SLONIP—A Slovenian Web-Based Interactive Research Platform on Water Isotopes in Precipitation

: Knowledge of how the isotopic composition (i.e., δ 2 H, δ 18 O and 3 H) of precipitation changes within an individual catchment allows the origins of surface and groundwater to be differ-entiated and the dynamic characteristics of water within individual water bodies to be traced. This paper presents the Slovenian Network of Isotopes in Precipitation (SLONIP), a research platform that has been operating since April 2020. The SLONIP platform currently contains 2572 isotope data points of monthly composite precipitation from eight locations obtained from various investigations performed since 1981. It also provides information about a sample’s location, analysis, and links to the relevant scientiﬁc papers. It also presents the data in numerical and graphical form, including monthly, seasonal, and annual means and local meteoric water lines, all calculated using a Python code made freely available on GitHub. The platform provides essential information for geograph-ically, climatologically, and geologically diverse regions like Slovenia and can help improve our understanding of the water cycle on a local and regional scale.


Introduction
Water is crucial for life and moves through the Earth's four spheres as part of the water cycle via evaporation, transpiration, condensation, precipitation, infiltration, and runoff. The natural water cycle is also influenced by anthropogenic activities, e.g., water extraction, including pumping for human, agricultural, and industrial use, water treatment, water storage, distribution, and wastewater treatment. Precipitation represents an integral part of the hydrological cycle, as it is the primary source of surface water and groundwater. Existing patterns of precipitation and storm events are likely to change in the future since precipitation is directly impacted by changes in atmospheric circulation, in addition to the increase in water vapor resulting from a warmer climate due to increased emissions of greenhouse gases [1]. Water molecules, as they move through the water cycle, i.e., from ocean to precipitation to surface and eventually groundwater, can be traced using the stable ( 16 O, 17 O, 18 O, 1 H, 2 H) and radioactive ( 3 H) isotopes, e.g., [2][3][4][5][6][7][8][9][10]. This is possible because the isotopic composition of precipitation and its temporal and spatial distribution are determined by local and regional geographical characteristics (e.g., latitude, altitude, distance from the oceans), origin and mixing of air masses, temperature, amount of precipitation, humidity, evaporation, and seasonal changes [3,4,6,11]. Therefore, understanding temporal and spatial variations in the amount and mode of precipitation and information regarding the stable isotopic composition of hydrogen (δ 2 H) and oxygen (δ 18 O) together with the activity concentration of tritium ( 3 H) is essential [6,8,[10][11][12][13].
In 1958, developments in analytical techniques allowing the isotopic composition of precipitation to be characterized [2] led the International Atomic Energy Agency (IAEA), in

The History of the SLONIP
Briefly, the first research on the isotopic composition of oxygen and hydrogen in precipitation in Slovenia was performed as part of tracer experiments in the karst Ljubljanica River drainage basin between 1972-1975 [39]. Unfortunately, the analyses were performed in Germany and Austria, and the raw data are not publicly available. The systematic monitoring of isotopes in precipitation in Slovenia began in 1981, but only in Ljubljana [26][27][28]37,[40][41][42][43][44]. In 2000, as a part of Mediterranean precipitation investigations [44], we identified the need for a more refined understanding of the isotopic variation of precipitation. In response, a program for collecting new data with higher spatial and temporal resolution in different parts of the country was initiated [44][45][46]. Since then, the number of sampling locations has grown into a countrywide Slovenian Network of Isotopes in Precipitation (SLONIP). However, during a review of past monitoring activities performed in the period 1981-2015 [37], recommendations were made regarding future monitoring based on several gaps identified, such as the limited information about past sampling locations and sampling methods. Also, researchers have used different approaches, and additional meteorological data and information on how these data were obtained are often missing. Isotope techniques and evaluation approaches have also changed and developed with time. Another issue is that numerical data is presented in graphical form in many cases, and only a limited amount of data collected at a particular station is publicly available [37], and only part of the data set provided by the JSI is available in the international GNIP database.
The idea of establishing the Slovenian Network of Isotopes of Precipitation (SLONIP) research platform following the GNIP model was presented for the first time in 2006 [47]. This initiative was followed by a lengthy transitional period, where various activities took place. Unfortunately, these were not unified at the national or inter-institute level [37]. Finally, in 2015, the infrastructure at the JSI was upgraded, which enabled the independent monitoring of water isotopes in precipitation at selected sites in Slovenia. Simultaneously, a need appeared to make the SLONIP platform publicly accessible and interactive. While part of the functionality of the SLONIP platform is related to that of GNIP (e.g., information on measurement stations along with monthly, annual, and long-term isotopic means and regression lines), SLONIP has added functionality enabling data processing and visualization (Section 3).

