{"id":"annual-runoff-depth","slug":"annual-runoff-depth","name":"Annual Runoff Depth Dataset","description":"River flow expressed as a depth of water over the land that produced it, in millimetres, for 1,699 United States catchments. The same unit as rainfall — which is the point: a rainfall record and a flow record become two halves of one account rather than two lines sharing a time axis.","methodology":"Each value is the year’s mean discharge multiplied by the seconds in that year and divided by the area draining to the gauge, giving the depth that volume of water would make over that area. Leap years are counted. The discharge is the publisher’s own annual mean and the area is the publisher’s own delineation — the contributing area where one is distinguished, the topographic area otherwise — and every value records which was used. Produced only where a gauge holds at least five years of annual mean discharge and an area is held for it; a gauge whose publisher states no drainage area produces nothing rather than a figure divided by a guess.","domains":["river-flow"],"metricIds":["runoff-annual"],"subjectScope":"United States gauging stations for which both an annual mean flow and a catchment area are held","cadence":"irregular","version":"1.0.0","released":"2026-08-31","derived":true,"changelog":[{"version":"1.0.0","date":"2026-08-31","summary":"First release, made possible by collecting the catchment areas.","methodVersion":"discharge-to-runoff-depth 1.0.0","additions":["Annual runoff depth for every gauge holding both a long enough flow record and a published catchment area."]}],"limitations":["This is not a water balance. Runoff is what left the catchment past the gauge, and the difference between the rain that fell and the runoff that emerged is not evaporation alone — it includes change in groundwater storage, water abstracted and taken elsewhere, and water imported by a transfer scheme. A catchment yielding more runoff than its rainfall is usually being fed from outside rather than measured wrongly.","Every value rests on the catchment area beneath it, which Hydrionis does not measure. A revised area, or a topographic area used where a contributing one was meant, rescales an entire series by the ratio between them.","A runoff depth is an average over a whole year and over the whole catchment. It says nothing about where within the catchment the water came from, or when within the year.","Comparing runoff with rainfall requires rainfall for the same catchment. A station total from a gauge that happens to be nearby is a point measurement and is not catchment rainfall."],"observationCount":144126,"seriesCount":1687,"subjectCount":1687,"from":1874,"to":2025,"sourceIds":["usgs-nwis"],"units":["mm"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), Annual Runoff Depth Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/annual-runoff-depth/","downloads":[{"file":"/downloads/observations-river-flow-usgs-nwis.csv","format":"CSV","label":"river-flow observations from USGS (483,211 rows)"}],"apiPath":"/api/v1/datasets/annual-runoff-depth.json","fields":[{"id":"runoff-annual","name":"Annual runoff","definition":"The depth of precipitation that becomes river flow rather than evaporating or being stored, over a year.","unit":"mm","nativeGeography":["catchment","grid-cell"]}]}