Karst aquifers provide drinking water for 10% of the world's population, support agriculture, groundwater-dependent activities, and ecosystems. These aquifers are characterised by complex groundwater-flow systems, hence, they are extremely vulnerable and protecting them requires an in-depth understanding of the systems. Poor data accessibility has limited advances in karst research and realistic representation of karst processes in large-scale hydrological studies. In this study, we present World Karst Spring hydrograph (WoKaS) database, a community-wide effort to improve data accessibility. WoKaS is the first global karst springs discharge database with over 400 spring observations collected from articles, hydrological databases and researchers. The dataset's coverage compares to the global distribution of carbonate rocks with some bias towards the latitudes of more developed countries. WoKaS database will ensure easy access to a large-sample of good quality datasets suitable for a wide range of applications: comparative studies, trend analysis and model evaluation. This database will largely contribute to research advancement in karst hydrology, supports karst groundwater management, and promotes international and interdisciplinary collaborations. Background & Summary Karst aquifers are essential sources of drinking water to about 10% of the world's population 1. In many regions across the globe, karst groundwater is also an indispensable resource for ecosystems, agriculture and, economic activities, as well as for tourism and recreation 2,3. For example, in Europe, 21.6% of the land surface is underlain by carbonate rock 4 which contributes up to 50% of supplied drinking water in some countries 5-7. However, groundwater flow in karst aquifers is characterised by a complex interplay of fast-flowing conduit and slow-flowing matrix systems 8,9. Hence, the storage capacity of karst aquifers is variable and systems are extremely vulnerable to climatic pressures, human impacts and contamination 10. In order to ensure adequate protection of karst water sources, in-depth hydrogeological knowledge is necessary. Large-scale modelling and comparative water resource research have shown the great value of large datasets in hydrology 11. Numerous studies have applied these large datasets for several purposes such as model evaluation, global parameter estimations, impact studies, statistical and comparative analyses. For instance, large-scale hydrological models such as WaterGAP 12 used discharge data from the Global Runoff Data Centre (https://www. bafg.de/GRDC) for parameter estimation. Likewise, streamflow data from the Model Parameter Estimation Experiment (MOPEX) 13 and the Global Runoff Data Centre (GDRC) were combined to derive global base flow indexes and recession constants 14. Streamflow observations of near-natural catchments obtained from UNESCO's European Water Archive (EWA) were used to investigate the streamflow trends across Europe and differentiated the impacts from climatic variability and anthropogenic dri...

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Groundwater pollution by agrochemicals such as nitrogen fertilizers can cause complex biogeochemical transformations to take place in groundwater-dependent ecosystems. To explore the interaction between nitrogen load and groundwater-dependent, spring-fed ecosystems, a study was conducted in Latvia in an area of suspected high nitrate (NO3−) vulnerability due to its geological settings. A map of NO3− vulnerability along the margins of the carbonate aquifer in Latvia is presented. The map is based on a conceptual model that was developed during an extensive case study involving hydrological, hydrochemical, and habitat investigation of springs discharging from a karst aquifer and spring-fed ecosystems. Areas that should be prime targets for restricting fertilizer application are highlighted on the map. Although the case study revealed increased nitrogen pollution (up to 51 mg L−1, standard deviation of 9 mg L−1, in the springs discharging from the karst aquifer), no clear evidence of adverse effects due to NO3− pollution on the groundwater-dependent ecosystems using biotic indicators was found, highlighting the resilience of spring-fed ecosystems against high nitrogen inputs. In the case study, downstream groundwater-dependent ecosystems retained 70% of the reactive nitrogen during the vegetation season, but only a small proportion during the cold season. Thus, NO3− pollution can be partly mitigated by restoring wetlands along valley slopes where natural groundwater discharge takes place. The conceptual model developed for groundwater NO3− vulnerability is applicable to other areas in the Baltic region and other places with similar climatic and geological conditions.

UDC 556.34:551.44(474.2) Oliver Koit, Nataša Ravbar, Andres Marandi & Jaanus Terasmaa: Treshold-controlled three-stage hydraulic behaviour of a mantled shallow carbonate aquifer (Tuhala karst area, North Estonia) As karst aquifers are often characterised by non-linear behaviour, ascertaining the turning points in their hydraulic regime may provide essential information on the functioning of the aquifer. These characteristics also apply to the Silurian-Ordovician aquifer system, composed of diverse carbonate rocks, in Estonia. The aquifer system is an important source of drinking water in northern Estonia. It also comprises the NabalaRakvere aquifer, a locally important groundwater resource that underlies the Tuhala karst area famous for an intermittently overflowing karst spring known as the Witch's Well. The water rich in humic substances of the Tuhala River recharges the Tuhala karst system, which is drained by two spring groups. In order to develop measures for the sustainable management of the aquifer in the future, the aim of this study was to enhance understanding of the hydraulic behaviour and hydrodynamic properties of the Tuhala karst system. From October 2014 to December 2016 an extensive field campaign was carried out comprising the observation of 22 surface-and groundwater monitoring points for water level and physico-chemical parameters and the performance of four quantitative tracer tests. The data obtained were evaluated in accordance with the inputoutput water level relation curves and a conceptual model of the system was set up. The results show that a strong hydraulic link exists, primarily through well-developed conduits between the recharge and discharge area of the karst system. Depending on the hydrological conditions, groundwater flow direc- IzvlečekUDK 556.34:551.44(474.2) Oliver Koit, Nataša Ravbar, Andres Marandi & Jaanus Terasmaa: Določanje mejnih vrednosti tristopenjskega hidravljičnega obnašanja pokritega plitvega karbonatnega vodonosnika (Tuhalski kras, severna Estonija) Kraški vodonosniki se pogosto obnašajo nelinearno, zato lahko z ugotavljanjem mejnih vrednosti v hidravličnem režimu zagotovimo bistvene informacije o delovanju vodonosnika. To velja tudi za silurijsko-ordovicijski vodonosni sistem v Estoniji, ki ga sestavljajo različne karbonatne kamnine in je pomemben vir pitne vode na severu države. Vključuje tudi vodonosnik Nabala-Rakvere, ki je lokalno pomemben vodni vir. Na tem območju se nahaja Tuhalski kras, ki je znan po občasno bruhajočem kraškem izviru Witch's Well (čarovničin vodnjak). Tuhalski vodonosnik napaja reka Tuhala, bogata z huminskimi snovmi, prazni pa se skozi dve skupini izvirov. Da bi v prihodnje razvili ukrepe za trajnostno upravljanje vodonosnika, je bil namen te raziskave izboljšati razumevanje hidravličnega obnašanja in hidrodinamičnih lastnosti kraškega sistema. Od oktobra 2014 do decembra 2016 smo opravili obsežno terensko delo, ki je obsegalo spremljanje vodostajev in fizikalno-kemijskih parametrov v 23 površinskih in podzemnih točkah ter izve...