Groundwater Flow Processes Research Articles

Identifying Groundwater Recharge Sites through Environmental Stable Isotopes in an Alluvial Aquifer

Environmental isotope tracers have been a useful tool in providing new insights into hydrologic processes. In Mexico, there have been several studies reporting different values for δ 18 O and δ 2 H for certain geographical areas. The objective of this study is to achieve the isotopic characterization of rainfall and groundwater and to report the comprehensive understanding of groundwater flow processes around and within the Calera aquifer and, consequently, its potential recharge sites. The samples used for the stable isotope analysis ( δ 18 O , δ 2 H ) were measured using a GV-Isoprime isotope-ratio mass spectrometer at the Isotopy Laboratory of the Water Center for Latin America and the Caribbean. The δ D of precipitation ranged between −110.20‰ and 10.11‰, with a mean of −55.67‰ ± 27.81‰. The δ 18 O ranged between −17.80‰ and 2.74‰, with a mean of −9.44‰ ± 4.74‰. The δ D of groundwater ranged between −81.92‰ and −36.45‰, with a mean of −66.05‰ ± 8.58‰. The δ 18 O ranged between −18.26‰ and −8.84‰, with a mean of −12.35‰ ± 2.12‰. The local meteoric water line of the Zacatecas state is δ D = − 2.03 + 5.68 δ 18 O . The groundwater samples were clustered into four groups. The clustering of the samples led to the finding that streamflows play a significant role in the hydrological balance as a source of local recharge to the aquifer.

Dynamic Evaluation on the Groundwater Flow Process in a River Basin Using the Ensemble Kalman Filter Method with Localization

Ordos Plateau is located in a semi-arid area in the northwest part of China, and groundwater water is the major water source to sustain the ecosystem. With the industrial construction and economic development, it is required to manage the groundwater usage reasonable. Several monitoring instruments have been installed to monitor the groundwater dynamics continuously. An ensemble Kalman filter based data assimilation method is developed to use the sequentially available monitoring data to characterize the groundwater system. To improve the algorithm stability and avoid the spurious correlation estimation, a localization function is introduced. The analysis results show the proposed EnKF method can reduce the estimation uncertainty and improve the predictability.

Geothermal assessment of the Pisa plain, Italy: Coupled thermal and hydraulic modeling

This paper explores the possibility of a development project with a geothermal well doublet in the Pisa plain, Italy. The performance of the system has been evaluated with a 3-dimensional field-scale numerical model that simulates the evolution of temperature and pressure conditions in the aquifer, under different exploitation scenarios. Coupled groundwater flow and thermal transport processes in the reservoir are considered together with non-Darcy fluid flow in the wellbores, and heat exchange between boreholes and surrounding rock formations. Calculations are performed with a parallelized version of the wellbore-reservoir simulator T2Well. This code allows for the efficient modeling of coupled hydraulic-thermal processes over a domain about 40 km2 wide and 1.5 km thick. Simulation results indicate that the energy of the reservoir is sufficient for the designed extraction rate (between 80 and 150 m3/h), but also suitable for much larger rates, up to 250 m3/h. Although aimed at assessing the long-term performance of a specific system, this modeling approach could be profitably applied for the design of similar projects elsewhere.

Bayesian calibration of groundwater models with input data uncertainty

AbstractEffective water resources management typically relies on numerical models to analyze groundwater flow and solute transport processes. Groundwater models are often subject to input data uncertainty, as some inputs (such as recharge and well pumping rates) are estimated and subject to uncertainty. Current practices of groundwater model calibration often overlook uncertainties in input data; this can lead to biased parameter estimates and compromised predictions. Through a synthetic case study of surface‐ground water interaction under changing pumping conditions and land use, we investigate the impacts of uncertain pumping and recharge rates on model calibration and uncertainty analysis. We then present a Bayesian framework of model calibration to handle uncertain input of groundwater models. The framework implements a marginalizing step to account for input data uncertainty when evaluating likelihood. It was found that not accounting for input uncertainty may lead to biased, overconfident parameter estimates because parameters could be over‐adjusted to compensate for possible input data errors. Parameter compensation can have deleterious impacts when the calibrated model is used to make forecast under a scenario that is different from calibration conditions. By marginalizing input data uncertainty, the Bayesian calibration approach effectively alleviates parameter compensation and gives more accurate predictions in the synthetic case study. The marginalizing Bayesian method also decomposes prediction uncertainty into uncertainties contributed by parameters, input data, and measurements. The results underscore the need to account for input uncertainty to better inform postmodeling decision making.