SLONIP Platform Architecture
The structure of the code for the interactive SLONIP platform is based on Django [48], i.e., a server-side web framework written in the programming language Python. Django represents the logical foundation of the code that serves as the platform's backend (i.e., the server-side) and is capable of communicating with the frontend (i.e., the client-side). The platform's frontend is the part of the code responsible for the graphical interface as seen by the web user. The backend takes care of the data displayed on the page. It makes sure it is safely stored in a database and that the data is readily available if instructed by the frontend (triggered by an action performed by a web user) and is where the statistical processing of the data is performed.
The architecture of the SLONIP platform is schematically depicted in Figure 1. The webpage is built in an admin-friendly way, i.e., the admin is not required to apply any explicit changes to the code to change the content displayed on the client-side. The content comprises numerical data and textual information (displayed in paragraphs on the website). The content manipulation of the page can be done through the website Django-admin interface. For example, an administrator can create a new station (object) and assign different attributes such as raw data files and supplementary info to it. Specifically, the raw data files (xls) administered via the website's admin page are statistically processed in the backend with the help of the same code that is an integral part of the offline program, publicly available on GitHub [38] (https://github.com/nyuhanc/Isotopes-in-precipitation-statistics) (accessed on 6 May 2022). Once processed, data are stored in the database, ready to be sent back to the frontend, where they are displayed as graphs and tables.
in the backend with the help of the same code that is an integral part of the offline program, publicly available on GitHub [38] (https://github.com/nyuhanc/Isotopes-in-precipitation-statistics) (accessed on 6 May 2022). Once processed, data are stored in the database, ready to be sent back to the frontend, where they are displayed as graphs and tables.

Figure 1.
A schematic representation of the SLONIP platform architecture.

Selection of Stations and Data Attributes
The selection of stations on the SLONIP platform was based on a review of the isotope in precipitation monitoring activities performed between 1981 and 2015 [37] and Slovenian climate characteristics. The selection, eight stations in total, was confirmed by a geostatistical evaluation of the network in the context of Slovenian-Hungarian research cooperation [26][27][28]. The locations of the eight stations (Ljubljana, Portorož, Kozina, Kredarica, Rateče, Zg. Radovna, Murska Sobota and Sv. Urban) are presented in Figure 2, and their main attributes, station ID, station name, latitude, longitude, altitude, and type of station, are given in Table 1. The "green" stations correspond to stations active until the end of 2021, while the stations colored "red" are inactive. The data for Ljubljana (Table 1), where the station was moved twice in the past [42,43], are merged into a single data set (i.e., Ljubljana) because no significant differences were observed in the published and evaluated dataset [42,43,49].
Thirty-two different data attributes were selected for each station ( Table 2). The data were then used for statistical calculations and uploaded to the website (see Section 3.1).

