An integrated approach for urban water modelling, linking a watershed hydrological model and a cyanobacteria dynamics model in urban lakes

International audienceIn the future, the frequency and the intensity of cyanobacteria blooms in urban lakes are expected to increase in response to climate change and expanding urbanization. In order to study the impacts of watershed changes on cyanobacteria dynamics in urban lakes, a modelling approach, in which an ecological lake model is connected to a hydrological watershed model, is proposed. To validate this approach, the water quality (temperature, transparency, dissolved oxygen and chlorophyll-a) was monitored at high frequency (5-30 min) on two study sites in contrasted regions (climate, land-use...). In the first part of this paper, we show the advantage of using high-frequency data to simulate cyanobacteria dynamics in Lake Enghien (France), a temperate urban lake. In a second part, the methodology used, to link the ecological lake model and the hydrological watershed model is explained for Lake Pampulha (Brazil), a tropical urban lake. Some preliminary results of the ecological modelling of Lake Pampulha are also presented. The integrated modelling approach proposed, will allow us to study the lake response to different future cenarios of the watershed evolution. Furthermore, high-frequency data are expected to provide a better understanding of the lake functioning during extreme meteorological conditions (e.g. heavy rainfall events or drought)

Empirical modelling of stream nutrients for countries without robust water quality monitoring systems

Water quality models are useful tools to understand and mitigate eutrophication processes. However, gaining access to high-resolution data and fitting models to local conditions can interfere with their implementation. This paper analyzes whether it is possible to create a spatial model of nutrient water level at a local scale that is applicable in different geophysical and land-use conditions. The total nitrogen and phosphorus concentrations were modeled by integrating Geographical Information Systems, Remote Sensing, and Generalized Additive and Land-Use Changes Modeling. The research was based on two case studies, which included 204 drainage basins, with nutrient and limnological data collected during two seasons. The models performed well under local conditions, with small errors calculated from the independent samples. The recorded and predicted concentrations of nutrients indicated a significant risk of water eutrophication in both areas, showing the impact of agricultural intensification and population growth on water quality. The models are a contribution to the sustainable land-use planning process, which can help to prevent or promote land-use transformation and new practices in agricultural production and urban design. The ability to implement models using secondary information, which is easily collected at a low cost, is the most remarkable feature of this approach.Universidad de la República. Comisión Sectorial de Investigación CientíficaAgencia Nacional de Investigación e InnovaciónPrograma de Desarrollo de las Ciencias Básicas - Geociencia