An investigation into the development of toppling at the edge of fractured rock plateaux using a numerical modelling approach

The mechanisms controlling the onset of minor slope instability at the edges of rocky plateaux exhibiting lateral spreading phenomena are yet to be fully understood. Hypotheses have recently been introduced to explain the influence of groundwater within these plateaux on geomorphological processes leading to slope instability. We present a back analysis of a recent landslide which occurred on 27th February 2014 in the town of San Leo, Italy. The role of the softening of basal clay shales and erosion due to seepage is investigated using finite element geomechanical models. Both processes were observed in the field and are related to groundwater discharging along the contact between the rocky slab and the clay-rich substratum. Fracture propagation paths involving pre-existing discontinuities and intact rock bridges failure were simulated using a simplified discrete fracture network (DFN) model coupled with a Voronoi polygonal mesh approach. Model results allow the failure to be classified as a secondary toppling phenomenon. Moreover, a critical amount of undermining was indicated by the models agreeing with field observations made prior to the failure. Based on the modelling results, an interpretation of the overall mechanism inducing failures at the edges of fractured rock slabs is given. In particular, the inter-relationships between groundwater flow and geomorphic processes acting within the rock masses are presented.

Simulation of temperature effects on groundwater flow, contaminant dissolution, transport and biodegradation due to shallow geothermal use

The quantitative prognosis and assessment of possible impacts of temperature changes in groundwater due to the geothermal use of the shallow subsurface in urban regions requires process-based numerical models of coupled non-isothermal groundwater flow, heat and mass transport processes and biogeochemical reactive processes. This work therefore aims at developing and implementing numerical methods as well as the required parameterizations to simulate the effects of temperature increases due to heat injection in a groundwater aquifer. Parameter and process models for fluid flow, solute transport, mass transfer processes between aqueous and non-aqueous phases, and microbial growth coupled to contaminant biodegradation are expressed as functions of temperature for this purpose. The developed model is implemented in the OpenGeoSys code and applied in a set of benchmark simulations, where thermal impacts of borehole heat exchangers (BHE) are simulated in an aquifer with a TCE contamination in a residual NAPL source zone. The thermal plumes emitted by the BHEs result in a focusing of groundwater flow due to a viscosity reduction of the heated water. The local increase in groundwater flow as well as an increase in TCE solubility with temperature leads to increased TCE emissions from the source zone. At the same time, increases in microbial growth rates allow for higher TCE degradation rates by reductive dechlorination. Results of the benchmark simulations allow insights into the interactions of the individual processes and potential benefits or conflicts of geothermal use of the subsurface and natural attenuation processes at contaminated sites. Also, the benchmark simulations can be used as test cases for intercomparison and validation of reactive transport codes.

Distinct groundwater recharge sources and geochemical evolution of two adjacent sub-basins in the lower Shule River Basin, northwest China