Isotopes in Precipitation Data Acquisition
Although sampling and analysis have changed with time [37], the most rational approach for monitoring isotopes in precipitation is to perform monthly composite sampling at meteorological stations that are part of a national network [5,37], such as the Slovenian National Meteorological Network maintained by the Slovenian Environmental Agency (ARSO; [50]), which provide reliable meteorological data. Therefore, isotopic monitoring was performed at ARSO's meteorological synoptic or precipitation stations (Table 1). Precipitation samples were collected by the staff of ARSO from a classical rain gauge collector three times (synoptic stations) or once per day (precipitation stations). In a few cases, sampling was conducted at stations maintained by the JSI (i.e., Ljubljana-JSI, Ljubljana-Reaktor [37,42,43]) or volunteers (i.e., Sv. Urban [51]). In all cases, the samples were collected as soon as possible after a precipitation event. Sampling details before 2015 are available on the SLONIP platform [52] and in the review of Vreča and Malenšek [37]. Since 2016, impurities (e.g., particulate matter) have been removed by filtration (12-25 μm pore size ashless filter papers). Samples for hydrogen and oxygen stable isotope measurements

Selection of Stations and Data Attributes
The selection of stations on the SLONIP platform was based on a review of the isotope in precipitation monitoring activities performed between 1981 and 2015 [37] and Slovenian climate characteristics. The selection, eight stations in total, was confirmed by a geostatistical evaluation of the network in the context of Slovenian-Hungarian research cooperation [26][27][28]. The locations of the eight stations (Ljubljana, Portorož, Kozina, Kredarica, Rateče, Zg. Radovna, Murska Sobota and Sv. Urban) are presented in Figure 2, and their main attributes, station ID, station name, latitude, longitude, altitude, and type of station, are given in Table 1. The "green" stations correspond to stations active until the end of 2021, while the stations colored "red" are inactive. The data for Ljubljana (Table 1), where the station was moved twice in the past [42,43], are merged into a single data set (i.e., Ljubljana) because no significant differences were observed in the published and evaluated dataset [42,43,49].
Thirty-two different data attributes were selected for each station ( Table 2). The data were then used for statistical calculations and uploaded to the website (see Section 3.1).

Isotopes in Precipitation Data Acquisition
Although sampling and analysis have changed with time [37], the most rational approach for monitoring isotopes in precipitation is to perform monthly composite sampling at meteorological stations that are part of a national network [5,37], such as the Slovenian National Meteorological Network maintained by the Slovenian Environmental Agency (ARSO; [50]), which provide reliable meteorological data. Therefore, isotopic monitoring was performed at ARSO's meteorological synoptic or precipitation stations (Table 1). Precipitation samples were collected by the staff of ARSO from a classical rain gauge collector three times (synoptic stations) or once per day (precipitation stations). In a few cases, sampling was conducted at stations maintained by the JSI (i.e., Ljubljana-JSI, Ljubljana-Reaktor [37,42,43]) or volunteers (i.e., Sv. Urban [51]). In all cases, the samples were collected as soon as possible after a precipitation event. Sampling details before 2015 are available on the SLONIP platform [52] and in the review of Vreča and Malenšek [37]. Since 2016, impurities (e.g., particulate matter) have been removed by filtration (12-25 µm pore size ashless filter papers). Samples for hydrogen and oxygen stable isotope measurements were stored in glass bottles (minimum 30 mL) and in high-density polyethylene bottles (minimum 300 mL) for activity concentration of tritium measurements.

The Graphical User Interface of the SLONIP Platform
The graphical user interface of the SLONIP platform [52] and its main content are presented in Figure 3. The website has four main navigation menus: Home, Stations, Data and References. The website has been operating since 1 April 2020 and continues to be maintained and gradually supplemented with the new data. In 2016 the infrastructure of the JSI laboratories was improved, and since then, the isotopic composition of hydrogen (δ 2 H) and oxygen (δ 18 O) has been determined using the H2-H2O [58] and CO2-H2O [59,60] equilibration technique. Measurements are performed on a dual inlet isotope ratio mass spectrometer (DI IRMS, Finnigan MAT DELTA plus, Finnigan MAT GmbH, Bremen, Germany) with an automated H2-H2O and CO2-H2O HDOeq 48 Equilibration Unit (custom built by M. Jaklitsch). The results are normalized to VSMOW/SLAP using LIMS (Laboratory Information Management System for Light Stable Isotopes) program. For details see [27,[61][62][63][64]. The overall uncertainties are estimated to be less than 1 ‰, 0.05 ‰ and 1.01 ‰ for δ 2 H, δ 18 O and deuterium excess, respectively. Activity concentration of tritium is determined using liquid scintillation counting (LSC) following electrolytic enrichment (EE) on a TriCarb 3170 TR/SL (Canberra Packard) or Quantulus 1220 (PerkinElmer, Waltham, MA, USA) following an accredited method (LP-090).

