George P Karatzas | Technical University of Crete (original) (raw)
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Papers by George P Karatzas
Mathematical Geosciences, 2024
Groundwater resources in Mediterranean coastal aquifers are under several threats including saltw... more Groundwater resources in Mediterranean coastal aquifers are under several threats including saltwater intrusion. This situation is exacerbated by the absence of sustainable management plans for groundwater resources. Management and monitoring of groundwater systems require an integrated approach and the joint interpretation of any available information. This work investigates how uncertainty can be integrated within the geo-modelling workflow when creating numerical three-dimensional aquifer models with electrical resistivity borehole logs, geostatistical simulation and Bayesian model averaging. Multiple geological scenarios of electrical resistivity are created with geostatistical simulation by removing one borehole at a time from the set of available boreholes. To account for the spatial uncertainty simultaneously reflected by the multiple geostatistical scenarios, Bayesian model averaging is used to combine the probability distribution functions of each scenario into a global one, thus providing
The purpose of this study was to model the Koiliaris River flow where flood phenomena appear from... more The purpose of this study was to model the Koiliaris River flow where flood phenomena appear from time to time. The Koiliaris River basin is located east of the City of Chania, Crete, (Greece). The basin is extended from the White Mountains (Lefka Ori) to the coastline. The geology of the basin is mainly constituted by carbonate (Karstic area), quaternary-neogenic deposits and flysch formation. The main volume of water is discharged from the karstic system of the White Mountains (Lefka Ori) through springs and temporary rivers. In order to calculate the flow from the springs the Maillet Karstic model is used. The main volume of water that is discharged to the springs through the karstic system comes from snow melt. In order to determine with high precision the rate of snow melt, a Snow melt model and the applications of GIS are combined. Using this approach the discharge from springs (for the time of the simulation) is determined. The time-series of the karstic discharge from the springs is entered in the code of the Hydrological Simulation Program-FORTRAN (HSPF) model, in order to calculate both the surface and subsurface discharge. The HSPF is a set of computer codes that simulate the hydrologic process. The hydrologic model of HSPF functions in the frame of the BASINS 4 model. The final step is the Calibration and the Sensitivity analysis of the HSPF model. The main objective of the present study is to develop a tool for the prediction and management of flood events that occur in the area.
Zenodo (CERN European Organization for Nuclear Research), Apr 30, 2021
Zenodo (CERN European Organization for Nuclear Research), Nov 24, 2020
Zenodo (CERN European Organization for Nuclear Research), Apr 30, 2021
<p&amp... more <p>In recent years, Artificial Neural Networks (ANNs) have proven their merit in being able to simulate the changes in groundwater levels, using as inputs other parameters of the water budget, e.g. precipitation, temperature, etc.. In this study, ANNs have been used to simulate hydraulic head in a large number of wells throughout the Danube River Basin, taking as inputs, precipitation, temperature, and evapotranspiration data in the region. Different ANN architectures have been examined, to minimize the simulation error of the testing data-set. Among the different training algorithms, Levenberg-Marquardt and Bayesian Regularization are used to train the ANNs, while the different activation functions of the neurons that were deployed include tangent sigmoid, logarithmic sigmoid and linear. The initial application comprised of data from 128 wells between 1 January 2000 and 31 October 2014. The best performance was achieved by the algorithm Bayesian Regularization with a error of the order  based on all observation wells. A second application, compared the results of the first one, with the results of an ANN used to simulate a single well. The pros and cons of the two approaches, and the synergies of using both of them is further discussed in order to distinguish the differences, and guide researchers in the field for further applications.</p>
Zenodo (CERN European Organization for Nuclear Research), May 31, 2021
EGU General Assembly Conference Abstracts, Apr 1, 2019
EGU General Assembly Conference Abstracts, May 1, 2014
EGU General Assembly Conference Abstracts, Apr 1, 2009
Water, Feb 13, 2023
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The present research work uses Residual Kriging to estimate the groundwater level of an unconfine... more The present research work uses Residual Kriging to estimate the groundwater level of an unconfined alluvial aquifer, as well as the trend function. The ground surface elevation is used as auxiliary variable in the trend model. Indicator Kriging is applied to detect potential vulnerable locations. Classical variogram functions are applied to determine the spatial correlation of the measurements. The risk of hydraulic head to lie below a threshold value is significant, mainly at the South and North parts of the aquifer, where the lower values of groundwater level are estimated, indicating that these areas require intense monitoring to ensure the water resources availability.