Based on analysis of groundwater hydrogeochemical and isotopic data, this study aims to identify the recharge sources and understand geochemical evolution of groundwater along the downstream section of the Shule River, northwest China, including two sub-basins. Groundwater samples from the Tashi sub-basin show markedly depleted stable isotopes compared to those in the Guazhou sub-basin. This difference suggests that groundwater in the Tashi sub-basin mainly originates from meltwater in the Qilian Mountains, while the groundwater in the Guazhou sub-basin may be recharged by seepage of the Shule River water. During the groundwater flow process in the Tashi sub-basin, minerals within the aquifer material (e.g., halite, calcite, dolomite, gypsum) dissolve in groundwater. Mineral dissolution leads to strongly linear relationships between Na+ and Cl− and between Mg2++ Ca2+ and SO4 2− + HCO3 −, with stoichiometry ratios of approximately 1:1 in both cases. The ion-exchange reaction plays a dominant role in hydrogeochemical evolution of groundwater in the Guazhou sub-basin and causes a good linear relationship between (Mg2++ Ca2+)–(SO4 2− + HCO3 −) and (Na++ K+)–Cl− with a slope of −0.89 and also results in positive chloroalkaline indices CAI 1 and CAI 2. The scientific results have implications for groundwater management in the downstream section of Shule River. As an important irrigation district in Hexi Corridor, groundwater in the Guazhou sub-basin should be used sustainably and rationally because its recharge source is not as abundant as expected. It is recommended that the surface water should be used efficiently and routinely, while groundwater exploitation should be limited as much as possible.

A hybrid simulation–optimization approach for solving the areal groundwater pollution source identification problems

Summary In this study, a new simulation–optimization approach is proposed for solving the areal groundwater pollution source identification problems which is an ill-posed inverse problem. In the simulation part of the proposed approach, groundwater flow and pollution transport processes are simulated by modeling the given aquifer system on MODFLOW and MT3DMS models. The developed simulation model is then integrated to a newly proposed hybrid optimization model where a binary genetic algorithm and a generalized reduced gradient method are mutually used. This is a novel approach and it is employed for the first time in the areal pollution source identification problems. The objective of the proposed hybrid optimization approach is to simultaneously identify the spatial distributions and input concentrations of the unknown areal groundwater pollution sources by using the limited number of pollution concentration time series at the monitoring well locations. The applicability of the proposed simulation–optimization approach is evaluated on a hypothetical aquifer model for different pollution source distributions. Furthermore, model performance is evaluated for measurement error conditions, different genetic algorithm parameter combinations, different numbers and locations of the monitoring wells, and different heterogeneous hydraulic conductivity fields. Identified results indicated that the proposed simulation–optimization approach may be an effective way to solve the areal groundwater pollution source identification problems.

Multi-tracer approach to characterize hydraulically induced sulfate rock dissolution processes below a weir lock

Numerous damages at structures and failures of dams can be ascribed to solution processes and attendant subsidence. Leaching in gypsum-bearing layers of the Middle Triassic has induced subsidence underneath the structure of the weir Hessigheim, River Neckar. The goals of this study were to gain a better understanding of the prevailing groundwater flow processes as well as the interaction between surface water and groundwater in the area of the weir lock and to verify the success of former remediation measures. In order to attain these objectives a combined groundwater and surface water tracer test in combination with hydraulic borehole tests were performed. In addition, a study of the dissolved sulfate distribution was carried out using sampling data from 1987 to 2013. Based on the results of these methods a strongly heterogeneous aquifer with main runoff occurring in few major flow paths could be characterized. Former remediation measures showed to be successful.

The effects of monsoons and climate teleconnections on the Niangziguan Karst Spring discharge in North China

Karst aquifers supply drinking water for 25 % of the world’s population, and they are, however, vulnerable to climate change. This study is aimed to investigate the effects of various monsoons and teleconnection patterns on Niangziguan Karst Spring (NKS) discharge in North China for sustainable exploration of the karst groundwater resources. The monsoons studied include the Indian Summer Monsoon, the West North Pacific Monsoon and the East Asian Summer Monsoon. The climate teleconnection patterns explored include the Indian Ocean Dipole, E1 Nino Southern Oscillation, and the Pacific Decadal Oscillation. The wavelet transform and wavelet coherence methods are used to analyze the karst hydrological processes in the NKS Basin, and reveal the relations between the climate indices with precipitation and the spring discharge. The study results indicate that both the monsoons and the climate teleconnections significantly affect precipitation in the NKS Basin. The time scales that the monsoons resonate with precipitation are strongly concentrated on the time scales of 0.5-, 1-, 2.5- and 3.5-year, and that climate teleconnections resonate with precipitation are relatively weak and diverged from 0.5-, 1-, 2-, 2.5-, to 8-year time scales, respectively. Because the climate signals have to overcome the resistance of heterogeneous aquifers before reaching spring discharge, with high energy, the strong climate signals (e.g. monsoons) are able to penetrate through aquifers and act on spring discharge. So the spring discharge is more strongly affected by monsoons than the climate teleconnections. During the groundwater flow process, the precipitation signals will be attenuated, delayed, merged, and changed by karst aquifers. Therefore, the coherence coefficients between the spring discharge and climate indices are smaller than those between precipitation and climate indices. Further, the fluctuation of the spring discharge is not coincident with that of precipitation in most situations. Karst spring discharge as a proxy can represent groundwater resource variability at a regional scale, and is more strongly influenced by climate variation.

