{"id":"river-flow-regime","slug":"river-flow-regime","name":"River Flow Regime Dataset","description":"How low, how high and how variable a river runs, calculated from the daily discharge records of gauges in Sweden and Finland. Flow duration curve percentiles — Q95, Q50 and Q10 — describe the shape of a whole record, and the annual maximum daily flow gives each water year its largest day.","methodology":"The duration curve is the gauge’s entire daily record sorted from largest to smallest: Q95 is the discharge equalled or exceeded on 95% of days, which is the standard low-flow measure used in abstraction licensing and environmental flow assessment; Q50 is the median, which on most rivers sits well below the mean because a few days of flood carry a large share of the year’s water; Q10 describes a high but unexceptional day. A curve is drawn only from a record of at least five years that holds at least nine tenths of the days between its first reading and its last, because a curve assumes the days held are a fair sample of the days that happened — an assumption that fails exactly where it matters if a gauge was out of service through a drought. The annual maximum is the largest daily mean within an October-to-September water year, produced only where the year holds at least 355 days with no absent run longer than five, since the largest day of a year nobody read is simply not in the record. Gauges whose discharge is signed — the sluices, locks and pumping stations where a negative value means water moving in reverse — are excluded from all of it, because a duration curve of a managed structure describes the operating regime rather than the river.","domains":["river-flow"],"metricIds":["river-flow-q95","river-flow-q50","river-flow-q10","river-flow-annual-max-daily"],"subjectScope":"The gauging stations Hydrionis holds at least five years of daily discharge for, which today are in Sweden and Finland","cadence":"irregular","version":"1.0.0","released":"2026-08-31","derived":true,"changelog":[{"version":"1.0.0","date":"2026-08-31","summary":"First release: flow duration curves and annual maxima from the daily records.","methodVersion":"flow-duration-percentile 1.0.0, annual-maximum-of-daily 1.0.0","additions":["Q95, Q50 and Q10 for every gauge with a long enough and dense enough daily record.","Annual maximum daily flow by water year, so a winter flood belongs to one year rather than being split across two."]}],"limitations":["The annual maximum here is the largest daily *mean*, not the instantaneous peak. A flood peak lasting hours is averaged across twenty-four, so this figure sits below the true peak on the same day — slightly on a large slow river, far below on a small fast-responding catchment. It is not a flood peak series and flood frequency analysis needs the publisher’s own instantaneous maxima, which Hydrionis does not yet hold.","A duration curve belongs to the span it was drawn over. It moves as a record lengthens, and one covering a run of years without a drought puts Q95 too high — which matters, because Q95 is the number abstraction licences are set against.","Flow belongs to a gauge on a reach, never to a river as a whole. Two gauges on the same river can differ by an order of magnitude, and none of these figures may be attributed to the river.","Rating curves are extrapolated at extreme discharges, so the largest values carry the largest measurement uncertainty. That applies to Q10 and to every annual maximum.","Coverage follows the daily records Hydrionis holds, which are Nordic. This is not a European or global picture of low flow, and the absence of a gauge is an absence of daily data rather than an absence of a river."],"observationCount":54519,"seriesCount":4567,"subjectCount":1252,"from":1847,"to":2024,"sourceIds":["fi-syke-hydrologia","se-smhi-hydro"],"units":["m³/s"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), River Flow Regime Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/river-flow-regime/","downloads":[{"file":"/downloads/observations-river-flow-fi-syke-hydrologia.csv","format":"CSV","label":"river-flow observations from Syke, part 1 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part2.csv","format":"CSV","label":"river-flow observations from Syke, part 2 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part3.csv","format":"CSV","label":"river-flow observations from Syke, part 3 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part4.csv","format":"CSV","label":"river-flow observations from Syke, part 4 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part5.csv","format":"CSV","label":"river-flow observations from Syke, part 5 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part6.csv","format":"CSV","label":"river-flow observations from Syke, part 6 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part7.csv","format":"CSV","label":"river-flow observations from Syke, part 7 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part8.csv","format":"CSV","label":"river-flow observations from Syke, part 8 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part9.csv","format":"CSV","label":"river-flow observations from Syke, part 9 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part10.csv","format":"CSV","label":"river-flow observations from Syke, part 10 of 11 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-fi-syke-hydrologia-part11.csv","format":"CSV","label":"river-flow observations from Syke, part 11 of 11 (355,335 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro.csv","format":"CSV","label":"river-flow observations from SMHI, part 1 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part2.csv","format":"CSV","label":"river-flow observations from SMHI, part 2 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part3.csv","format":"CSV","label":"river-flow observations from SMHI, part 3 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part4.csv","format":"CSV","label":"river-flow observations from SMHI, part 4 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part5.csv","format":"CSV","label":"river-flow observations from SMHI, part 5 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part6.csv","format":"CSV","label":"river-flow observations from SMHI, part 6 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part7.csv","format":"CSV","label":"river-flow observations from SMHI, part 7 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part8.csv","format":"CSV","label":"river-flow observations from SMHI, part 8 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part9.csv","format":"CSV","label":"river-flow observations from SMHI, part 9 of 10 (1,000,000 rows)"},{"file":"/downloads/observations-river-flow-se-smhi-hydro-part10.csv","format":"CSV","label":"river-flow observations from SMHI, part 10 of 10 (534,002 rows)"}],"apiPath":"/api/v1/datasets/river-flow-regime.json","fields":[{"id":"river-flow-q95","name":"Q95 low flow","definition":"The flow exceeded 95% of the time — a standard measure of low-flow conditions.","unit":"m³/s","nativeGeography":["river-gauge"]},{"id":"river-flow-q50","name":"Q50 median flow","definition":"The flow exceeded half of the time — the middle of the flow duration curve.","unit":"m³/s","nativeGeography":["river-gauge"]},{"id":"river-flow-q10","name":"Q10 high flow","definition":"The flow exceeded 10% of the time — the wet end of the flow duration curve, describing a high but not exceptional day.","unit":"m³/s","nativeGeography":["river-gauge"]},{"id":"river-flow-annual-max-daily","name":"Annual maximum daily flow","definition":"The highest daily mean discharge recorded at a gauging station in a water year.","unit":"m³/s","nativeGeography":["river-gauge"],"comparabilityNotes":"Comparable between years at the same gauge, and between gauges only in the loose sense that any two discharges are. It is not comparable with an instantaneous annual maximum from any publisher, because the averaging makes it a systematically smaller number rather than a noisier one. The water year runs October to September, so a winter flood is not split across two annual values."}]}