Reference

Glossary

Definitions of reservoir, hydrology and water-data terms used across the site.

Reservoir: An artificial impoundment of water created by damming a river or stream. Used for water supply, hydropower, flood control, irrigation or recreation.
Dam: The physical structure that impounds the water. The reservoir is the lake of water held back by the dam.
Capacity (total / gross): Maximum volume the reservoir can hold at its normal operating level, typically in million cubic metres (Mm³ / hm³) or billion cubic metres (BCM).
Live storage (useful capacity): The portion of total capacity available for active management — the water that can actually be released for use. Excludes dead storage below the lowest outlet.
Dead storage: Water below the lowest outlet that cannot be released through normal operations. Reserved for sediment accumulation, emergencies or environmental flow.
Fill rate / fill percentage: Current storage divided by total capacity, expressed as a percentage. The single most-watched indicator of reservoir status.
Hectómetre cubic (hm³)(also: Mm³): One million cubic metres (1,000,000 m³). The standard unit for reservoir storage in Spain, Italy, France and most of Latin America. 1 hm³ = 1 Mm³.
Billion cubic metres (BCM)(also: km³): 1,000 hm³ = 10⁹ m³. Used for very large reservoirs (Aswan, Three Gorges, Bratsk).
Acre-foot: The volume of one acre of surface area to a depth of one foot — 1,233 m³. Standard unit in US reservoir data.
Run-of-river hydropower: A hydropower plant with little or no reservoir storage — water flows through turbines as it arrives. Run-of-river plants are excluded from most reservoir storage statistics because they have no live storage.
Pumped storage: A pair of reservoirs at different elevations used to store energy: water is pumped up at off-peak hours and released through turbines at peak hours. Net storage cycles daily.
Large dam (ICOLD): The International Commission on Large Dams defines a large dam as 15+ m height OR 5–15 m height with storage 3+ hm³. This is a structural, not capacity-based, definition.
Reservoir Transparency Index (RTI): The semi-annual reservoirs.earth ranking of 194 countries on the openness of their national reservoir data. 0–100 score across 7 weighted dimensions. See RTI Methodology.
Coverage (RTI): The capacity-weighted share of a country's national reservoir storage that has public data. The most heavily weighted RTI dimension (30%).
SAIH (Spain): Sistema Automático de Información Hidrológica — Spain's real-time hydrological monitoring network, operated by each of the 9 federal river basin authorities (Ebro, Tajo, Júcar, Duero, etc.).
BD-Embalses (Spain): MITECO's reservoir database covering 401 peninsular reservoirs with weekly readings since 1988.
USGS NWIS (USA): National Water Information System operated by the U.S. Geological Survey — the largest public hydrological dataset in the world, with daily storage records back to the early 20th century for many stations.
Bureau of Reclamation (BOR): U.S. federal agency operating major dams in the 17 western states (Lake Mead, Lake Powell, Shasta, etc.). Publishes data via the RISE platform.
CDEC (California): California Data Exchange Center — operated by CA Dept. of Water Resources, publishes near-real-time storage for all major California reservoirs.
ANA (Brazil): Agência Nacional de Águas e Saneamento Básico — Brazil's national water agency.
ONS (Brazil): Operador Nacional do Sistema Elétrico — publishes daily reservoir storage for all hydropower reservoirs in the national grid (SIN).
CWC (India): Central Water Commission — publishes the Weekly Reservoir Bulletin covering 166 strategic reservoirs (~71% of national live storage capacity).
NVE (Norway): Norwegian Water Resources and Energy Directorate — publishes Magasinstatistikk covering virtually all 490 Norwegian hydropower reservoirs.
ENTSO-E: European Network of Transmission System Operators for Electricity — under EU Reg. 543/2013 publishes weekly aggregate reservoir storage in MWh (energy equivalent) per market zone.
Open Government Data (OGD): Government data published in machine-readable formats under licences that allow free reuse. Most reservoir data on reservoirs.earth qualifies.
Stage / water level: The elevation of the water surface, typically in metres above sea level (msnm) or metres above a local gauge zero. A reservoir's storage volume is derived from the stage via its elevation-volume curve.
Elevation-volume curve: The mathematical relationship between water surface elevation and stored volume, unique to each reservoir based on its bathymetry. Required to convert stage readings into volume.
Bathymetric survey: Underwater topographic mapping of a reservoir's basin. Re-surveying after years of sediment accumulation produces the updated elevation-volume curve.
Siltation: Gradual accumulation of sediment in a reservoir, reducing its effective storage over time. India loses ~1% of national capacity per decade to siltation.