Numerical investigation of coupled density‐driven flow and hydrogeochemical processes below playas

AbstractNumerical modeling approaches with varying complexity were explored to investigate coupled groundwater flow and geochemical processes in saline basins. Long‐term model simulations of a playa system gain insights into the complex feedback mechanisms between density‐driven flow and the spatiotemporal patterns of precipitating evaporites and evolving brines. Using a reactive multicomponent transport model approach, the simulations reproduced, for the first time in a numerical study, the evaporite precipitation sequences frequently observed in saline basins (“bull's eyes”). Playa‐specific flow, evapoconcentration, and chemical divides were found to be the primary controls for the location of evaporites formed, and the resulting brine chemistry. Comparative simulations with the computationally far less demanding surrogate single‐species transport models showed that these were still able to replicate the major flow patterns obtained by the more complex reactive transport simulations. However, the simulated degree of salinization was clearly lower than in reactive multicomponent transport simulations. For example, in the late stages of the simulations, when the brine becomes halite‐saturated, the nonreactive simulation overestimated the solute mass by almost 20%. The simulations highlight the importance of the consideration of reactive transport processes for understanding and quantifying geochemical patterns, concentrations of individual dissolved solutes, and evaporite evolution.

Using noble gas tracers to constrain a groundwater flow model with recharge elevations: A novel approach for mountainous terrain

AbstractEnvironmental tracers provide information on groundwater age, recharge conditions, and flow processes which can be helpful for evaluating groundwater sustainability and vulnerability. Dissolved noble gas data have proven particularly useful in mountainous terrain because they can be used to determine recharge elevation. However, tracer‐derived recharge elevations have not been utilized as calibration targets for numerical groundwater flow models. Herein, we constrain and calibrate a regional groundwater flow model with noble‐gas‐derived recharge elevations for the first time. Tritium and noble gas tracer results improved the site conceptual model by identifying a previously uncertain contribution of mountain block recharge from the Coast Mountains to an alluvial coastal aquifer in humid southwestern British Columbia. The revised conceptual model was integrated into a three‐dimensional numerical groundwater flow model and calibrated to hydraulic head data in addition to recharge elevations estimated from noble gas recharge temperatures. Recharge elevations proved to be imperative for constraining hydraulic conductivity, recharge location, and bedrock geometry, and thus minimizing model nonuniqueness. Results indicate that 45% of recharge to the aquifer is mountain block recharge. A similar match between measured and modeled heads was achieved in a second numerical model that excludes the mountain block (no mountain block recharge), demonstrating that hydraulic head data alone are incapable of quantifying mountain block recharge. This result has significant implications for understanding and managing source water protection in recharge areas, potential effects of climate change, the overall water budget, and ultimately ensuring groundwater sustainability.

