The isotope signature of cave waters provides an excellent opportunity to better understand the recharge in karst regions and the complexity of drainage systems in the vadose zone. We have developed ...a cave isotope hydrological model (ISODRIP) that requires entering basic hydrometeorological information and a precipitation δ18O record to simulate the discharge and δ18O signals of different drip sites. The model includes four different modules to simulate various flow route regimes: continuous and discontinuous drips under diffuse or preferential flows. We use precipitation and cave water δ18O records that were obtained in Eagle Cave (central Spain) during a 5-year period to test our model and to better understand the dynamics of karst aquifers. Eagle Cave waters do not record evaporation. The δ18O signals do not have seasonality, although they record intra-annual and inter-annual variability. Additionally, cave water δ18O signal falls within the range of the annual average weighted isotope composition of precipitation. Well-mixed cave waters, that characterize diffuse flows, record 1‰ δ18O variability, whereas partially-mixed waters, that flow along preferential drainage routes, have up to 3‰ δ18O variability. The results suggest that precipitation takes on average 15 months to reach the cave through the diffuse flow network, whereas under preferential flow the transit time is highly variable depending on the previous condition of the system. ISODRIP includes a soil layer above the vadose zone that controls large recharge events, together with direct recharge components that bypass the soil layer enabling at least some recharge all year round. Thus, the simulations reproduce the observed lack of seasonal bias in the cave water δ18O composition in relation to the average weighted isotope composition of precipitation. This research highlights the importance of understanding recharge dynamics and the configuration of particular drips sites to properly interpret speleothem δ18O records.
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•ISODRIP is a new hydrological model to simulate δ18O records of cave waters.•The model includes preferential flows within the soil layer as part of recharge.•ISODRIP was applied to Eagle cave reproducing 5-year record of drip δ18O values.•A residence time of 15 months was calculated for drips from diffuse flow networks.•The model design supports that karst recharge could occur during all seasons.
Karst regions represent 7–12% of the Earth's continental area, and about one quarter of the global population is completely or partially dependent on drinking water from karst aquifers. Climate ...simulations project a strong increase in temperature and a decrease of precipitation in many karst regions in the world over the next decades. Despite this potentially bleak future, few studies specifically quantify the impact of climate change on karst water resources. This review provides an introduction to karst, its evolution, and its particular hydrological processes. We explore different conceptual models of karst systems and how they can be translated into numerical models of varying complexity and therefore varying data requirements and depths of process representation. We discuss limitations of current karst models and show that at the present state, we face a challenge in terms of data availability and information content of the available data. We conclude by providing new research directions to develop and evaluate better prediction models to address the most challenging problems of karst water resources management, including opportunities for data collection and for karst model applications at so far unprecedented scales.
Key Points
We elaborate the importance of karst water resourcesWe provide a detailed overview of karst modeling approachWe present new methods and directions for their improvement
Karst terrain confounds efforts to parameterize and calibrate The Soil and Water Assessment Tool (SWAT). The purpose of this study was to use SWAT to simulate long-term natural (e.g. climate) and ...human (e.g. land use) impacts to streamflow, sediment and nutrient loading in a karst catchment. Sub-objectives were to: (1) perform a rigorous model performance assessment of SWAT, and (2) apply SWAT to highlight critical source areas of NPS pollutant loading in a large (HUC-8 spatial scale) karst basin of the central USA. Long-term (25-years) of daily streamflow data collected at seven nested U.S. Geological Survey flow gaging sites were used to calibrate and validate SWAT. Annual estimates of spring flow contributions from dominant springs were disintegrated to daily timeseries' dependent on baseflow separation results calculated from observed streamflow data. Model calibration results of daily streamflow showed percent bias (PBAIS) ranged from 1.1 to 19.3%, and Nash-Sutcliff efficiency (NSE) values ranged from 0.25 to 0.63. Model validation results of daily streamflow showed PBIAS ranged from −5.1 to −32.8%, and NSE values ranged from 0.44 to 0.78. Results highlighted potential critical source areas of pollutant loading from pasture sources. Simulated overland (i.e. not including channel processes) sediment, TN, and TP yields were 108 Mg km−2 yr−1, 14.1 kg ha−1 yr−1, 0.68 kg ha−1 yr−1, respectively, at the HUC-8 spatial scale. The greatest N and P nutrient yields were associated with a waste water treatment plant (WWTP), and WWTP loading contributions persisted over 100 km downstream ultimately accounting for approximately 37% of TP loading contributions at the basin outlet. Results have implications for end users tasked with creating physically realistic models of karst watershed hydrology using SWAT, and point to an ongoing need for improved sub-surface storage and flow routing SWAT modeling routines.
