Integrated hydrological modelling of a managed coastal Mediterranean wetland (Rhone delta, France): initial calibration

This paper presents a model of a heavily managed coastal Mediterranean wetland. The hydrosystem studied , called ``Ile de Camargue', is the central part of the Rhone river delta. It comprises flat agricultural drainage basins, marshes, and shallow brackish lagoons whose connection to the sea is managed. This hydrosystem is subject to strong natural hydrological variability due to the combination of a Mediterranean climate and the artificial hydrological regime imposed by flooded rice cultivation. To quantify the hydrological balance at different spatial and temporal scales, a simplified model is developed — including the basin and the lagoons — using a time step that enables the temporal dynamic to be reproduced that is adapted to data availability. This modelling task takes into account the functioning of the natural and anthropogenic components of the hydrosystem. A conceptual approach is used for modelling drainage from the catchment, using a GIS to estimate water input for rice irrigation. The lagoon system is modelled using a two-dimensional finite element hydrodynamic model. Simulated results from the hydrodynamic model run under various hydro-climatic forcing conditions (water level, wind speed and direction, sea connection) are used to calculate hydraulic exchanges between lagoon sub units considered as boxes. Finally, the HIC ('Hydrologie de l’Ile de Camargue') conceptual model is applied to simulate the water inputs and exchanges between the different units, together with the salt balance in the hydrosystem during a calibration period.</p> <p style='line-height: 20px;'><b>Keywords: </b>water management,conceptual hydrological model, hydrodynamic model, box model, GIS, Rhone delta, Camargue

Hydrological processes controlling flow generation in a small Mediterranean catchment under karstic influence

The hydrological behaviour of the River Vene (67 km(2), south of France) was assessed through a coupled approach based on field observations and modelling. The origins of runoff were identified by analysing discharge and water conductivity data. During rainfall events, natural areas did not contribute to runoff. In these areas, infiltration fed the karstic aquifer and contributed to spring discharges. Urban areas were characterized by a rather constant runoff coefficient whose value compares well with the extent of urbanisation and explains the total volume of small floods. Agricultural areas did not contribute directly to the flood peak, and subsurface flow in unsaturated areas was the major hydrological process during flood recession. A mathematical model, based on this "perceptual model", confirmed the hydrological processes identified and helped quantify the functioning thresholds in this intermittent river

Integrated hydrological modelling of a managed coastal Mediterranean wetland (Rhone delta, France): initial calibration

International audienceThis paper presents a model of a heavily managed coastal Mediterranean wetland. The hydrosystem studied , called ``Ile de Camargue", is the central part of the Rhone river delta. It comprises flat agricultural drainage basins, marshes, and shallow brackish lagoons whose connection to the sea is managed. This hydrosystem is subject to strong natural hydrological variability due to the combination of a Mediterranean climate and the artificial hydrological regime imposed by flooded rice cultivation. To quantify the hydrological balance at different spatial and temporal scales, a simplified model is developed — including the basin and the lagoons — using a time step that enables the temporal dynamic to be reproduced that is adapted to data availability. This modelling task takes into account the functioning of the natural and anthropogenic components of the hydrosystem. A conceptual approach is used for modelling drainage from the catchment, using a GIS to estimate water input for rice irrigation. The lagoon system is modelled using a two-dimensional finite element hydrodynamic model. Simulated results from the hydrodynamic model run under various hydro-climatic forcing conditions (water level, wind speed and direction, sea connection) are used to calculate hydraulic exchanges between lagoon sub units considered as boxes. Finally, the HIC ("Hydrologie de l'Ile de Camargue") conceptual model is applied to simulate the water inputs and exchanges between the different units, together with the salt balance in the hydrosystem during a calibration period. Keywords: water management,conceptual hydrological model, hydrodynamic model, box model, GIS, Rhone delta, Camargue

Detecting trend on ecological river status – how to deal with short incomplete bioindicator time series? Methodological and operational issues

