Uncertainties in assessing annual nitrate loads and concentration indicators. Part 1: Impact of sampling frequency and load estimation alogorithms

International audienceThe objectives of this study are to evaluate the uncertainty in annual nitrate loads and concentrations (such as annual average and median concentrations) as induced by infrequent sampling and by the algorithms used to compute fluxes. A total of 50 watershed-years of hourly to daily flow and concentration data gathered from nine watersheds (5 to 252 km2) in Brittany, France, were analyzed. Original (high frequency) nitrate concentration and flow data were numerically sampled to simulate common sampling frequencies. Annual fluxes and concentration indicators calculated from the simulated samples were compared to the reference values calculated from the high-frequency data. The uncertainties contributed by several algorithms used to calculate annual fluxes were also quantified. In all cases, uncertainty increased as sampling intervals increased. Results showed that all the tested algorithms that do not use continuous flow data to compute nitrate fluxes introduced considerable uncertainty. The flowƒ]weighted average concentration ratio method was found to perform best across the 50 annual datasets. Analysis of the bias values suggests that the 90th and 95th percentiles and the maximum concentration values tend to be systematically underestimated in the long term, but the load estimates (using the chosen algorithm) and the average and median concentrations were relatively unbiased. Great variability in the precision of the load estimation algorithms was observed, both between watersheds of different sizes and between years for a particular watershed. This has prevented definitive uncertainty predictions for nitrate loads and concentrations in this preliminary work, but suggests that hydrologic factors, such as the watershed hydrological reactivity, could be a key factor in predicting uncertainty levels

Histoire de Bèarn et Navarre

Imp. tomado de colofó

Mesure de la dénitrification et modélisation spatialisée des flux d'azote à l'échelle d'un petit bassin versant d'élevage

Notre objectif était de quantifier l'importance relative de la dénitrification hétérotrophe dans le devenir des excédents azotés à l'échelle du bassin versant. Nous avons mesuré la dénitrification sur 15 sites pendant un an sur un bassin versant en Bretagne, France. En utilisant la technique du blocage à l'acétylène, des mesures de dénitrification sur carottes de sol et de dénitrification potentielle (DEA) ont été réalisées sur 2 horizons de sol : 0-20 et 20-40 cm. TNT2, un modèle agro-hydrologique distribué a été utilisé pour spatiailser les mesures. Les taux de dénitrification de 0 à 417 mg N. m-2.jour-1, sont plus élevés dans les zones ripariennes. Les valeurs de DEA mesurées varient de 76.48 à 530.63 ng N.g-1 sol sec.h-1. L'humidité du sol semble être le principal facteur limitant. Le ratio N20/(N2 + N2) moyen, d'environ 60 % paraît être constant sur l'année. Les simulations montrent des émissions liées à la dénitrification égales aux flux d'azote dans le cours d'eau.Our objective was to assess the relative importance of heterotrophic denitrification on the fate of excess nitrogen at the catchment scale, and to quantitify the relative importance of riparian areas ont the N20 emissions. We measured denitrificatin at 15 sites over one year on a farming water catchment area in Brittany France. Using C2H2 inhibition, denitrification analysed on soil core and denitrifying enzyme activity (DEA) were each determined, for samples taken from 2 soil layers : 0-20 and 20-40 cm. Denitrification rates, ranging from 0 to 417 mg N.m-2.d-1 were significantly higher in riparian areas. The measured DEA values ranged from 76.48 ng N.g-1 dry soil.h-1 to 530.63 ng N.g-1 dry soil.h-1. Soil moisture appeared to be the main limiting factor. The ratio N20/(N20+N2) was about 60 % and seemed to be constant throughout the year. The results simulations suggest that the at the studied water catchment area scale, denitrification is a major route for nitrogen removal.RENNES-Agrocampus-CRD (352382323) / SudocSudocFranceF

Effects of bottom sediment restoration on interstitial habitat characteristics and benthic macroinvertebrate assemblages in a headwater stream

International audienceThe restoration of in-stream habitats by structural improvement of stream beds is more and more frequent, but the ecological consequences of such works are still little known.We have examined the influence of the deposit of a 15 cm gravel layer over the stream bottom on the chemical characteristics of the interstitial water, the sediment grain size and the composition of the benthic assemblages. We have compared a restored reach to an upstream control over three years and at three seasons each year. Dissolved oxygen, ammonium, nitrite and nitrate contents were measured in both surface and interstitial (15 cm deep) waters, together with the depth of anoxia estimated using wooden stakes and fine sediment content at the surface. During the same period and seasons, benthic invertebrates were sampled at five points in each reach. The restoration induced an increase in vertical exchanges of water between surface and interstitial habitats, with an increase in the depth of hypoxia. Changes were observed in the composition of invertebrate communities, but not in the density or in the taxonomic richness of assemblages. These changes in assemblages were fragile: a local disturbance (such as a drying period) diminished the beneficial effect of the restoration with the disappearance of several organisms. The viability of such restoration works may be associated with catchment management designed to reduce fine sediment inputs to the river

Concert'eau: Task 4. Deliverable 4.1.1. Report on diagnosis related to DPSIR. Analyse par la méthode DPSIR des enjeux environnementaux sur la zone test "Gers-amont"

[Departement_IRSTEA]GT [TR1_IRSTEA]RURAMEN / CERESLe document présente la méthode D.P.S.I.R. développée pour l'étude de la pollution diffuse liée à l'agriculture sur le territoire du Gers-amont. Cette zone d'application a été choisie par les groupes d'acteurs dans le cadre de la tâche 4 du projet Life Concert'eau, pour la construction de la Plateforme Technologique Collaborative(CTP). Il est proposé une liste des principaux indicateurs relevant de chacun des composantes du DPSIR, ainsi qu'une méthode de calcul adaptée au territoire considéré. Des scénarios de mesures correctives (de type Bonnes pratiques agricoles ) sont ensuite suggérés en vue de la modélisation (modèle SWAT) de leurs effets à moyen terme sur la qualité de l'eau

Contrôle des pollutions diffuses par les pesticides. Une approche coût-efficacité spatialement distribuée

International audienc

A spatially-distributed cost-effectiveness analysis framework for controlling water pollution

International audienc