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•Simulating water quantity and quality is especially challenging in karst watersheds•SWAT model performance was quantified in a HUC-8 karst basin of the central U.S.•A baseflow separation technique captured rapid delivery of groundwater to streams•Pasture land use dominated 62% sediment, 75% TN, and 66% of TP loading's•Legacy TP contributions from wastewater treatment plants were 33% of TP loading
Karst regions offer a variety of natural resources such as freshwater and biodiversity, and many cultural resources. The World Karst Aquifer Map (WOKAM) is the first detailed and complete global ...geodatabase concerning the distribution of karstifiable rocks (carbonates and evaporites) representing potential karst aquifers. This study presents a statistical evaluation of WOKAM, focusing entirely on karst in carbonate rocks and addressing four main aspects: (1) global occurrence and geographic distribution of karst; (2) karst in various topographic settings and coastal areas; (3) karst in different climatic zones; and (4) populations living on karst. According to the analysis, 15.2% of the global ice-free continental surface is characterized by the presence of karstifiable carbonate rock. The largest percentage is in Europe (21.8%); the largest absolute area occurs in Asia (8.35 million km
2
). Globally, 31.1% of all surface exposures of carbonate rocks occur in plains, 28.1% in hills and 40.8% in mountains, and 151,400 km or 15.7% of marine coastlines are characterized by carbonate rocks. About 34.2% of all carbonate rocks occur in arid climates, followed by 28.2% in cold and 15.9% in temperate climates, whereas only 13.1 and 8.6% occur in tropical and polar climates, respectively. Globally, 1.18 billion people (16.5% of the global population) live on karst. The highest absolute number occurs in Asia (661.7 million), whereas the highest percentages are in Europe (25.3%) and North America (23.5%). These results demonstrate the global importance of karst and serve as a basis for further research and international water management strategies.
Increasing groundwater contamination across the globe triggered the concept of “aquifer vulnerability”, which has been extensively used worldwide during past three to four decades by researchers and ...policy makers for protecting groundwater from pollution. However, only a few recent studies have focused on the performance evaluation of two or more vulnerability assessment methods. Some of these studies have resulted in contrasting findings. Given this fact and considering growing threat of groundwater contamination due to increasing human activities across the globe, it is necessary to critically review existing methods, understand current research trends, and identify major challenges associated with the assessment of aquifer vulnerability. Hence, the aim of this study is to present a comprehensive review of the methods and approaches used for the evaluation of aquifer vulnerability for ‘resource’ and ‘source’ protection. First, the concept and types of aquifer vulnerability along with the definitions evolved over the years are presented, and then the methods for assessing aquifer vulnerability are suitably classified and briefly discussed. Second, the concept of vulnerability assessment for ‘source’ protection is highlighted, and the evolution of groundwater vulnerability evaluation methods is presented with an enlightening block diagram. Third, current research trends and critiques on past studies are discussed. Fourth, the major challenges of vulnerability assessment are highlighted and a way forward is suggested. It is concluded that the progress of vulnerability evaluation methods has not kept pace with the advancement of knowledge and tools/techniques. There is an urgent need for developing a scientifically robust and somewhat versatile methodology for the evaluation of ‘intrinsic’ and ‘specific’ groundwater vulnerability for ‘resource’ and ‘source’ protection under varying hydrogeologic and hydro-climatic conditions. It is emphasized that more studies should be devoted to vulnerability assessment for ‘source’ protection using ‘Source-Pathway-Receptor/Target’ approach. Also, spatial decision support systems should be developed using modern tools/techniques including artificial intelligence to improve decision-making process for protecting vital groundwater resources.
•This study presents a review of methods used for mapping of groundwater vulnerability to pollution.•Methods widely used for assessing aquifer vulnerability are classified into suitable groups and their overview is provided.•We illustrated evolution of groundwater vulnerability methods by an enlightening block diagram, and methods are critiqued.•Future challenges are identified and a way forward is suggested.•This study emphasizes need of a robust and versatile method using modern tools for groundwater vulnerability evaluation.
•Employ the LSTM network to fill missing data of groundwater level over 50 years.•Approach requires no calibration for the time-lag in data processing.•Implementations rely only on the groundwater ...level to retrieve missing data.
Monitoring groundwater level (GWL) over long time periods is critical in understanding the variability of groundwater resources in the present context of global changes. However, in Normandy (France) for example, GWLs have only been systematically monitored for ~20 to 50 years. This study evaluates Long Short-Term Memory (LSTM) neural network modeling to reconstruct GWLs, fill gaps and extend existing time-series. The approach is illustrated by using available monitoring fluctuations in piezometers implanted in the chalk aquifer in the Normandy region, Northern France. Here GWL data recorded over 50 years at 31 piezometers in northwestern Normandy is employed to perform GWL prediction. To optimize the network performance, the most influential factors that impact the accuracy of prediction are first determined, such as the network architecture, data quantity and quality. The resulting network is adopted to reconstruct measurements in the piezometers step by step with an increment of missing observation time. The approach requires no calibration for the time-lag in data processing and the implementation relies only on the groundwater level fluctuations to retrieve missing data in the targeted piezometers.
•We studied the runoff coefficient response to various climate and watershed characteristics.•A highly fragmented and heterogeneous landscape greatly reduced runoff coefficient.•Runoff coefficient ...significantly decreased with karst coverage.•The PLSR approach is a useful model for solving co-linear problems.
