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It is a challenge to describe the hydrogeological characteristics of karst aquifers due to the complex structure with extremely high heterogeneity. As the response of karst aquifers to rainfall events, spring discharge variations after precipitation can be used to identify the internal structure of karst systems. In this study, responses of spring discharge to different kinds of precipitations are investigated by continuously monitoring precipitation and karst spring flow at a single-conduit karst aquifer in western Hunan province, China. Recession curves were used to analyze hydrodynamic behaviors and separate recession stages. The results show that the shape of the recession curve was changed under different rainfall conditions. Recession processes can be divided in to three recession stages under heavy rain conditions due to water drainage mainly from conduits, fracture, and matrix at each stage, but only one recession stage representing drainage mainly from matrix in the case of light rain. With the change in amount and intensity of precipitation, the calculated recession coefficient at each stage changes in an order of magnitude. The influence of precipitation on the recharge coefficient and the discharge composition at each recession are discussed, and then the conceptual model diagram of water filling and releasing in the single-conduit karst aquifers is concluded. The findings provide more insight understand on hydraulic behaviors of karst spring under different types of rainfall events and provide support for water resource management in karst regions.

Recharge, storage and flow characteristics of a karst aquifer were identified by correlating the results acquired from hydrogeological and speleological studies, stream/spring hydrographs, dye tracer tests, and hydrogeochemical analyses. Drinking water needs of the city of Kocaeli (Turkey) are partly supplied by karst springs (discharge 20–843 L/s) and the karst aquifer provides baseflow to streams that flow into Yuvacık Reservoir. A multi-variate dataset for characterization of the hydrological behavior of the karst aquifer was created. Stream hydrographs indicated that the aquifer comprises interconnected multipartite reservoirs, exhibiting groundwater residence times of 11–125 days. Recession coefficients of these reservoirs, obtained from master baseflow-recession curves, were 0.008–0.092 day−1. Dye tracer tests showed that groundwater velocities between recharge and discharge points were 483–1,328 m/day. Conduit flow characteristics were generally dominant in the upper reservoirs of the karst aquifer, characterized with high recession coefficients (0.017–0.092 day−1). Deeper reservoirs had a small baseflow recession coefficient (0.008 day−1). Autogenic point infiltration mechanisms were important in the recharge of many karst springs that discharged at high elevations (603–830 m amsl). However, autogenic diffusive infiltration regions (e.g. karst plateaus at high elevations) contributed to the recharge. Dye tracer tests and cave mapping revealed that depression points, allowing fast recharge, require special attention for protection and sustainable management of the karst aquifer. The results obtained from multiple methods and their correlation help to characterize the complex hydrodynamics of karst systems, and they can guide local authorities when assigning protection zones to locally important karst aquifers.

Considering the importance of fractured rock aquifers in the hydrogeologic process, this research aimed to analyze the flow regime, internal degree of karstification, and estimate storage volume in fractured rock aquifers of the Germi Chai Basin in northwest Iran, which is attributed to its active tectonics, erosion, and the lithological diversity. Given the geological setting, the hypothesis is that this basin is characterized by a high degree of karstification and diffuse or intermediate flow regime leading to variation in discharge flow rate. The hydrodynamic and hadrochemical analysis was conducted on 9 well-distributed springs across the basin from 2019 to 2020. The maximum flow rate in most of the springs appeared in the early wet season despite their different levels of fluctuations on the monthly discharge time series. Analyzing the spring recession curve form revealed an aquifer containing multiple micro-regimes with α recession coefficients and a degree of karstification ranging between 0.001 to 0.06 and 0.55 to 2.61, respectively. These findings indicated a dominant diffuse and intermediate flow system resulting from the development of a high density of fractures in this area. The electrical conductivity of the spring changes inversely proportional to the change in flow discharge, indicating the reasonable hydrological response of the aquifer to rainfall events. Hydrograph analysis revealed that the delay time of spring discharge after rainfall events mostly varies between 10 to 30 days. The total dynamic storage volume of the spring for a given period (2019–2020) was estimated to be approximately 1324 million cubic meters reflecting the long-term drainage potential and high perdurability of dynamic storage. Estimating the maximum and minimum ratio revealed that the springs recharging system in Germi Chai Basin comes under the slow aquifers category. This finding provides valuable insight into the hydrogeological properties of fractured rock aquifers contributing to effective water management strategy.

