45 research outputs found
Vers une modélisation hydrologique adaptée à la prévision opérationnelle des crues éclair. Application à de petits bassins versants du sud de la France
La prĂ©sente thĂšse traite de la prĂ©vision des crues dĂ©vastatrices engendrĂ©es par des alĂ©as extrĂȘmes et se situant principalement dans des rĂ©gions mĂ©diterranĂ©ennes françaises, souvent qualifiĂ©es de crues « Ă©clair ». Une analyse des diffĂ©rents processus hydrologiques principalement responsables de la genĂšse et de la propagation de telles crues montre qu'une modĂ©lisation distribuĂ©e Ă base physique Ă l'Ă©chelle temporelle de la crue (moins de 2 jours) et Ă l'Ă©chelle spatiale de petits bassins versants mĂ©diterranĂ©ens (moins de 1 000 km2) apparaĂźt comme pertinente. Parmi les modĂšles dĂ©jĂ existants, rĂ©pondant au moins partiellement aux critĂšres requis et disponibles au moment de l'Ă©tude, TOPMODEL et HECHMS ont Ă©tĂ© mis en oeuvre. Les rĂ©sultats obtenus ne sont pas pleinement satisfaisants, soit car ces modĂšles nĂ©cessitent des chroniques pluie - dĂ©bit correspondant Ă ces Ă©vĂ©nements particuliers -chroniques rarement disponibles-, soit car la variabilitĂ© spatiale et les caractĂ©ristiques des processus physiques influents ne sont pas suffisamment prises en compte par ces modĂšles. Afin de palier ces inconvĂ©nients, un modĂšle (MARINE pour ModĂ©lisation pour l'Anticipation du Ruissellement et des Inondations pour des Ă©vĂ©Nements ExtrĂȘmes) a Ă©tĂ© conçu dans le cadre de cette thĂšse. Il s'agit d'un modĂšle pluie â dĂ©bit qui intĂšgre l'imagerie satellitale et les pluies issues du RADAR mĂ©tĂ©orologique et qui ne nĂ©cessite pas de longues chroniques pluie - dĂ©bit pour son calage. Afin que MARINE puisse ĂȘtre adaptĂ© d'une part aux Ă©chelles des donnĂ©es issues de l'imagerie satellitale et d'autre part Ă l'Ă©chelle objectif Ă laquelle les prĂ©visions doivent ĂȘtre rĂ©alisĂ©es, diffĂ©rentes modĂ©lisations hydrologiques ont dĂ» ĂȘtre mises en cascades pour franchir, par une mĂ©thode agrĂ©gative, ce saut d'Ă©chelles. Ce modĂšle a fourni des rĂ©sultats trĂšs encourageants sur la crue de l'Aude de novembre 1999 pour diffĂ©rents sous bassins et Ă l'Ă©chelle du bassin entier, mais aussi pour des crues survenues dans le dĂ©partement du Gard en 1995. Son implĂ©mentation en 2002 dans un service d'annonce des crues français (dans le cadre du projet PACTES) et sa future implantation opĂ©rationnelle au SCHAPI offrent de rĂ©elles applications temps rĂ©el de ce code
Benefits and limitations of data assimilation for discharge forecasting using an event-based rainfallârunoff model
Mediterranean catchments in southern France are threatened by potentially devastating fast floods which are difficult to anticipate. In order to improve the skill of rainfall-runoff models in predicting such flash floods, hydrologists use data assimilation techniques to provide real-time updates of the model using observational data. This approach seeks to reduce the uncertainties present in different components of the hydrological model (forcing, parameters or state variables) in order to minimize the error in simulated discharges. This article presents a data assimilation procedure, the best linear unbiased estimator (BLUE), used with the goal of improving the peak discharge predictions generated by an event-based hydrological model Soil Conservation Service lag and route (SCS-LR). For a given prediction date, selected model inputs are corrected by assimilating discharge data observed at the basin outlet. This study is conducted on the Lez Mediterranean basin in southern France. The key objectives of this article are (i) to select the parameter(s) which allow for the most efficient and reliable correction of the simulated discharges, (ii) to demonstrate the impact of the correction of the initial condition upon simulated discharges, and (iii) to identify and understand conditions in which this technique fails to improve the forecast skill. The correction of the initial moisture deficit of the soil reservoir proves to be the most efficient control parameter for adjusting the peak discharge. Using data assimilation, this correction leads to an average of 12% improvement in the flood peak magnitude forecast in 75% of cases. The investigation of the other 25% of cases points out a number of precautions for the appropriate use of this data assimilation procedure
Correcting the radar rainfall forcing of a hydrological model with data assimilation: application to flood forecasting in the Lez Catchment in Southern France