MODELING AND MANAGEMENT OF WATER QUANTITY AND QUALITY IN COLD-CLIMATE PRAIRIE WATERSHEDS

Saskatchewan’s surface and ground water sources are vital to life in the province, not only as the supply of safe drinking water for the residents, but also as a key driver of economic activity. The Qu'Appelle and Assiniboine River Basins are among the highly valued water resources in the province as they supply water for more than one-third of the population of Saskatchewan and contain a chain of eight lakes that are major recreational and economically valued resources in the region. The health of several watersheds within these highly valued river basins is being degraded by intensive agricultural and other developmental activities. The decision making processes for sustainable water management in these watersheds is stunted by limited observed field data. As a result, for Saskatchewan watersheds in general, and the Qu’Appelle and Assiniboine River Basins in particular, a better understanding is required of the type, extent and sources of pollutant loadings, and effects of potential alternative management practices may have to mitigate water quality problems. Modeling approaches that have the capacity to analyze the quantity and quality of water resources, identify existing and potential watershed stressors, and the relative importance of best management options are therefore needed. With the intention of helping decision makers in the province, this thesis focuses on developing an eco-hydrological model, which is suitable for Canadian prairie watersheds and capable of simulating the long term effects of management practices. Following a review of several models, the Soil and Water Assessment Tool (SWAT) has been selected for this study. In order achieve the objectives, the SWAT model has been modified to suit site specific characteristics of the Canadian prairies. The first such modification was to incorporate the numerous landscape depressions that vary in storage capacity into SWAT. This was done by representing depression storage heterogeneity using a probability distribution using an algorithm called “Probability Distributed Landscape Depressions (PDLD)”. The modified model, called SWAT-PDLD, was tested over two prairie watersheds: the Assiniboine and Moose Jaw watersheds. An improved simulation for streamflow was achieved for both case study watersheds as compared to the original SWAT lumped storage approach. The other modification to SWAT was the incorporation of seasonally varying soil erodibility due to the cold climate conditions. This was done using a sediment module with a time variant soil erodibility factor that allows the value of soil erodibility to vary between seasons. The modified SWAT-PDLD along with seasonally varying soil erodibility was tested for sediment export simulation for the same two case study watersheds: the Assiniboine and Moose Jaw watersheds. Results show an improved sediment simulation for both case study watersheds when seasonally varying soil erodibility factors are considered as compared to the original SWAT model sediment module, which uses annual values of soil erodibility. The modified model was also used to simulate phosphorous and nitrogen export from the Assiniboine watershed and a satisfactorily model performance was obtained. In addition, the developed model was used to assess the impacts of three different management practices on the export of pollutants for the Assiniboine watershed. The scenarios considered were conservation tillage, a cover crop, and filter strips. Model results show that both the filter strips and cover crops decreased sediment, phosphorous, and nitrogen export, while conservation tillage increased phosphorous export in the study watershed. Finally, the study investigated the different sources of modeling uncertainty for the developed model. Parameter as well as precipitation, observed discharge, and model structure uncertainty of the SWAT-PDLD model was evaluated. Parameter uncertainty was quantified using three different techniques that include GLUE, ParaSol, and SUFI-2. Model structure uncertainty was assessed using a framework that combines the Bayesian Model Averaging (BMA) and Shuffled Complex Evolution (SCE). Results suggest that ignoring either input error or model structure uncertainty will lead to unrealistic model simulations and incorrect uncertainty bounds. The study also shows that prediction uncertainty bounds, posterior parameter distribution, and final parameter values vary between methods

Modelling non-point source pollution of rivers in the UK and Colombia

Mathematical fate models have been developed and validated to simulate the transport of contaminants in temperate regions but little is known about their applicability in the tropics. Different models were applied to simulate brominated flame retardants in Colombia and the UK and to identify differences in model application and drivers of emissions in both regions. Emissions of decabromodiphenyl ether (decaBDE) in Colombia and the UK were estimated and suggested large releases to wastewater textile back-coating and waste management stages. Emission data were used to study the partitioning of the flame retardant with a fugacity approach. Fugacity results from Colombia were in agreement with sediment concentrations from the literature for the outlet of the River Magdalena. GREAT-ER was also applied to simulate decaBDE emissions in the Calder catchment; the model showed good potential for the simulation of the flame retardant. Monitoring of polybrominated diphenyl ethers (PBDEs) in sediments in the Calder showed that decaBDE represented the vast majority of PBDEs analysed (>90%) with increasing concentrations moving downstream. A modelling framework with field-scale models using MACRO was developed to simulate transport of six contrasting herbicides targeted by a management programme in the Wensum catchment in eastern England. The catchment-scale model SPIDER was also used for comparison. Preferential flow was the main driver of pesticide transport to water. A fairly good simulation of the flow was achieved (model efficiency, E = 0.6 for MACRO and 0.4 for SPIDER) but variability in pesticide simulations was observed due to uncertainties in input parameters. In-stream processes had little effect on pesticide simulations from either model. Modelling showed that most of the observed reductions in pesticide transport to the river (ca. 80% decrease between 2006 and 2011) can be explained by changes in weather and flow in the catchment during the study period, but an influence on management practices cannot be excluded. AnnAGNPs was applied to simulate triazine loss to the River Cauca from sugarcane, maize and sorghum in the Cauca Valley of Colombia. Runoff was the main driver of pesticide emissions to water. Satisfactory simulation and validation of the hydrology was achieved after little calibration (E = 0.7). A fairly good simulation of pesticides was generally achieved, but some patterns in the measured data could not be simulated. Use of grab samples resulted in uncertainty in measured concentrations. Implementing best management practices was predicted to result in a 78% reduction in triazine losses, whilst replacing triazine herbicides resulted in an 87% reduction when expressed as a proportion of the total pesticide applied. Uncertainty analyses of sensitive input parameters were carried out for the applied models. Their impact on simulations was chemical- and situation-specific. Recommendations for future research are provided to improve modelling of chemical fate in contrasting situations