As one of the largest continuous karst areas in the world, southwest China is experiencing a lack of surface water resources which have negative impacts on the development of the economy and the life of people. Determining the relationship between climate, watershed characteristics and the runoff coefficient is thus important to manage water resources. However, the dominant factors influencing runoff coefficient is still unclear in karst areas (e.g., southwest China). The objective of this study was to examine how the runoff coefficient is related to climate and watershed characteristics (i.e., land use composition, landscape, lithology, soil properties, and topography) for 95 selected karst watersheds. Since climatic and watershed variables are highly codependent, partial least squares regression (PLSR) was used to explain the linkages between the runoff coefficient and 36 selected variables. First-order controls were identified by calculating the variable importance for the projection (VIP). Results showed that the climate and watershed characteristics played an important role in the runoff coefficient and could explain 72% of the total variance in runoff coefficient for the 95 selected watersheds. According to the VIP values, the dominant first-order factors controlling runoff coefficient were as follows: precipitation, elevation, the areal percentage of forestland, largest patch index, Shannon’s diversity index, the areal percentage grassland, aggregation index, gravel content, hypsometric integral and karst coverage. Greater precipitation, gravel content, and the areal percentage of forestland significantly enhanced the runoff coefficient, while the elevation, Shannon’s diversity index, and karst coverage had negative effects on runoff coefficient. As partially eliminating the co-dependency of the variables, the PLSR approach conveniently determined factors controlling runoff coefficient, and could provide quantitative information to enable decision makers to make better watershed management planning. This effective approach could be recommended to a variety of other karst watersheds.
•Tunnel excavation-induced karst groundwater drawdown changed hydrological processes.•Tunnel excavation changed the distribution patterns of karst water resources.•Tunnel excavation altered the ...physical and chemical properties of karst soils.•Tunnel excavation changed physiological processes and growth rates of karst plants.•Tunnel excavation induced karst geological hazards.
Due to a lack of awareness of environmental protection at the beginning of tunnel excavation and the unique ecological vulnerability of karst areas, tunnel excavation is beginning to have increasingly serious negative effects on the ecological environments in karst areas, leading to challenges related to regional water resources and ecological security. The groundwater drawdown caused by tunnel drainage has had far-reaching impacts on the hydrology, ecology and environment in karst areas. The most prominent effects of the recent advances in drainage techniques include the destruction of aquifer structures, changes in the distribution patterns of water resources and the groundwater flow field and even the initiation of geological disasters (i.e., collapses) in karst areas. In addition, karst water circulation and hydrogeochemical processes have also been affected. However, due to the complex geological conditions and limited observational data in karst areas, the impacts of tunnel drainage on ecological systems remain poorly understood. With increased understanding, researchers have found that the physical and chemical properties of the soil, the speed of soil erosion, the physiological processes and growth rates of plants, and even the compositions of plant communities are gradually changing in tunnel-affected karst areas, although the understanding of these processes and mechanisms remains far from sufficient. Based on the progress made regarding the understanding of water resource and hydrological process issues resulting from tunnel excavation in karst areas, we expect to experience a worldwide increase in investigations of the eco-hydrogeological effects of tunnel excavation in the future.
•ERA-Interim SM well reflected the interannual change of observational SM.•SM in the karst areas of China declined in 1982–2015.•Precipitation is more important for SM changes in the karst areas than ...temperature.•SM may be affected on a macro-scale by exposed rocks redistributing rainfall.
Soil moisture is one of the restricting factors in a fragile karst ecological environment. However, its spatiotemporal evolution characteristics in the karst areas of China remain poorly understood. Thus, based on soil moisture from reanalysis (ERA-Interim product) and ground stations, this study used the Mann-Kendall test, the Theil-Sen slope estimator, sensitivity analysis and stepwise regression and obtained the following results: 1) ERA-Interim soil moisture well reflected the interannual change of observational soil moisture at 0–7, 7–28 and 28–100 cm. 2) The reanalysis and station data showed that soil at various depths in the karst areas was dominated by a drying trend in 1982–2015. 3) Soil moisture in karst areas of southern China was high but decreased fastest. In the karst areas of northern China, soil moisture was low and declined quickly. Nevertheless, soil wetting was observed in the central karst areas of Qinghai-Tibet Plateau. 4) Changes of soil moisture throughout the karst region of China and its subareas were mainly affected by precipitation, followed by temperature. 5) In Qinghai-Tibet Plateau and southern China, soil moisture in karst areas is overall higher than that in non-karst areas when under low vegetation coverage levels (NDVI ≤ 0.3) of some climate zones, possibly caused by the centralized allocation of precipitation in karst areas due to exposed rocks. In conclusion, climate, vegetation, and geological background make the spatiotemporal distributions of soil moisture differ within the karst region, while the soil drying trend in recent decades and global climate change are not conducive to the ecological restoration of vulnerable karst areas.
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