Slovenia Reservoir Transparency

ARSO publishes real-time water-level readings (vodostaj) every 30 minutes from automated stations on rivers, reservoir canals, and lake surfaces via the 'Stanje voda' portal. Key reservoirs in the covered subset (Drava cascade: Dravograd, Mariborski Otok, Zlatoličje, Formin, Ptuj, Borl; Sava: Moste; Soča: Most na Soči) are gauged. However, what is published is water elevation (masl), not volumetric storage or fill percentage. No national dashboard translates levels to fill % or volume for the public. The annual Hydrological Yearbook includes mean daily water levels for reservoir stations, but volumetric reservoir storage is not a routinely published open dataset. ENTSO-E 16.1.D weekly aggregate filling data is submitted by ELES for the electricity system but covers only hydropower-relevant storage and has documented year-level gaps.

No documented public REST API exists for ARSO hydrological data covering the 9 reservoirs in the covered subset. The 'Stanje voda' portal delivers data as HTML tables and AMP pages; the archive (vode.arso.gov.si/hidarhiv/) provides per-station data downloadable as Excel files by year. The GIS server (gis.arso.gov.si/arcgis/rest/services) runs ArcGIS Server 10.9 but uses token-based security and does not expose hydrological time-series layers openly. The national open-data portal (podatki.gov.si) lists the ARSO surface-water measurement network via an ATOM web-service, but this describes station metadata rather than time-series water levels. Third-party developers have relied on reverse-engineered XML scraping for meteorological data; no equivalent clean programmatic interface exists for reservoir levels. ENTSO-E provides a REST API for the aggregated electricity-sector reservoir data, but this is not a hydrological reservoir-management API.

Conservative coverage estimation applied 2026-05-29. Of approximately 30 reservoirs above 10 hm³ active capacity in Slovenia, ARSO 'Stanje voda' provides level (masl) for ~7 reservoir stations on the Drava cascade and the upper Sava/Soča, plus 2 additional reservoirs via DEM/HSE submissions to ENTSO-E. Conservative coverage = 25 (down from 30) reflects that the published metric is water elevation rather than volumetric storage, that no national portal translates levels to fill % or storage volume, and that smaller flood-retention and agricultural reservoirs (Šmartinsko, Slivniško, Perniško, Ledavsko, Gajševsko, Vogršček, Klivnik, Mola) receive no public hydrological storage reporting. Run-of-river dominance further limits the operational relevance of public fill data.

Scoped to the 9 covered reservoirs, the ARSO hydrological archive (vode.arso.gov.si/hidarhiv/) holds records going back to the early 20th century for the longest-running gauging stations on the Drava and Sava, with systematic continuous coverage from the 1960s onwards for most reservoir-adjacent stations. Daily water-level series for the major reservoir stations (e.g., HE Formin canal, station 2140) are freely downloadable year by year in tabular form back to at least 2000. The annual Hydrological Yearbook, published continuously, provides mean daily and monthly water-level tables for all active stations including reservoir lakes. The median historical depth across the 9 covered reservoirs is approximately 40 years. The main limitation is that records relate to water elevation not volumetric fill, and some shorter-lived or refurbished reservoir stations have discontinuities.

Automated hydrological stations at the 9 covered reservoirs transmit readings at 30-minute intervals; new data are typically published on the ARSO 'Stanje voda' portal five minutes after each half-hour mark. This sub-hourly refresh is among the better update cadences in Europe for national hydrological portals. ENTSO-E 16.1.D reservoir filling data from ELES is published weekly, conforming to the EU Regulation 543/2013 requirement. The annual Hydrological Yearbook closes within roughly 12–18 months of the reference year. The principal limitation affecting this score is that the high-frequency water-level data is not the same as reservoir fill/storage updates; volumetric data, to the extent it exists, is only implicitly derivable from level-volume curves that are not publicly provided.

ARSO's measurement methodology is partially documented in the Hydrological Yearbook prefaces and in agency publications (including a freshwater biodiversity metadata document hosted on freshwaterbiodiversity.eu and the ADO/Eurac Research dataset descriptions), which describe gauging station types, rating curves, and data quality flags. The open-data portal entry for surface-water measurements references the national monitoring network and legal basis. However, reservoir capacity figures, bathymetric surveys, and the level-to-volume conversion curves for the 9 covered reservoirs are not published in open, machine-readable form. There is no unified methodology document in English covering all major reservoirs' operational capacities, dead volumes, and fill-calculation methods. The ARSO note that automated station data has 'provisional and informational significance only' further underscores the absence of formal uncertainty documentation for real-time readings.

ARSO operates a partial English translation of its main portal (arso.gov.si/en/water/ and hydro.si/en/), covering real-time surface-water conditions and water-level pages. The Hydrological Yearbook is published bilingually (Slovenian / English summaries) in some editions but the bulk of data tables and the archive interface at vode.arso.gov.si/hidarhiv/ are entirely in Slovenian. The national open-data portal (podatki.gov.si) is Slovenian-only. No English-language user guide or API documentation for the hydrological data exists. Researchers can access ENTSO-E data fully in English for electricity-sector reservoir data, and some ARSO environmental-indicator pages (kazalci.arso.gov.si/en/) have English versions. Overall, a non-Slovenian researcher can reach the real-time water-level portal but cannot navigate the archive or interpret station metadata without knowledge of Slovenian.