The Graphical User Interface of the SLONIP Platform
The graphical user interface of the SLONIP platform [52] and its main content are presented in Figure 3. The website has four main navigation menus: Home, Stations, Data and References. The website has been operating since 1st April 2020 and continues to be maintained and gradually supplemented with the new data. The Home menu provides basic information about the SLONIP, including an overview of the isotopic composition of water as part of the hydrological cycle, possible applications, the purpose of the portal and its Terms of Use.
The Stations menu summarizes information about the monitoring stations ( Figure 2). It also gives sampling frequency and information about how the isotopic composition of precipitation is determined and has links to the relevant sources of information.
At the core of the SLONIP platform is the Data menu, divided into two parts: the Stations data and the Supplementary data use info. When logged in, the Stations data gives access to the graphical presentation of monthly meteorological and isotope data and calculations available for individual stations obtained by the Python script [38]. The Supplementary data use info part includes five submenus. First, the terms of use and privacy policy are explained in the Data use menu, and then the accessibility of the meteorological data is defined in the Meteorological data menu. In the third, Isotope data submenu, information about raw data, normalization procedures and other information related to reporting of isotope data in the database will eventually be included. Perhaps the most important is The Home menu provides basic information about the SLONIP, including an overview of the isotopic composition of water as part of the hydrological cycle, possible applications, the purpose of the portal and its Terms of Use.
The Stations menu summarizes information about the monitoring stations ( Figure 2). It also gives sampling frequency and information about how the isotopic composition of precipitation is determined and has links to the relevant sources of information.
At the core of the SLONIP platform is the Data menu, divided into two parts: the Stations data and the Supplementary data use info. When logged in, the Stations data gives access to the graphical presentation of monthly meteorological and isotope data and calculations available for individual stations obtained by the Python script [38]. The Supplementary data use info part includes five submenus. First, the terms of use and privacy policy are explained in the Data use menu, and then the accessibility of the meteorological data is defined in the Meteorological data menu. In the third, Isotope data submenu, information about raw data, normalization procedures and other information related to reporting of isotope data in the database will eventually be included. Perhaps the most important is the fourth Evaluation of data submenu, where the statistical data analysis is presented. All of the numeric data seen on the Stations data menu, i.e., the local meteoric water lines (LMWLs), the mean monthly, seasonal and annual data, are displayed and calculated dynamically by default using pre-written formulas.
Although statistical procedures should be universal, different laboratories often use different statistical formulas and methods. Consequently, if the methods used are not clearly reported, it can lead to the results being misinterpreted. An example is the calculation of LMWLs using different regression methods [65]. To make the calculations in the Evaluation of data submenu more accessible, a Python script, "Isotopes in precipitation-statistics", was developed and is publicly available on the GitHub repository [38]. Before performing any calculations, the data need to be parsed as described in the README.md file, which gives an example for the Murska Sobota station. Finally, the fifth submenu is where the Metadata are listed. The References menu enables access to relevant scientific papers, a list of projects, cooperating institutions, team members ,and related websites.