The event runoff coefficient ( ) and the recession coefficient ( ) are of theoretical importance for understanding catchment response and of practical importance in hydrological design. We analyse 57 event periods in the period 2013 to 2015 in the 66 ha Austrian Hydrological Open Air Laboratory (HOAL), where the seven subcatchments are stratified by runoff generation types into wetlands, tile drainage and natural drainage. Three machine learning algorithms (Random forest (RF), Gradient Boost Decision Tree (GBDT) and Support vector machine (SVM)) are used to estimate and from 22 event based explanatory variables representing precipitation, soil moisture, groundwater level and season. The model performance of the SVM algorithm in estimating and is generally higher than that of the other two methods, measured by the coefficient of determination , and the performance for is higher than that for . The relative importance of the explanatory variables for the predictions, assessed by a heatmap, suggests that of the tile drainage systems is more strongly controlled by the weather conditions than by the catchment state, while the opposite is true for natural drainage systems. Overall, model performance strongly depends on the runoff generation type.

It is widely accepted that spring hydrographs are an effective tool for evaluating the internal structure of karst aquifers because they depict the response of the whole aquifer to recharge events. The spring hydrograph is affected by various factors such as flow regime, geometry, type of recharge, and hydraulic properties of conduit. However, the effect of conduit network geometry received less attention and required more comprehensive research studies. The present study attempted to highlight the impact of the two most frequent patterns of karst conduits (i.e., branchwork and network maze) on the characteristic of the spring hydrograph. Therefore, two conduit patterns, branchwork and network maze, were randomly generated with MATLAB codes. Then, MODFLOW-CFP was used to quantify the effect of conduit pattern, conduit density, and diffuse or concentrated recharge on the spring hydrograph. Results reveal that peak discharge, fast-flow recession coefficient, and the return time to baseflow are mainly affected by conduit network pattern, conduit network density, and recharge, respectively. In contrast, the time to reach peak flow only reacts to recharge conditions. Large variations in conduit density produce tangible changes in the baseflow recession coefficient.

A large share of surface water resources in Sahelian countries originates from Guinea’s Fouta Djallon highlands, earning the area the name of “the water tower of West Africa”. This paper aims to investigate the recent dynamics of the Fouta Djallon’s hydrological functioning. The evolution of the runoff and depletion coefficients are analyzed as well as their correlations with the rainfall and vegetation cover. The latter is described at three different space scales and with different methods. Twenty-five years after the end of the 1968–1993 major drought, annual discharges continue to slowly increase, nearly reaching a long-term average, as natural reservoirs which emptied to sustain streamflows during the drought have been replenishing since the 1990s, explaining the slow increase in discharges. However, another important trend has been detected since the beginning of the drought, i.e., the increase in the depletion coefficient of most of the Fouta Djallon upper basins, as a consequence of the reduction in the soil water-holding capacity. After confirming the pertinence and significance of this increase and subsequent decrease in the depletion coefficient, this paper identifies the factors possibly linked with the basins’ storage capacity trends. The densely populated areas of the summit plateau are also shown to be the ones where vegetation cover is not threatened and where ecological intensification of rural activities is ancient.

This paper aims at characterizing the groundwater flow in a highly dynamic karst aquifer using a global modeling approach based on rainfall and spring discharge time series. The Dardennes aquifer (SE France) was studied as it is used for drinking water supply and it also produces karst flash floods that increase the flood hazard downstream in urban areas. Three years of data were available, including a normal rainy year, a wet year and a dry year. Modeling was performed with the new platform KarstMod, a rainfall-discharge model with calibration tools. The Dardennes aquifer model was structured with three interconnected reservoirs: Epikarst, Matrix, and Conduit. Using this modeling approach, we were able to determine the groundwater hydrograph separation of the karst spring discharge, at the annual scale and at the event scale (flood). This gives insight into the low flow (Matrix) available for the drinking water demand and the fast flow (Conduit) that generates flash floods. In such a dynamic aquifer, part of the water budget cannot be accounted for by water resources as fast flow is not stored within the aquifer and is not available for the drinking water demand. The results were compared with the current groundwater management to determine whether the withdrawal is sustainable. Depending on whether it is a wet or a dry year, the proportion of slow flow ranges from 27 to 61% of the total discharge, respectively. During floods in high water periods, the proportion of quickflow increases drastically up to more than 90% of the spring discharge. In the case of a 300 mm/d simulated Mediterranean rainfall event, the mean daily peak value may reach 74 m3/s. This discharge can be reduced if the aquifer is previously depleted, which increases the storage within the aquifer. Coupling the geological context and the model results opens up future perspectives for the active management of the karst aquifer.