Groundwater flow processes and mixing in active volcanic systems: the case of Guadalajara (Mexico)

Abstract. Groundwater chemistry and isotopic data from 40 production wells in the Atemajac and Toluquilla valleys, located in and around the Guadalajara metropolitan area, were determined to develop a conceptual model of groundwater flow processes and mixing. Stable water isotopes (δ2H, δ18O) were used to trace hydrological processes and tritium (3H) to evaluate the relative contribution of modern water in samples. Multivariate analysis including cluster analysis and principal component analysis were used to elucidate distribution patterns of constituents and factors controlling groundwater chemistry. Based on this analysis, groundwater was classified into four groups: cold groundwater, hydrothermal groundwater, polluted groundwater and mixed groundwater. Cold groundwater is characterized by low temperature, salinity, and Cl and Na concentrations and is predominantly of Na-HCO3-type. It originates as recharge at "La Primavera" caldera and is found predominantly in wells in the upper Atemajac Valley. Hydrothermal groundwater is characterized by high salinity, temperature, Cl, Na and HCO3, and the presence of minor elements such as Li, Mn and F. It is a mixed-HCO3 type found in wells from Toluquilla Valley and represents regional flow circulation through basaltic and andesitic rocks. Polluted groundwater is characterized by elevated nitrate and sulfate concentrations and is usually derived from urban water cycling and subordinately from agricultural return flow. Mixed groundwaters between cold and hydrothermal components are predominantly found in the lower Atemajac Valley. Twenty-seven groundwater samples contain at least a small fraction of modern water. The application of a multivariate mixing model allowed the mixing proportions of hydrothermal fluids, polluted waters and cold groundwater in sampled water to be evaluated. This study will help local water authorities to identify and dimension groundwater contamination, and act accordingly. It may be broadly applicable to other active volcanic systems on Earth.

Hydrochemical evolution and groundwater flow processes in the Galilee and Eromanga basins, Great Artesian Basin, Australia: A multivariate statistical approach

The Galilee and Eromanga basins are sub-basins of the Great Artesian Basin (GAB). In this study, a multivariate statistical approach (hierarchical cluster analysis, principal component analysis and factor analysis) is carried out to identify hydrochemical patterns and assess the processes that control hydrochemical evolution within key aquifers of the GAB in these basins.The results of the hydrochemical assessment are integrated into a 3D geological model (previously developed) to support the analysis of spatial patterns of hydrochemistry, and to identify the hydrochemical and hydrological processes that control hydrochemical variability. In this area of the GAB, the hydrochemical evolution of groundwater is dominated by evapotranspiration near the recharge area resulting in a dominance of the Na–Cl water types. This is shown conceptually using two selected cross-sections which represent discrete groundwater flow paths from the recharge areas to the deeper parts of the basins. With increasing distance from the recharge area, a shift towards a dominance of carbonate (e.g. Na–HCO3 water type) has been observed. The assessment of hydrochemical changes along groundwater flow paths highlights how aquifers are separated in some areas, and how mixing between groundwater from different aquifers occurs elsewhere controlled by geological structures, including between GAB aquifers and coal bearing strata of the Galilee Basin. The results of this study suggest that distinct hydrochemical differences can be observed within the previously defined Early Cretaceous–Jurassic aquifer sequence of the GAB. A revision of the two previously recognised hydrochemical sequences is being proposed, resulting in three hydrochemical sequences based on systematic differences in hydrochemistry, salinity and dominant hydrochemical processes.The integrated approach presented in this study which combines different complementary multivariate statistical techniques with a detailed assessment of the geological framework of these sedimentary basins, can be adopted in other complex multi-aquifer systems to assess hydrochemical evolution and its geological controls.

Improved understanding of groundwater flow in complex superficial deposits using three-dimensional geological-framework and groundwater models: an example from Glasgow, Scotland (UK)

