{"id":"us-reservoir-storage","slug":"us-reservoir-storage","name":"United States Reservoir Storage Dataset","description":"Monthly mean storage for long-record United States reservoirs and lakes, some from the 1920s. The data family that makes drought legible: rainfall says what fell, flow says what ran off, and storage says what is actually available to supply people.","methodology":"Aggregated by Hydrionis from USGS daily reservoir storage. Values are published in acre-feet and converted to millions of cubic metres using the exact definition of the acre-foot. A month is published only where at least twenty days of record exist. Reservoirs need twenty-five years of record and five thousand daily values, and at most twelve are taken from each state so that coverage is national rather than Californian. Storage is published as a volume and not as a percentage of capacity: the capacities published alongside do not state whether they are gross, usable or conservation storage, and the three differ substantially.","domains":["reservoir"],"metricIds":["reservoir-storage-volume"],"subjectScope":"United States reservoirs with 25+ years of record, capped at twelve per state","cadence":"monthly","version":"1.0.0","released":"2026-08-25","changelog":[{"version":"1.0.0","date":"2026-08-25","summary":"First release. Hydrionis holds reservoir observations for the first time.","additions":["Monthly mean storage for long-record United States reservoirs."]}],"limitations":["Reservoir storage is managed as well as observed. A drawdown may record a flood-control rule curve or a scheduled release rather than a shortage, and no statistic here can distinguish the two.","Volume without capacity cannot be read as fullness. A reservoir holding 400 Mm³ may be nearly full or nearly empty.","Coverage is western and plains United States, following where the USGS gauges reservoirs, and is not a national inventory."],"observationCount":54117,"seriesCount":92,"subjectCount":92,"from":1914,"to":2026,"sourceIds":["usgs-nwis"],"units":["Mm³"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), United States Reservoir Storage Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/us-reservoir-storage/","downloads":[{"file":"/downloads/observations-reservoir-usgs-nwis.csv","format":"CSV","label":"reservoir observations from USGS (62,892 rows)"}],"apiPath":"/api/v1/datasets/us-reservoir-storage.json","lastFetched":"2026-10-01","fields":[{"id":"reservoir-storage-volume","name":"Reservoir stored volume","definition":"Volume of water held in a reservoir at a given time.","unit":"Mm³","nativeGeography":["reservoir-site"],"comparabilityNotes":"Storage is strongly seasonal and is managed, not merely observed: an operator drawing a reservoir down for flood control produces the same reading as a drought would. A month should be compared with the same month in other years, and a fall in storage should not be read as a supply signal without knowing the operating rule."}]}