Among the various parameters monitored in river monitoring networks, bioindicators provide very informative data. Analysing time variations in bioindicator data is tricky for water managers because the data sets are often short, irregular, and non-normally distributed. It is then a challenging methodological issue for scientists, as it is in Saône basin (30 000 km2, France) where, between 1998 and 2010, among 812 IBGN (French macroinvertebrate bioindicator) monitoring stations, only 71 time series have got more than 10 data values and were studied here. Combining various analytical tools (three parametric and non-parametric statistical tests plus a graphical analysis), 45 IBGN time series were classified as stationary and 26 as non-stationary (only one of which showing a degradation). Series from sampling stations located within the same hydroecoregion showed similar trends, while river size classes seemed to be non-significant to explain temporal trends. So, from a methodological point of view, combining statistical tests and graphical analysis is a relevant option when striving to improve trend detection. Moreover, it was possible to propose a way to summarise series in order to analyse links between ecological river quality indicators and land use stressors

Reperméabiliser les sols urbains : une démarche d’amélioration de la qualité de eaux pluviales qui tarde à se mettre en place à Montpellier ?

International audienceUrban pavements, one of the objectives of which is to evacuate rainwater as quickly as possible, have become, in a short time, the source of many problems. They generate large amounts of runoff that aggravate flooding and degrade the quality of surface water. They also contribute to rising temperatures in cities - as well as hindering biodiversity. Our article, which is based on interdisciplinarity, evokes the situation of the Metropolis of Montpellier (Southern France), which is experiencing one of the strongest demographic and urban growth in France. It raises the question of the impact of increasing construction on the quality of water in a small and fragile coastal catchment area by the Mediterranean. It also questions local public policies in an attempt to identify the brakes and levers on the depaving actions encouraged by certain government agencies - which promote alternative green techniques (and the return to pervious urban soils) for managing runoff more efficiently. To do so, it mobilises empirical material collected during a sociological survey - in addition to hydrological and chemical measurements. We conclude our reflection by suggesting ways to democratically think about the dismantling of modern coating infrastructures, to slow down urban water flows, and thus to improve the habitability of our cities.L’imperméabilisation des sols urbains, dont l'un des objectifs est d'évacuer au plus vite le ruissellement pluvial, est devenu, en peu de temps, la source de nombreux problèmes. Cela contribue à augmenter le volume ruisselé et par là même aggrave le risque d’inondation et dégrade la qualité des eaux de surface. Cette imperméabilisation, le plus souvent réalisé en bitume noir ou en béton gris, contribue également à la hausse des températures dans les villes - ainsi qu'à la baisse de la biodiversité. Notre article, qui s'appuie sur l'interdisciplinarité, évoque la situation de la Métropole de Montpellier (Sud de la France), qui connaît l'une des plus fortes croissances démographiques et urbaines de France. Elle pose la question de l'impact de l'augmentation des constructions sur la qualité de l'eau dans un bassin versant côtier petit et fragile du bord de la Méditerranée. Elle interroge également les politiques publiques locales pour tenter d'identifier les freins et les leviers aux actions de désimperméabilisation encouragées par les agences gouvernementales - qui promeuvent des techniques vertes alternatives (et le retour aux sols urbains perméables) pour mieux gérer les ruissellements. Pour ce faire, il mobilise du matériel empirique collecté lors d'une enquête sociologique - en plus de mesures hydrologiques et chimiques. Notre réflexion permet de suggérer des pistes pour penser démocratiquement le démantèlement des sols urbains imperméables, pour diminuer et freiner les ruissellements des zones urbaines, et ainsi améliorer à la fois l'habitabilité de nos villes et la qualité des eaux côtières

Modelling nitrogen loads at the catchment scale under the influence of land use.