Groundwater models are useful in improving knowledge of groundwater flow processes, both for testing existing hypotheses of how specific systems behave and predicting the response to various environmental stresses. The recent advent of highly detailed three-dimensional (3D) geological-framework models provides the most accurate representation of the subsurface. This type of modelling has been used to develop conceptual understanding of groundwater in the complex Quaternary deposits of Glasgow, Scotland (UK). Delineating the 3D geometry of the lithostratigraphical units has allowed the most detailed conceptualisation of the likely groundwater flow regime yet attempted for these superficial deposits. Recharge and groundwater flow models have been developed in order to test this conceptual understanding. Results indicate that the direction of groundwater flow is predominantly convergent through the permeable, relatively thick Quaternary deposits of the Clyde valley towards the River Clyde, which runs through Glasgow, with some indication of down-valley flow. A separate nearby system with thick and potentially permeable Quaternary deposits, the Proto-Kelvin Valley, may also be a significant conveyor of groundwater towards the River Clyde, although the absence of local data makes any conclusions conjectural. To improve the robustness of the current model there is a need for an overall increase in good quality groundwater-level data, particularly outside central Glasgow. A prototype groundwater-monitoring network for part of Glasgow is an encouraging development in this regard. This would allow the development of a time-variant groundwater model which could be used to study future modelling scenarios.

Potential of MODFLOW to Model Hydrological Interactions in a Semikarst Floodplain of the Ozark Border Forest in the Central United States

Abstract Riparian shallow groundwater and nutrient movement is important for aquatic and forest ecosystem health. Understanding stream water (SW)–shallow groundwater (GW) interactions is necessary for proper management of floodplain biodiversity, but it is particularly confounding in underrepresented semikarst hydrogeological systems. The Modular Three-Dimensional Finite-Difference Ground-Water Flow Model (MODFLOW) was used to simulate shallow groundwater flow and nutrient transport processes in a second-growth Ozark border forest for the 2011 water year. MODFLOW provided approximations of hydrologic head that were statistically comparable to observed data (Nash–Sutcliffe = 0.47, r2 = 0.77, root-mean-square error = 0.61 cm, and mean difference = 0.46 cm). Average annual flow estimates indicated that 82% of the reach length was a losing stream, while the remaining 18% was gaining. The reach lost more water to the GW during summer (2405 m3 day−1) relative to fall (2184 m3 day−1), spring (2102 m3 day−1), and winter (1549 m3 day−1) seasons. Model results showed that the shallow aquifer had the highest nitrate loading during the winter season (707 kg day−1). A Particle-Tracking Model for MODFLOW (MODPATH) revealed significant spatial variations between piezometer sites (p = 0.089) in subsurface flow path and travel time, ranging from 213 m and 3.6 yr to 197 m and 11.6 yr. The current study approach is novel with regard to the use of transient flow conditions (as opposed to steady state conditions) in underrepresented semikarst geological systems of the U.S. Midwest. This study emphasizes the significance of semikarst geology in regulating SW–GW hydrologic and nutrient interactions and provides baseline information and modeling predictions that will facilitate future studies and management plans.

Effect of groundwater--lake interactions on arsenic enrichment in freshwater beach aquifers.

Field measurements combined with numerical simulations provide insight into the water exchange, groundwater flow, and geochemical processes controlling the mobility of arsenic (As) in freshwater beach aquifers. Elevated dissolved As (up to 56 μg/L) was observed 1-2 m below the shoreline at two sandy beaches on Lake Erie, Ontario, Canada. Water and solid-phase analyses suggest that Fe (hydr)oxides present below the shoreline accumulate As, creating a risk of high As in the beach aquifer. Groundwater flow simulations combined with vertical hydraulic gradient measurements indicate that wave-induced flow recirculations across the groundwater-lake interface are significant. These recirculations, which vary with wave intensity and lake water level fluctuations, set up redox and pH gradients, where Fe precipitates and subsequently sequesters As. The elevated As concentrations observed at both beaches, combined with the distribution of other dissolved species, suggest that the As enrichment may be naturally occurring. Regardless of the As source, the interacting hydrologic and geochemical processes revealed may have important implications for the flux of As and also other oxyanions, such as phosphate, across the groundwater-lake interface in nearshore areas of the Great Lakes.