The present study explores the application of a data assimilation (DA) procedure to correct the radar rain- fall inputs of an event-based, distributed, parsimonious hy- drological model. An extended Kalman filter algorithm was built on top of a rainfall-runoff model in order to assimilate discharge observations at the catchment outlet. This work fo- cuses primarily on the uncertainty in the rainfall data and considers this as the principal source of error in the sim- ulated discharges, neglecting simplifications in the hydro- logical model structure and poor knowledge of catchment physics. The study site is the 114 km2 Lez catchment near Montpellier, France. This catchment is subject to heavy oro- graphic rainfall and characterised by a karstic geology, lead- ing to flash flooding events. The hydrological model uses a derived version of the SCS method, combined with a Lag and Route transfer function. Because the radar rainfall in- put to the model depends on geographical features and cloud structures, it is particularly uncertain and results in signifi- cant errors in the simulated discharges. This study seeks to demonstrate that a simple DA algorithm is capable of ren- dering radar rainfall suitable for hydrological forecasting. To test this hypothesis, the DA analysis was applied to estimate a constant hyetograph correction to each of 19 flood events. The analysis was carried in two different modes: by assimi- lating observations at all available time steps, referred to here as reanalysis mode, and by using only observations up to 3 h before the flood peak to mimic an operational environment, referred to as pseudo-forecast mode. In reanalysis mode, the resulting correction of the radar rainfall data was then com- pared to the mean field bias (MFB), a corrective coefficient determined using rain gauge measurements. It was shown that the radar rainfall corrected using DA leads to improved discharge simulations and Nash-Sutcliffe efficiency criteria compared to the MFB correction. In pseudo-forecast mode, the reduction of the uncertainty in the rainfall data leads to a reduction of the error in the simulated discharge, but un- certainty from the model parameterisation diminishes data assimilation efficiency. While the DA algorithm used is this study is effective in correcting uncertain radar rainfall, model uncertainty remains an important challenge for flood fore- casting within the Lez catchment
Impact environnemental de la diffusion de produits phytosanitaires par ruissellement
Lâagriculture moderne sâest appuyĂ©e sur lâutilisation croissante de produits phytosanitaires. Toutefois, la contamination des milieux comme les impacts sanitaires sont aujourdâhui avĂ©rĂ©s. Le transfert de ces produits vers les eaux de surface ou souterraines pourrait conduire Ă lâexposition de la population gĂ©nĂ©rale rĂ©sidant Ă proximitĂ© des zones agricoles. Notre objectif vise Ă dĂ©finir une mĂ©thodologie pour caractĂ©riser lâalĂ©a de pollution rĂ©sultant tant de lâĂ©pandage direct de produits phytosanitaires que de leur transfert par ruissellement au sein dâun bassin versant. Cette mĂ©thodologie est testĂ©e sur deux zones oĂč les prĂ©cipitations intenses provoquent dâimportants ruissellements, lâune en Languedoc-Roussillon et lâautre Ă lâextrĂȘme Nord du Cameroun. La premiĂšre phase consiste Ă quantifier la pression phytosanitaire au niveau des communes de la zone dâĂ©tude grĂące Ă lâIndicateur de FrĂ©quence des Traitements (IFT). Ensuite, une analyse hydrologique, qui sâappuie sur lâadaptation de la mĂ©thode cartographique IRIP (Indicateur de lâalĂ©a inondation par Ruissellement Intense Pluvial), permet de caractĂ©riser les zones propices Ă la production ou Ă lâaccumulation du ruissellement. Enfin, lâutilisation dâun SystĂšme dâInformation GĂ©ographique (SIG) permet, en recoupant lâIFT et ces zones ruisselantes, de dĂ©finir diffĂ©rents niveaux dâalĂ©a de pollution au sein du bassin versant. Cette Ă©tude devrait permettre dâaffiner la dĂ©finition de lâexposition humaine dans les analyses Ă©pidĂ©miologiques en population gĂ©nĂ©rale. Cependant, lâabsence de prise en compte de la propagation aĂ©rienne et souterraine de ces produits constitue une de ses principales limites. Enfin, elle pourrait ĂȘtre complĂ©tĂ©e par la prise en compte des transferts dâeau souterrains pour mieux Ă©valuer la pollution au sein de lâhydrosystĂšme.Modern agriculture is based on the increasing use of pesticides. Environmental contamination by pesticides as well as the health impacts are now proven. The