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Universidade do Minho (2011)

Modelling of water quality in the Alqueva Reservoir, Portugal

Fontes, Cláudia Lindim

Titre : Modelling of water quality in the Alqueva Reservoir, Portugal

Auteur : Fontes, Cláudia Lindim

Université de soutenance : Universidade do Minho

Grade : Doutoramento em Engenharia Civil 2011

Eutrophication is a serious environmental problem in lakes and reservoirs worldwide. The eutrophic Alqueva reservoir (Portugal) is the largest western European reservoir and constitutes a vital regional water resource. The river Guadiana, which is the main tributary to the Alqueva reservoir, imports large nutrient loads leading to eutrophication being an issue in this waterbody. But despite its importance and problems, few scientific studies concerning the Alqueva exist. This work aims at contributing to foster the understanding of water ecology in the Alqueva reservoir through the use of data analysis techniques and numerical modelling. The results contained herein can be used to assist management decisions in this waterbody and the modelling effort can be used to obtain forecasts in this and other reservoirs and improve understanding of ecological behavior there. Data analysis methods, namely time series analysis, were applied to monitoring data collected between 2003-2009 at several locations and depths in the Alqueva reservoir in order to infer possible spatial and temporal patterns. The monitoring data comprised climatology, hydrology and water quality data from different sources. Data analysis showed that the Alqueva behavior presents high interannual variability. This is mainly a consequence of the high variability of precipitation, nutrient loads and Guadiana hydrological regimen. It was found that the system is P-limited and that nutrients input is mostly dependent on the main tributary input loads. Therefore management schemes aimed at improving the trophic level in the Alqueva should focus on reducing phosphorus in the Guadiana inflow. Numerical modelling main goals were to develop and apply tools to simulate the main ecological traits in the Alqueva reservoir. A new numerical model to simulate eutrophication processes in lakes and reservoirs based on a hybrid deterministic-stochastic approach was developed. A plain methodology to estimate nutrient loads in a basin was also developed. These tools were used together with the finite element based hydrodynamic model RMA-10 to perform simulations of currents, thermal structure and eutrophication in the Alqueva reservoir. The models were successfully calibrated and validated in both two-dimensional and three-dimensional versions. Model performance was assessed by comparing simulation results with in situ measured data. It was found that the models reproduced Alqueva thermal structure quite accurately and eutrophication related trends reasonably well. The performance of the eutrophication tool was constrained by the availability and quality of input and forcing data. It was shown that the particular geomorphological and hydrological characteristics of the reservoir together with local climate features are responsible for the existence of two distinct ecological regions within the reservoir whose boundary can be placed at a transect south of monitoring station 3 : - The upper part of the reservoir is a shallow channel like region with riverine characteristics that is interrupted by a few scattered deeper pools. This area receives the major nutrients input and is eutrophic. Hydrodynamic in this area is governed by hydraulically induced currents from the Guadiana. - The lower part of the reservoir is a deep lacustrine area that presents stable thermal and oxygen stratification in summer (April – October). In this region, wind induced currents and thermal stratification are the dominant hydrodynamic traits. Wind is dominant over hydraulic flow during all year and affects mostly the surface circulation. Model results indicate that the velocities in the reservoir are always smaller than 0.25 m/s, with the higher values occurring in the upper reservoir area. The Alqueva was found to present a monomictic behavior with a seasonal summer stratification that is responsible for generating anoxic bottom waters during dry season. An autumnal overturn leads to a fully mixed water column in winter season. Model findings, corroborated by data, indicate that phytoplankton in the Alqueva peaks in spring and in autumn with productivity in the upper reservoir area presenting values much higher than in the lower part. Dry season ecology seems to be ruled by stratification while wet season ecology main driving force appears to be the nutrients load through the main tributary. The Alqueva was build to boost economic development in the region and provide irrigation water for agriculture activities. It can be inferred from the results of the present work that the major problem it may face is the impacts of the poor water quality coming from the Guadiana. The Guadiana river carries wastewater with a high level of nutrients from a large Spanish population, from industries and from agriculture activities. The development of effective water quality management in this reservoir should therefore focus on nutrient containment strategies for the Guadiana river inflow.

Mots clés : Alqueva reservoir ; Modeling ; Eutrophication ; Alqueva reservoir thermal stratification


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Page publiée le 23 octobre 2013, mise à jour le 12 juin 2017