Functioning of the SLONIP Portal
The SLONIP platform is intended primarily for viewing data and providing users with summary information for a particular station. Graphical and numerical data also provide users with insight into the variability of the isotopic composition of precipitation at different locations in Slovenia and is important for further climatological, hydrological, hydrogeological, and other applications, such as determining the provenience of food.
The SLONIP research portal functions best in terms of user experience when using Google Chrome or Mozilla Firefox. The Home, Stations, and Reference menus are publicly available, while the individual station's data, monthly meteorological and isotope data, and different calculations are accessible only upon login.
The Station data includes the long-term means for each station and a summary of all eight stations is presented in Table 3, where the climatological and isotope differences among the stations are clearly shown. The longest data set (1981-2010) is for Ljubljana; however, it contains data from three different locations [42,43]. Since there are no significant differences between the data [49], it is reported as a single dataset. The datasets for other stations are shorter. Also, except for Kozina, all stations were in operation until the end of 2021 and will continue to be supplemented with new data. Table 3. Available data in SLONIP: long-term means of precipitation amount (P), air temperature (T), the isotopic composition of oxygen (δ 18 O), the isotopic composition of hydrogen (δ 2 H), deuterium excess (d) and activity concentration of tritium ( 3 H are also displayed separately (see table), and by clicking the colored buttons above the plot, differences among the LMWLs are revealed. The LMWLs represent the site-specific covariation of hydrogen and oxygen stable isotope ratios and have practical utility as a hydrologic framework and a benchmark for evaluating hydroclimatic processes in isotope-enabled climate models [66]. The slopes of SLONIP LMWLs presented here vary around eight units, while the intercepts vary between 6.54 (RMA at Sv. Urban) and 19.71 (PWMA at Kredarica) and differ among the stations depending on the regression method used. Therefore, it is important when performing future hydrological studies to precisely describe the regression methods applied, particularly since precipitation-weighted regression LMWLs better represent the hydrological influence of precipitation [65]. Further, the scatter plot of δ 2 H versus δ 18 O is presented (Figure 4), including all monthly data as well as different LMWLs, i.e., precipitation amount unweighted or weighted (PW) major axis (MA) or reduced major axis (RMA) regression lines. The lines are also displayed separately (see table), and by clicking the colored buttons above the plot, differences among the LMWLs are revealed. The LMWLs represent the site-specific covariation of hydrogen and oxygen stable isotope ratios and have practical utility as a hydrologic framework and a benchmark for evaluating hydroclimatic processes in isotope-enabled climate models [66]. The slopes of SLONIP LMWLs presented here vary around eight units, while the intercepts vary between 6.54 (RMA at Sv. Urban) and 19.71 (PWMA at Kredarica) and differ among the stations depending on the regression method used. Therefore, it is important when performing future hydrological studies to precisely describe the regression methods applied, particularly since precipitation-weighted regression LMWLs better represent the hydrological influence of precipitation [65].  (blue dots) and different LMWLs, i.e., precipitation-amount unweighted or weighted major axis (MA) or reduced major axis (RMA) regression lines for the Murska Sobota station data [52].
A climate histogram and isotope data plots (Raw data button) are also available graphically and numerically by scrolling along the plot. In some cases, the isotope data are missing, either because precipitation was too low (e.g., December 2016 for the Murska Sobota station) or the sample was not collected or stored correctly prior to analysis (e.g., October 2016 and July 2017 for the Murska Sobota station). The whole data sets are available on request but will be made available for download in the future.
Finally, monthly, seasonal, and annual data are presented. The user can select different display options by clicking the "box" buttons Show/Hide data or plot. In addition, the user can compare the differences between the unweighted and precipitation amount weighted data. For example, δ 18 O annual means at Murska Sobota in 2016 show no significant difference between unweighted and weighted data, whereas differences are pronounced for 2017 and 2018 ( Figure 5). The different δ 18 O annual means indicate the importance of not uniformly distributed precipitation, which changes monthly and annually.  (blue dots) and different LMWLs, i.e., precipitation-amount unweighted or weighted major axis (MA) or reduced major axis (RMA) regression lines for the Murska Sobota station data [52].
A climate histogram and isotope data plots (Raw data button) are also available graphically and numerically by scrolling along the plot. In some cases, the isotope data are missing, either because precipitation was too low (e.g., December 2016 for the Murska Sobota station) or the sample was not collected or stored correctly prior to analysis (e.g., October 2016 and July 2017 for the Murska Sobota station). The whole data sets are available on request but will be made available for download in the future.
Finally, monthly, seasonal, and annual data are presented. The user can select different display options by clicking the "box" buttons Show/Hide data or plot. In addition, the user can compare the differences between the unweighted and precipitation amount weighted data. For example, δ 18