transfer of these products into surface waters or groundwater could lead to exposure of the general population living near agricultural areas. The study aims to provide a useful methodology to characterize the pollution risk resulting from the direct application of plant protection products and their transfer by runoff within a watershed. This methodology is tested on two areas where intense rainfall are causing major runoff, one in Languedoc-Roussillon (France) and the other in the extreme north of Cameroon. The first step is to quantify the pest pressure within the municipalities in the studied areas with the Treatment Frequency Index (IFT). Then in a second time a hydrological analysis, based on the adaptation of the cartographic method IRIP (Indicator of the flood hazard by Intense Pluvial runoff), allows a characterization of areas suitable for the production or accumulation of runoff. Finally, the use of a Geographic Information System (GIS) allows, by matching the IFT and the streaming areas to define different levels of pollution hazard within the watershed. This study should refine the definition of human exposure in epidemiological analyzes in the general population. However, the lack of consideration of the air and underground propagation of these products is one of its major limitations. Finally, it could be supplemented by the inclusion of underground water transfers to better assess the pollution in the river system
Produits Phytosanitaires et Ruissellement : Méthodologie pour une caractérisation de l'impact environnemental.
International audienc
Evolution hydrogĂ©ologique de lâhydrosystĂšme karstique Cesse-Pouzols (Minervois, France) au cours du quaternaire
LâhydrosystĂšme karstique Cesse-Pouzols, en cours dâexploitation, se situe dans le Languedoc-Roussillon, dans le Minervois. La gĂ©ologie de la zone est complexe et a suscitĂ© un intĂ©rĂȘt pluridisciplinaire (gĂ©ologie, hydrogĂ©ologie et gĂ©omorphologie), sans pour autant apporter un schĂ©ma de fonctionnement hydrogĂ©ologique global. A partir de la rĂ©interprĂ©tation des travaux antĂ©rieurs guidĂ©e par de nouvelles observations en particulier sur les directions de fracturation et de drains karstiques, son Ă©volution gĂ©omorphologique et hydrogĂ©ologique au cours du Quaternaire a pu ĂȘtre Ă©tablie. Sur la base de cette Ă©volution complexe et polyphasĂ©e de lâhydrosystĂšme Cesse-Pouzols, un schĂ©ma de fonctionnement hydrogĂ©ologique cohĂ©rent avec son comportement hydrodynamique actuel est proposĂ©, afin dâamĂ©liorer les connaissances et la gestion de cet hydrosystĂšme.The Cesse-Pouzols karst hydrosystem, currently exploited, is located in the Languedoc-Roussillon in the Minervois area. The complex geology of this area arose a pluridisciplinary interest (geology, hydrogeology and geomorphology), without bringing a global hydrogeologic functioning. From the analysis between precedent works and new observations about the direction of fractures and karstic conduits, the geomorphology and hydrogeology evolution during Quaternary was performed. Based on this complex and polyphased evolution of the hydrosystem, a hydrogeologic functioning scheme is proposed, in order to improve the knowledge and the management of this hydrosystem
Evolution hydrogĂ©ologique de lâhydrosystĂšme karstique Cesse-Pouzols (Minervois, France) au cours du quaternaire
International audienc
Impact environnemental de la diffusion de produits phytosanitaires par ruissellement.: Deux exemples contrastés en Europe et en Afrique
International audienceModern agriculture is based on the increasing use of pesticides. Environmental contamination by pesticides as well as thehealth impacts are now proven. The transfer of these products into surface waters or groundwater could lead to exposure of thegeneral population living near agricultural areas. The study aims to provide a useful methodology to characterize the pollution riskresulting from the direct application of plant protection products and their transfer by runoff within a watershed. This methodologyis tested on two areas where intense rainfall are causing major runoff, one in Languedoc-Roussillon ( France) and the other in theextreme north of Cameroon. The first step is to quantify the pest pressure within the municipalities in the studied areas with theTreatment Frequency Index (IFT). Then in a second time a hydrological analysis, based on the adaptation of the cartographicmethod IRIP (Indicator of the flood hazard by Intense Pluvial runoff), allows a characterization of areas suitable