Induced Temperature Gradients to Examine Groundwater Flowpaths in Open Boreholes

Techniques for characterizing the hydraulic properties and groundwater flow processes of aquifers are essential to design hydrogeologic conceptual models. In this study, rapid time series temperature profiles within open-groundwater wells in fractured rock were measured using fiber optic distributed temperature sensing (FO-DTS). To identify zones of active groundwater flow, two continuous electrical heating cables were installed alongside a FO-DTS cable to heat the column of water within the well and to create a temperature difference between the ambient temperature of the groundwater in the aquifer and that within the well. Additional tests were performed to examine the effects of pumping on hydraulic fracture interconnectivity around the well and to identify zones of increased groundwater flow. High- and low-resolution FO-DTS cable configurations were examined to test the sensitivities of the technique and compared with downhole video footage and geophysical logging to confirm the zones of active groundwater flow. Two examples are presented to demonstrate the usefulness of this new technique for rapid characterization of fracture zones in open boreholes. The combination of the FO-DTS and heating cable has excellent scope as a rapid appraisal tool for borehole construction design and improving hydrogeologic conceptual models.

Hydrologic implications of errors in bias-corrected regional reanalysis data for west central Florida

Summary This study investigated the limitations associated with using dynamically-downscaled, bias-corrected reanalysis data (i.e. regional reanalysis data) to predict hydrologic behavior of low-relief rainfall driven systems using an integrated surface/groundwater model. Four different sets of global reanalysis data (NCEP/NCAR-R1, NCEP-DOE-R2, ERA40, and 20CR) that were previously downscaled using two RCMs (MM5 and RSM) were obtained, bias-corrected on a daily basis using the CDF-mapping approach, and used to drive an integrated hydrologic model (INTB) that was previously calibrated and verified for the Tampa Bay region. All raw dynamically-downscaled reanalysis datasets accurately estimated the annual cycle of daily maximum and minimum temperature, except the NCEP/NCAR R1+MM5 data which consistently underestimated daily maximum temperature. All raw regional reanalysis precipitation data significantly overestimated precipitation, particularly for the dry season. Bias-correction using the CDF-mapping approach effectively removed biases in the temporal mean and standard deviation of both the daily precipitation and temperature predictions. Biases in the mean monthly and mean annual precipitation totals were removed by CDF-mapping on a daily basis, but the standard deviation of the monthly and annual precipitation totals were not accurately reproduced. Furthermore inaccuracies in actual daily precipitation time series aggregated into monthly and annual rainfall total time series that showed significant and temporally persistent errors. Precipitation timing errors produced by regional reanalysis data were propagated and enhanced by non-linear streamflow generation, groundwater flow and storage processes in the hydrologic model and produced significant errors in both actual and mean daily, monthly and annual streamflow and groundwater level predictions. These results show that improvement in large-scale reanalysis products and regional climate models may be required before dynamically downscaled bias-corrected reanalysis data can be used as a surrogate for observational data in hydrologic model applications for low-relief, rainfall driven systems.

Variability of groundwater flow and transport processes in karst under different hydrologic conditions

Significance of hydrological variability in karst is presented, which also discusses factors inducing such variability and consequences it may cause. Groundwater flow in karst aquifers is often characterized by strong variability of flow dynamics in response to different hydrologic conditions within a short time period. Consequently, water table fluctuations are often in the order of tens of meters, differences in flow velocities between low- and high-flow conditions can reach ten or even more times. In dependence to respective hydrologic conditions groundwater flow also results in variations of flow directions, and thus in contribution of different parts of the aquifer to a particular spring. The described hydrological variability has many implications for contaminant transport, groundwater availability and vulnerability. Groundwater level rising reduces thickness of the unsaturated zone and decreases protectiveness of the overlying layers. Higher water flow velocities reduce underground retention. Due to more turbulent flow, transport and remobilization of solute and insoluble matter is more effective. During high-flow conditions there is usually more surface flow and hence more concentrated infiltration underground. Particularly in karst systems that show very high hydrologic variability, this should be considered to correctly characterize, understand or predict the aquifers’ hydrological behaviour and to prepare proper protection strategies.Keywords: karst aquifer, temporal hydrological variability, groundwater flow, transport processes, karst spring, water source protection.DOI: 10.3986/ac.v42i2.644