Papers by Vasileios Christelis
Hydrological Sciences Journal
Water Resources Management
Variable-fidelity modelling has been utilized in several engineering optimization studies to cons... more Variable-fidelity modelling has been utilized in several engineering optimization studies to construct surrogate models. However, similar approaches have received much less attention in coastal aquifer management problems. A variable-fidelity optimization framework was developed utilizing a lower-fidelity and computationally cheap model of seawater intrusion, based on the sharp interface assumption, and a simple correction process. The variable-fidelity method was compared to the direct optimization with the high-fidelity variable density and salt transport model and to conventional surrogate-based optimization. The surrogate-based approaches were embedded into the operations of an evolutionary algorithm to implement an efficient online update of the surrogate models and control the feasibility of the optimal solutions. Multiple independent optimization runs were performed to provide more insightful comparison outcomes. Although the variable-fidelity method found a better optimum than the conventional approach, the overall sample statistics showed that the surrogate-based optimization frameworks performed equally well and provided good approximations to the high-fidelity solution. Despite the potential for an improved exploration of the optimal search space by using the variable-fidelity method, the conventional approach had a 30% faster average convergence time.
Environmental Modelling & Software
Highlights 1) Distributed groundwater models were linked together with a river model via the Open... more Highlights 1) Distributed groundwater models were linked together with a river model via the OpenMI software platform. 2) Kriging metamodels were applied to facilitate analysis with the integrated models. 3) The overall computational savings were in the range of 70-90%. 4) Monte Carlo simulations demonstrate that metamodels were in good agreement with the responses of the integrated models. 5) Sensitivity Analysis using the metamodels accurately identified the important parameters.
Journal of Hydroinformatics
The computationally expensive variable density and salt transport numerical models hinder the imp... more The computationally expensive variable density and salt transport numerical models hinder the implementation of simulation-optimization routines for coastal aquifer management. To reduce the computational cost, surrogate models have been utilized in pumping optimization of coastal aquifers. However, it has not been previously addressed whether surrogate modelling is effective given a limited number of numerical simulations with the seawater intrusion model. To that end, two surrogate-based optimization (SBO) frameworks are employed and compared against the direct optimization approach, under restricted computational budgets. The first, a surrogate-assisted algorithm, employs a strategy which aims at a fast local improvement of the surrogate model around optimal values. The other, balances global and local improvement of the surrogate model and is applied for the first time in coastal aquifer management. The performance of the algorithms is investigated for optimization problems of m...
Journal of Arid Environments, 2017
Water Resources Management, 2016
Water Resources Management, 2015
Conceptual hydrological models have been widely used as efficient tools for simulating the hydrol... more Conceptual hydrological models have been widely used as efficient tools for simulating the hydrological processes at river basin scale. Medbasin-D is a daily conceptual hydrological model included in the Medbasin software package. The model is oriented towards applications under the conditions of the Mediterranean. For a reliable prediction of streamflow, a successful calibration of the fourteen model parameters is required, based on measured streamflow data. Medbasin software includes a generic semi-automatic iterative calibration routine, based on the minimisation of a selected objective function. This procedure is time consuming, requires subjective decisions by the user and it is considered inefficient for exploring the optimal search space. It is the aim of this study to improve this calibration procedure by including a batch-processing module, to enable the implementation of coupled simulation-optimisation routines and the capability to interact with other widely used software packages. The enhanced Medbasin calibration module is presented in this paper along with a demonstration of the model calibration in a real world case study in the island of Crete (Greece). The Shuffled Complex Evolution (SCE) and the Genetic algorithms (GA) are two widely known global optimisation approaches that are tested with the Medbasin-D model. Several objective functions are used to test the performance of the calibration process under various hydrological conditions. Due to the stochastic nature of the global optimisers used for the calibration, a series of optimisation replicates are produced for assessing the statistics of each calibration procedure. Finally, the performance of the calibrated model over the validation period is assessed and discussed. Based on the case analysed, the results reveal that the SCE is more robust than the GA for all tested objective functions.
The problem of pumping optimization of coastal unconfined aquifers is investigated. The Sequentia... more The problem of pumping optimization of coastal unconfined aquifers is investigated. The Sequential Quadratic Programming (SQP) optimization method is combined with a numerical flow model based on the sharp interface assumption and Ghyben-Herzberg approximation. The objective is to maximize the total pumping rate, considering aquifer spatial variability and drought conditions due to climate changes, while protecting the aquifer from seawater intrusion. Using Monte-Carlo simulation, optimal pumping rates were calculated for different statistical properties of hydraulic conductivity and for various drought recharge scenarios. The results indicate that the maximum allowed pumping rates are significantly affected by the variance and correlation length of hydraulic conductivity random fields while recharge reduction leads to a significant reduction of maximum pumping.
Environmental Processes, 2016
Coastal aquifer management often involves aquifers of finite dimensions where optimal pumping rat... more Coastal aquifer management often involves aquifers of finite dimensions where optimal pumping rates must be calculated through a combined simulation-optimization procedure. Variable-density numerical models are considered more exact than sharp interface models as they better describe the governing flow and transport equations. However, such models are not always preferable in pumping optimization studies, due to their complexity and computational burden. On the other hand, sharp interface models are approximate and can lead to large errors if they are not applied properly, particularly when model boundaries are not treated correctly.
The problem of pumping optimization of coastal unconfined aquifers is investigated. The Sequentia... more The problem of pumping optimization of coastal unconfined aquifers is investigated. The Sequential Quadratic Programming (SQP) optimization method is combined with a numerical flow model based on the sharp interface assumption and Ghyben-Herzberg approximation. The objective is to maximize the total pumping rate, considering aquifer spatial variability and drought conditions due to climate changes, while protecting the aquifer from seawater intrusion. Using Monte-Carlo simulation, optimal pumping rates were calculated for different statistical properties of hydraulic conductivity and for various drought recharge scenarios. The results indicate that the maximum allowed pumping rates are significantly affected by the variance and correlation length of hydraulic conductivity random fields while recharge reduction leads to a significant reduction of maximum pumping.
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Papers by Vasileios Christelis