Future Development and the Importance of the SLONIP Platform
The first step in developing the SLONIP platform will be to update the database with archived isotope data following the open data principles. By doing this, Slovenian isotopes in precipitation research will become accessible to different levels of society. In addition, a threshold of 48 months of observations will be used to balance the spatial coverage as recommended by Putman et al. [66]. This criterion is now fulfilled in the SLONIP portal only for the Ljubljana and Portorož stations, and it is vital that the website continues to be updated and the whole network remains in operation [26]. Although challenging, expanding the network to include information from other locations would also be beneficial. The problem is that important information was not always recorded sufficiently, and publicly available data is limited [37]. Therefore, a Slovenian national collaborative network is planned among research institutions and other stakeholders. Networking on a regional level is vital to properly evaluate and interpret the data [26][27][28]. Equally, connecting the platform to similar platforms, like the GNIP [5], is also anticipated.
The data can also improve precipitation isotope time-series predictions in Europe [29]. The available Piso.AI model adequately describes the monthly time series for Ljubljana and Portorož. However, the minimum and maximum values are mostly underor overestimated, e.g., up to 9 ‰ in δ 18 O for Ljubljana. Previous investigations also suggest that the community could establish more realistic isoscapes of precipitation by merging national with regional networks [26,31]. The final goal is to establish appropriate isoscapes through long-term cooperation with neighboring countries.
SLONIP has already proven invaluable in many investigations. These studies are presented in the Reference part of the portal along with other sources (e.g., Google Scholar refers to "isotopes in precipitation, Ljubljana" more than 3700 times). For example, it has been used in the evaluation of GNIP data [11,67], in investigations of precipitation [26][27][28]49,68,69] and many hydrological and hydrogeological investigations [62,68,[70][71][72][73][74][75][76][77][78][79], moisture recycling in Alpine regions [80], bottled water provenance [81] and identification, and characterization of nitrate pollution sources [82]. The SLONIP data, in combination with Hungarian and other neighboring networks, were used for detailed geostatistical evaluation of of the design of the precipitation stable-isotope monitoring network and proved to be a well-represented network [26]. The results were used also for determination of the "altitude" and "continental" isotope effects [27] and to derive the gridded (1 km × 1 km) amount-weighted annual precipitation isoscapes [28] for the Adriatic-Pannonian region.
Knowledge of monthly, seasonal, annual, and daily variations in the isotopic composition of precipitation is essential for understanding climate change and the redistribution of precipitation due to extreme future events. Therefore, the SLONIP platform and the offline Python script can be an excellent basis for further use of isotopes in water resources research, studying the effects of climatic factors on isotopes in precipitation, water vapor