Zenodo (CERN European Organization for Nuclear Research), Jun 2, 2022
Zenodo (CERN European Organization for Nuclear Research), Sep 9, 2022
The objective of this study is to provide an overview of local climate change over the Mediterran... more The objective of this study is to provide an overview of local climate change over the Mediterranean (MED) area under the scope of the InTheMED project, EU funded in the framework of the PRIMA programme. Future precipitation and temperature projections are assessed until the end of this century for five different pilot sites, located in the MED region. To this end, the outputs of 17 Regional Climate Models under the RCP4.5 and RCP8.5 scenarios are used. For each pilot site, the raw climate model data were downscaled at each monitoring station location and bias-corrected on the basis of observations recorded in a 30-year historical period. The changes in the annual precipitation are heterogeneous across the five pilot sites: a negligible variation is expected for some areas and a decrease of up to 30% for others. On the contrary, a significant increase in temperature is expected for all sites, confirming the ongoing warming in the MED region.
Journal of Hydrology, 2019
Soil Science, 2016
Acheloos is the second longest and the largest, in terms of discharge, native river in Greece, su... more Acheloos is the second longest and the largest, in terms of discharge, native river in Greece, supplying three hydroelectric dams along its route. The Kremasta dam, which forms the largest artificial lake in Greece, is the first dam fed by Acheloos and two other rivers. Sediment accumulation in such large reservoirs is of major concern, as it reduces storage capacity and hydropower production. In this study, the reservoir is utilized as a live record of constantly renewed sediment deposition data, which is used as a means of assessing the accumulated sediment loads originating from the watershed. The Soil and Water Assessment Tool model was combined with data from previous field surveys, which estimated the volume of deposited sediments in the reservoir. After modeling the discharge of three rivers into the Kremasta reservoir using available monthly field data from 1965 to 2001, the Soil and Water Assessment Tool was successfully calibrated against the deposited sediment mass data, accumulated at the reservoir during the first 34 years of its operation. Simulation results for a set of International Panel on Climate Change "A1B" climate change scenarios suggested a 14.6% decrease in the average rainfall on the watershed, between the 2016-2055 and 2056-2095 time periods, which induces a proportional (19.5%) decrease in flow and a milder (7.9%) decrease in the deposited sediment mass. It was also estimated that by the year 2100 the deposited sediment volume will occupy 6.1% of the effective volume of the reservoir, a value much lower than the estimated dead volume of the dam (17.4%).
Hydrological Sciences Journal-journal Des Sciences Hydrologiques, Mar 30, 2011
This paper presents a viable approach for flood management strategy in a river basin based on the... more This paper presents a viable approach for flood management strategy in a river basin based on the European Floods Directive. A reliable flood management plan has two components: (a) a proper flood management strategy, and (b) the determination of the flood-hazard areas. A method to evaluate the benefits of a flood warning system is presented herein, as well as a
AGUFM, Dec 1, 2001
The cost-effective remediation designs that involve removal of contaminants from the subsurface g... more The cost-effective remediation designs that involve removal of contaminants from the subsurface given risk-based constraints and the requirement of contaminant-plume containment can be addressed via a new multi-stage dynamic approach using pump-and-treat systems. In this approach, the remedial design is modified from time to time in order to more effectively employ the available pumping wells. The duration of each period and the pumping rates at the various target wells are considered interactively in determining the least-cost design. At each stage, the pumping rates are modified to accommodate the dynamic nature of the plume. In response to the changing plume geometry, the well discharges may increase or decrease in order to produce the most cost-effective design that satisfies the specified constraints active during the overall design horizon. The utilization of this approach at a field location illustrates the effectiveness of the proposed strategy.