In this study, a synthetic modeling approach is proposed to quantify the effect of the amount and direction of the exchange flow on the karstic spring discharge fluctuations under different hydrologic conditions corresponding to high and low flow conditions. We hypothesis that the spring discharge fluctuations constitute a valuable proxy to understand the internal processes of the karst system. An ensemble of spring hydrographs was synthetically produced to highlight the effect of exchange flow by exploring the plausible range of variability of coefficients of exchange flow, conduit diameter, and matrix hydraulic conductivity. Moreover, the change of the rate of point recharge through the karst conduit allows for the quantifying of the sensibility of the spring hydrograph to the directions of exchange flow. We show that increasing the point recharge lies to a remarkable linear recession coefficient (β) as an indication of the conduit flow regime. However, a reduction in and/or lack of the point recharge caused the recession coefficient to change to exponential (α) due to the dominant effect of the matrix restrained flow regime and/or conduit-influenced flow regime. The simulations highlight that the exchange flow process from the conduit to the matrix occurred in a short period and over a restricted part of the conduit flow regime (CFR). Conversely, the exchange flow dumped from the matrix to the conduit occurs as a long-term process. A conceptual model is introduced to compare spring hydrographs’ characteristics (i.e., the peak discharge, the volume of baseflow, and the slope of the recession curve) under the various flow conditions with the directions of the exchange flow between the conduit and the matrix.

Drainage of a cylindrical water-filled tank-reservoir has been analysed by different physical models, providing relative discharge-time equations. The process has been simulated (1) in a condition of free-flow discharge, where no energy is lost during the process, and (2) where friction forces and water viscosity take effect. Simulation (1) is considered to be a Torricelli reservoir, characterised by a linear decrease of discharge; (2) is based on Darcy’s law or on Poiseuille’s law, where discharge decreases exponentially with time, giving a straight line in the semilogarithm plot. For the Darcy’s law simulation, the tank tube was filled with sand. The cylindrical water-filled tank-reservoir drainage analysis has been applied to simulation of the actual shape of karst spring hydrographs. It has been determined that the recession coefficient, α , is proportional to a hydraulic constant, c , which represents the hydraulic characteristics during the baseflow recession, and α is inversely proportional to the product of the water-table area with the effective porosity. This product expresses the area of the aquifer filled by free-flowing water along the water table and can vary during the aquifer drainage.

Water scarcity is becoming the biggest threat to the global population due to unpredictable rainfall, glaciers melt, and other anthropogenic activities. This study focuses on the analysis of monitored high-frequency continuous spring discharge and rainfall data in the contact and fracture type Mathamali spring located in the Garhwal Himalaya. Discharge from the spring and its storage behavior has been studied by analyzing recession components and flow duration curves. Analyzed discharge data revealed that the spring can generate maximum volume during monsoon as compared to winter due to aquifer properties and tendencies to store and transmit water. Springshed intervention practices were implemented in early April 2017. The measured average flow was 16.9 lpm but soon after the interventions, the average flow increased by 2.6 times. The minimum average spring flow was 2.3 lpm which increased by 5 times whereas the average maximum flow increased by 1.8 times. Post-intervention, storage duration has increased by 16%, decaying from 143 lpm (peak flow) to 12.7 lpm (baseflow). The preliminary findings from this spring can be considered as a check for establishing benchmarks for sustainable development of springsheds, climate change adaptation, and development plans to cope up with growing water insecurity in the rural Himalayas.