Future Development and the Importance of the SLONIP Platform
The first step in developing the SLONIP platform will be to update the database with archived isotope data following the open data principles. By doing this, Slovenian isotopes in precipitation research will become accessible to different levels of society. In addition, a threshold of 48 months of observations will be used to balance the spatial coverage as recommended by Putman et al. [66]. This criterion is now fulfilled in the SLONIP portal only for the Ljubljana and Portorož stations, and it is vital that the website continues to be updated and the whole network remains in operation [26]. Although challenging, expanding the network to include information from other locations would also be beneficial. The problem is that important information was not always recorded sufficiently, and publicly available data is limited [37]. Therefore, a Slovenian national collaborative network is planned among research institutions and other stakeholders. Networking on a regional level is vital to properly evaluate and interpret the data [26][27][28]. Equally, connecting the platform to similar platforms, like the GNIP [5], is also anticipated.
The data can also improve precipitation isotope time-series predictions in Europe [29]. The available Piso.AI model adequately describes the monthly time series for Ljubljana and Portorož. However, the minimum and maximum values are mostly under-or overestimated, e.g., up to 9 ‰ in δ 18 O for Ljubljana. Previous investigations also suggest that the community could establish more realistic isoscapes of precipitation by merging national with regional networks [26,31]. The final goal is to establish appropriate isoscapes through long-term cooperation with neighboring countries.
SLONIP has already proven invaluable in many investigations. These studies are presented in the Reference part of the portal along with other sources (e.g., Google Scholar refers to "isotopes in precipitation, Ljubljana" more than 3700 times). For example, it has been used in the evaluation of GNIP data [11,67], in investigations of precipitation [26][27][28]49,68,69] and many hydrological and hydrogeological investigations [62,68,[70][71][72][73][74][75][76][77][78][79], moisture recycling in Alpine regions [80], bottled water provenance [81] and identification, and characterization of nitrate pollution sources [82]. The SLONIP data, in combination with Hungarian and other neighboring networks, were used for detailed geostatistical evaluation of of the design of the precipitation stable-isotope monitoring network and proved to be a well-represented network [26]. The results were used also for determination of the "altitude" and "continental" isotope effects [27] and to derive the gridded (1 km × 1 km) amount-weighted annual precipitation isoscapes [28] for the Adriatic-Pannonian region.
Knowledge of monthly, seasonal, annual, and daily variations in the isotopic composition of precipitation is essential for understanding climate change and the redistribution of precipitation due to extreme future events. Therefore, the SLONIP platform and the offline Python script can be an excellent basis for further use of isotopes in water resources research, studying the effects of climatic factors on isotopes in precipitation, water vapor source, and inflow corridors of precipitation in Slovenia and in other fields, e.g., food authentication. The data will also be used in the future to derive the temporal and spatial distribution of stable oxygen and hydrogen isotopic compositions.

Conclusions
Stable and radioactive water isotopes are potent tools for tracking the path of water molecules over the whole water cycle; for example, tracing the source of water, its flow pathways, and quantifying the exchange of water, solutes and particulates between hydrological compartments during hydrological processes. In the last decade, several studies have been published on water isotopes. This number is increasing due to the development of new measurement techniques such as laser absorption spectroscopy that allow for measurements of stable isotope ratios at high resolutions.
The current paper describes the SLONIP, a web-based interactive research platform that collects existing, accessible data on the isotopic composition of precipitation in Slovenia obtained in the framework of research at the JSI since 1981, thereby improving access to the data for scientific research groups and other interested users. The platform is in line with FAIR data concepts, and the functionalities and tools available are especially valuable for lay people interested in isotope precipitation data but not experienced in data treatment, analysis, and visualization. Moreover, "raw" data is available to the scientific community upon request.
The platform [52] is available at https://slonip.ijs.si/ (accessed on 10 April 2022) and contains 2572 monthly isotope data points for composite precipitation samples from eight locations. SLONIP also provides location, sampling, and analysis (including references) information and isotopic data in numerical and graphical form. Monthly, seasonal, and annual means and precipitation amount weighted means and LMWLs calculated using different regression methods are presented. In addition, the Python code [38] enabling the respective calculations available on the platform is deposited on GitHub (https://github. com/nyuhanc/Isotopes-in-precipitation-statistics) (accessed on 6 May 2022).
In the future, SLONIP will be updated with new data and, if possible, data from other Slovenian institutions. Finally, to follow the FAIR data concept, it will be upgraded to a SlovenIan Water Isotopes Portal (SIWIP) that will provide isotope information for the complete water cycle (e.g., in precipitation, snow, surface and groundwater, tap and bottled water).

Acknowledgments:
The work of J. Pezdič, who initiated monitoring of the isotopic composition of precipitation in Ljubljana and all colleagues from JSI and other institutions who participated in developing SLONIP is greatly acknowledged. We also thank the IAEA and ARSO for their scientific and technical support and our project partners for their valuable discussions. The Slovenian team developed the SLONIP platform in the context of Slovenian-Hungarian cooperation (N1-0054 and SSN118205).

Conflicts of Interest:
The authors declare no conflict of interest.