Mathematical Geosciences, 2024
Groundwater resources in Mediterranean coastal aquifers are under several threats including saltw... more Groundwater resources in Mediterranean coastal aquifers are under several threats including saltwater intrusion. This situation is exacerbated by the absence of sustainable management plans for groundwater resources. Management and monitoring of groundwater systems require an integrated approach and the joint interpretation of any available information. This work investigates how uncertainty can be integrated within the geo-modelling workflow when creating numerical three-dimensional aquifer models with electrical resistivity borehole logs, geostatistical simulation and Bayesian model averaging. Multiple geological scenarios of electrical resistivity are created with geostatistical simulation by removing one borehole at a time from the set of available boreholes. To account for the spatial uncertainty simultaneously reflected by the multiple geostatistical scenarios, Bayesian model averaging is used to combine the probability distribution functions of each scenario into a global one, thus providing
The purpose of this study was to model the Koiliaris River flow where flood phenomena appear from... more The purpose of this study was to model the Koiliaris River flow where flood phenomena appear from time to time. The Koiliaris River basin is located east of the City of Chania, Crete, (Greece). The basin is extended from the White Mountains (Lefka Ori) to the coastline. The geology of the basin is mainly constituted by carbonate (Karstic area), quaternary-neogenic deposits and flysch formation. The main volume of water is discharged from the karstic system of the White Mountains (Lefka Ori) through springs and temporary rivers. In order to calculate the flow from the springs the Maillet Karstic model is used. The main volume of water that is discharged to the springs through the karstic system comes from snow melt. In order to determine with high precision the rate of snow melt, a Snow melt model and the applications of GIS are combined. Using this approach the discharge from springs (for the time of the simulation) is determined. The time-series of the karstic discharge from the springs is entered in the code of the Hydrological Simulation Program-FORTRAN (HSPF) model, in order to calculate both the surface and subsurface discharge. The HSPF is a set of computer codes that simulate the hydrologic process. The hydrologic model of HSPF functions in the frame of the BASINS 4 model. The final step is the Calibration and the Sensitivity analysis of the HSPF model. The main objective of the present study is to develop a tool for the prediction and management of flood events that occur in the area.
Zenodo (CERN European Organization for Nuclear Research), Apr 30, 2021
Zenodo (CERN European Organization for Nuclear Research), Nov 24, 2020
Zenodo (CERN European Organization for Nuclear Research), Apr 30, 2021
<p&amp... more <p>In recent years, Artificial Neural Networks (ANNs) have proven their merit in being able to simulate the changes in groundwater levels, using as inputs other parameters of the water budget, e.g. precipitation, temperature, etc.. In this study, ANNs have been used to simulate hydraulic head in a large number of wells throughout the Danube River Basin, taking as inputs, precipitation, temperature, and evapotranspiration data in the region. Different ANN architectures have been examined, to minimize the simulation error of the testing data-set. Among the different training algorithms, Levenberg-Marquardt and Bayesian Regularization are used to train the ANNs, while the different activation functions of the neurons that were deployed include tangent sigmoid, logarithmic sigmoid and linear. The initial application comprised of data from 128 wells between 1 January 2000 and 31 October 2014. The best performance was achieved by the algorithm Bayesian Regularization with a error of the order  based on all observation wells. A second application, compared the results of the first one, with the results of an ANN used to simulate a single well. The pros and cons of the two approaches, and the synergies of using both of them is further discussed in order to distinguish the differences, and guide researchers in the field for further applications.</p>
Zenodo (CERN European Organization for Nuclear Research), May 31, 2021
EGU General Assembly Conference Abstracts, Apr 1, 2019
EGU General Assembly Conference Abstracts, May 1, 2014
EGU General Assembly Conference Abstracts, Apr 1, 2009
Water, Feb 13, 2023
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The present research work uses Residual Kriging to estimate the groundwater level of an unconfine... more The present research work uses Residual Kriging to estimate the groundwater level of an unconfined alluvial aquifer, as well as the trend function. The ground surface elevation is used as auxiliary variable in the trend model. Indicator Kriging is applied to detect potential vulnerable locations. Classical variogram functions are applied to determine the spatial correlation of the measurements. The risk of hydraulic head to lie below a threshold value is significant, mainly at the South and North parts of the aquifer, where the lower values of groundwater level are estimated, indicating that these areas require intense monitoring to ensure the water resources availability.