Land use data are essential for water quality models. Pollutant inputs to streams are indeed a direct function of human activities that can be represented, at least approximately, in terms of land use. Remote sensing is a valuable data source to determine the land use on a catchment. However the land use data obtained by this kind of information are subject to significant uncertainties, including misclassification or categorical uncertainty. This paper presents a method to analyse the impact of the land use categorical uncertainty on the responses of a nitrogen load model at the outlet of a catchment. We use the POL model, a semi-distributed event-based model on a French Mediterranean rural catchment and we focus on agricultural land use. First, the sensitivity analysis realised by simulations considering a uniform land use on the catchment, shows a great sensitivity of the estimated load to the land use change. Second, the categorical land use uncertainty is analysed on a total nitrogen load prediction set calculated with randomly generated land use maps consistent with the confusion matrix that characterizes misclassification of land use. Thus, from 1% to 10% of misclassified agricultural area results in a variation of almost 40% on nitrogen loads for the three studied events. Misclassified areas explain from 46% to 75% of the variance of the estimated nitrogen load. These first results illustrate the importance of sensitivity and uncertainty analyses to improve the confidence of a water quality model and need to be extended to other input data sets

Utilisation et qualification d'un débitmètre à effet doppler (ADC) sur une rivière intermittente

Intermittent rivers pose different challenges to stream rating due to high spatial and temporal gradients. Long dry periods, cut by short duration flush flood events explain the difficulty to obtain reliable discharge data, for low flows as well as for floods: problems occur with standard gauging, zero flow period, etc. Our study aims to test the use of an acoustic Doppler current meter (ADC) for improving stream rating curves in small catchments subject to large variations of discharge, solid transport and high eutrophisation levels. The study is conducted at the outlet of the river Vette, a small coastal river (67 km2) located close to the city of Montpellier (France). The low flow period lasts for more than 6 month; during this period the river flow is sustained by effluents from urban sewage systems, which allows development of algae and macrophytes in the riverbed. The ADC device (Sontek ODArgonaut SW) is a pulsed Doppler current profiling system designed for measuring water velocity profiles and levels that are used to compute volumetric flow rates. It is designed for shallow waters (less than 4 meter depth). Its main advantages are its low cost and high accuracy (1% of the measured velocity or 0.05 m/sec, as reported by the manufacturer). The study evaluate the improvement in rating curves in an intermittent flow context and the effect of differences in sensitivity between low and high water level, by comparing mean flow velocity obtained by ADC to direct discharges measurements. The study also report long-term use of ADC device, by considering effects of biofilms, algae and macrophytes, as well as solid transport on the accuracy of the measurements. In conclusion, we show that the equipment is reliable and is able to provide additional information. However we show also that it is always difficult to measure low flow

Implication of two in-stream processes in the fate of nutrients discharged by sewage system into a temporary river

The aim of this study was to better understand the fate of nutrients discharged by sewage treatment plants into an intermittent Mediterranean river, during a low-flow period. Many pollutants stored in the riverbed during the low-flow period can be transferred to the downstream environments during flood events. The study focused on two processes that affect the fate and the transport of nutrients, a physical process (retention in the riverbed sediments) and a biological process (denitrification). A spatial campaign was carried out during a low-flow period to characterize the nutrient contents of both water and sediments in the VSne River. The results showed high nutrient concentrations in the water column downstream of the treated wastewater disposal (up to 13,315 mu g N/L for ammonium and 2,901 mu g P/L for total phosphorus). Nutrient concentrations decreased rapidly downstream of the disposal whereas nutrient contents in the sediments increased (up to 1,898 and 784 mu g/g for total phosphorus and Kjeldahl nitrogen, respectively). According to an in situ experiment using sediment boxes placed in the riverbed for 85 days, we estimated that the proportion of nutrients trapped in the sediments represents 25% (respectively 10%) of phosphorus (respectively nitrogen) loads lost from the water column. In parallel, laboratory tests indicated that denitrification occurred in the VSne River, and we estimated that denitrification likely coupled to nitrification processes during the 85 days of the experiment was significantly involved in the removal of nitrogen loads (up to 38%) from the water column and was greater than accumulation processes