for the productionor accumulation of runoff. Finally, the use of a Geographic Information System (GIS) allows, by matching the IFT and the streamingareas to define different levels of pollution hazard within the watershed. This study should refine the definition of human exposure inepidemiological analyzes in the general population. However, the lack of consideration of the air and underground propagationof these products is one of its major limitations. Finally, it could be supplemented by the inclusion of underground water transfers tobetter assess the pollution in the river system.Lâagriculture moderne sâest appuyĂ©e sur lâutilisation croissante de produits phytosanitaires. Toutefois, la contamination desmilieux comme les impacts sanitaires sont aujourdâhui avĂ©rĂ©s. Le transfert de ces produits vers les eaux de surface ou souterrainespourrait conduire Ă lâexposition de la population gĂ©nĂ©rale rĂ©sidant Ă proximitĂ© des zones agricoles. Notre objectif vise Ă dĂ©finir unemĂ©thodologie pour caractĂ©riser lâalĂ©a de pollution rĂ©sultant tant de lâĂ©pandage direct de produits phytosanitaires que de leurtransfert par ruissellement au sein dâun bassin versant. Cette mĂ©thodologie est testĂ©e sur deux zones oĂč les prĂ©cipitations intensesprovoquent dâimportants ruissellements, lâune en Languedoc-Roussillon et lâautre Ă lâextrĂȘme Nord du Cameroun. La premiĂšrephase consiste Ă quantifier la pression phytosanitaire au niveau des communes de la zone dâĂ©tude grĂące Ă lâIndicateur deFrĂ©quence des Traitements (IFT). Ensuite, une analyse hydrologique, qui sâappuie sur lâadaptation de la mĂ©thode cartographiqueIRIP (Indicateur de lâalĂ©a inondation par Ruissellement Intense Pluvial), permet de caractĂ©riser les zones propices Ă la productionou Ă lâaccumulation du ruissellement. Enfin, lâutilisation dâun SystĂšme dâInformation GĂ©ographique (SIG) permet, en recoupant lâIFTet ces zones ruisselantes, de dĂ©finir diffĂ©rents niveaux dâalĂ©a de pollution au sein du bassin versant. Cette Ă©tude devrait permettredâaffiner la dĂ©finition de lâexposition humaine dans les analyses Ă©pidĂ©miologiques en population gĂ©nĂ©rale. Cependant, lâabsencede prise en compte de la propagation aĂ©rienne et souterraine de ces produits constitue une de ses principales limites. Enfin,elle pourrait ĂȘtre complĂ©tĂ©e par la prise en compte des transferts dâeau souterrains pour mieux Ă©valuer la pollution au sein delâhydrosystĂšme
Conceptualization and classification of groundwaterâsurface water hydrodynamic interactions in karst watersheds: Case of the karst watershed of the Coulazou River (Southern France)
International audienceOver the last two decades, groundwater systems and surface water bodies are being more and more considered as a same entity which constitutes a single resource within the hydrologic system; understanding groundwater surface water (GWâSW) interactions is necessary to effectively manage the water resources. Concepts, methods of analysis and classifications of these hydrodynamic interactions have thus been developed, mostly in case of porous aquifer. Today, GWâSW interactions are still often poorly understood in many watersheds, particularly in the case of complex aquifers like karst aquifers.This study focuses on the influence of groundwater on the genesis and propagation of surface floods in the case of a highly karstified watershed. GWâSW hydrodynamic interactions are analysed (i) at a local scale focussing on hydrodynamic interactions between the karst aquifer in the vicinity of the river and the river, and (ii) at a larger scale focussing on hydrodynamic interactions between a karst spring and sinkholes in the riverbed. Hydrodynamic time series are described using the karst watershed of the Coulazou River (western Montpellier, France) as a case study. Results are used to adapt GWâSW classifications that are usually devoted to porous media. In case of the studied karst aquifer, it is shown that the initial groundwater level may be used to forecast the type of hydraulic connection between the river and the saturated zone during flood. However, initial water level in the karst aquifer does not give any information about the flow direction (gaining or losing stream) during a flood. The latter is found to be also controlled by the limited discharge capacity of karst conduits. A conceptual model of karst/river interactions during flood is given, with concepts that are derived from the analysis of the Coulazou watershed but that can be easily applied to most of the karst watersheds