Zenodo (CERN European Organization for Nuclear Research), Jun 2, 2022
Zenodo (CERN European Organization for Nuclear Research), Sep 9, 2022
The objective of this study is to provide an overview of local climate change over the Mediterran... more The objective of this study is to provide an overview of local climate change over the Mediterranean (MED) area under the scope of the InTheMED project, EU funded in the framework of the PRIMA programme. Future precipitation and temperature projections are assessed until the end of this century for five different pilot sites, located in the MED region. To this end, the outputs of 17 Regional Climate Models under the RCP4.5 and RCP8.5 scenarios are used. For each pilot site, the raw climate model data were downscaled at each monitoring station location and bias-corrected on the basis of observations recorded in a 30-year historical period. The changes in the annual precipitation are heterogeneous across the five pilot sites: a negligible variation is expected for some areas and a decrease of up to 30% for others. On the contrary, a significant increase in temperature is expected for all sites, confirming the ongoing warming in the MED region.
Journal of Hydrology, 2019
Soil Science, 2016
Acheloos is the second longest and the largest, in terms of discharge, native river in Greece, su... more Acheloos is the second longest and the largest, in terms of discharge, native river in Greece, supplying three hydroelectric dams along its route. The Kremasta dam, which forms the largest artificial lake in Greece, is the first dam fed by Acheloos and two other rivers. Sediment accumulation in such large reservoirs is of major concern, as it reduces storage capacity and hydropower production. In this study, the reservoir is utilized as a live record of constantly renewed sediment deposition data, which is used as a means of assessing the accumulated sediment loads originating from the watershed. The Soil and Water Assessment Tool model was combined with data from previous field surveys, which estimated the volume of deposited sediments in the reservoir. After modeling the discharge of three rivers into the Kremasta reservoir using available monthly field data from 1965 to 2001, the Soil and Water Assessment Tool was successfully calibrated against the deposited sediment mass data, accumulated at the reservoir during the first 34 years of its operation. Simulation results for a set of International Panel on Climate Change "A1B" climate change scenarios suggested a 14.6% decrease in the average rainfall on the watershed, between the 2016-2055 and 2056-2095 time periods, which induces a proportional (19.5%) decrease in flow and a milder (7.9%) decrease in the deposited sediment mass. It was also estimated that by the year 2100 the deposited sediment volume will occupy 6.1% of the effective volume of the reservoir, a value much lower than the estimated dead volume of the dam (17.4%).
Hydrological Sciences Journal-journal Des Sciences Hydrologiques, Mar 30, 2011
This paper presents a viable approach for flood management strategy in a river basin based on the... more This paper presents a viable approach for flood management strategy in a river basin based on the European Floods Directive. A reliable flood management plan has two components: (a) a proper flood management strategy, and (b) the determination of the flood-hazard areas. A method to evaluate the benefits of a flood warning system is presented herein, as well as a
AGUFM, Dec 1, 2001
The cost-effective remediation designs that involve removal of contaminants from the subsurface g... more The cost-effective remediation designs that involve removal of contaminants from the subsurface given risk-based constraints and the requirement of contaminant-plume containment can be addressed via a new multi-stage dynamic approach using pump-and-treat systems. In this approach, the remedial design is modified from time to time in order to more effectively employ the available pumping wells. The duration of each period and the pumping rates at the various target wells are considered interactively in determining the least-cost design. At each stage, the pumping rates are modified to accommodate the dynamic nature of the plume. In response to the changing plume geometry, the well discharges may increase or decrease in order to produce the most cost-effective design that satisfies the specified constraints active during the overall design horizon. The utilization of this approach at a field location illustrates the effectiveness of the proposed strategy.
