outflows or evaluating the sedimentation processes associated to particulate outflows. Considering the adequacy of our models, we validate the hypothesis that river bank erosion and water table behavior are the driving processes that govern losses of particulate and solute forms of P, in the studied extensive agriculture conditions

OPALE: Operational assessment of landscape water eco-functionalities

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