16th International Congress of the Geological Society of Greece 17-19 October, 2022 - Patras, Greece Bulletin of the Geological Society of Greece, Sp. Publ. 10 Ext. Abs. GSG2022-137, 2022
Groundwater resources in Mediterranean coastal aquifers are under threat due to overexploitation ... more Groundwater resources in Mediterranean coastal aquifers are under threat due to overexploitation of the water resources. Efficient management and monitoring of groundwater systems require interpreting all available data sources. Geostatistics and geophysics can be successfully combined to interpretating the hydrogeological characteristics of an aquifer system. This work is based on geological and geophysical surveys held in the coastal alluvial aquifer of the Tympaki basin in the south and central part of the island of Crete, Greece. This research work aims to develop a set of
plausible 3D geological models combining data of geological maps and mapping, 1D and 2D geophysical profiles, spatial well data analytics, and geostatistical simulation techniques. The resulting set of models represents possible scenarios of the structure of the coastal aquifer system under investigation and identify regions associated with higher
uncertainty.
Management and monitoring of groundwater systems require a sufficient understanding of hydrogeology and the subsurface configuration. This work employs the development of a 3D geological model by using two different approaches to assess the impact that each one has on the groundwater flow simulation. The first approach is based on geophysical data using geostatistical and simulation techniques such as 3D Kriging and Sequential Gaussian Simulation (SGS) to provide the structure of the coastal aquifer system. A geophysical survey (Pipatpan and Blindow 2005) provided data in the study area that were inverted to hydraulic conductivity. Therefore, the inverted geophysical data were used to estimate the spatial distribution of the hydraulic conductivity in the study area.
The modified Box–Cox technique was applied to transform the data close to normal distribution to allow the application of the SGS method. SGS is a stochastic approach for producing equiprobable realizations (maps) of spatial distribution of a variable on a grid by means of kriging methodology. Variogram analysis was employed to investigate the spatial
dependence of the monitoring data, while the simulation analysis provided the spatial distribution of hydraulic conductivity considering the bounds of estimations uncertainty.
The second approach uses the lithostratigraphic interpretation of the available geophysical and water-well data to define the subsurface structure of the alluvial aquifer and the associated fault patterns (Panagopoulos et al., 2021). Then, the geological model is further populated with resistivity and lithological values by applying stochastic algorithms of gaussian random function simulation and the sequential indicator simulation, respectively (Schlumberger, 2016).
Finally, the two approaches were combined to run a new groundwater flow simulation and assess the benefits of using a more detailed subsurface geological model as the basis for the simulation. The proposed methodologies exploit the spatial information from geophysical surveys to provide the spatial distribution of the hydrogeological structure of the coastal aquifer system in three dimensions inside the convex hull of the
measurements. The resulting models will be imported under the same parameterization (e.g., boundary conditions) to a numerical groundwater flow model (Feflow), considering time series of groundwater level in 20 monitoring wells and 6 observation wells for validation, to determine the groundwater level field in the study area.
The 3D property distribution of hydraulic conductivity, resistivity and lithologies have been constructed to understand the aquifer’s behavior better. The property model helps to identify the areas where the coarser material is expected as well as areas of low resistivity that could be related to seawater intrusion.
Comparing the two approaches will help identify similarities and differences in groundwater flow modeling using different data interpretation tools. In addition, it will provide valuable information on the hydrogeological structure detail which is required to acquire optimal groundwater flow modelling results. Such approaches can help develop
typical hydrogeological models, which will aid the management and monitoring of the area's groundwater resources.
This work will support the development of a reliable groundwater flow model to investigate future groundwater level fluctuations in the study area under climate change scenarios.
References
Panagopoulos, G.; Soupios, P.; Vafidis, A.; Manoutsoglou, E.: Integrated use of well and geophysical data for constructing 3D geological models in shallow aquifers: a case study at the Tymbakion basin, Crete. Greece. Environm. Earth Sci. 80, 142 (2021).