{"id":"us-groundwater-depth","slug":"us-groundwater-depth","name":"United States Groundwater Monitoring Dataset","description":"Monthly mean depth to the water table at long-record United States observation wells, the earliest reaching back to 1943. Groundwater is the slowest part of the water cycle and the part a drought is felt in last and longest: rainfall can return to normal while the water table stays low for another year.","methodology":"Aggregated by Hydrionis from the daily mean depth to water published by the USGS through its water data interface. Values are published in feet below the land surface and converted to metres using the exact definition of the international foot. A month is published only where at least fifteen days of record exist. Wells need twenty-five years of record, and at most twelve are taken from each state so that coverage is national rather than concentrated in the few states that drilled the most observation wells. Depth is stored rather than groundwater elevation because it is the quantity the United States actually publishes for most of its network, and the two are opposite measurements that are never combined.","domains":["groundwater"],"metricIds":["groundwater-depth-below-surface"],"subjectScope":"United States observation wells with 25+ years of record, capped at twelve per state; 103 wells across 35 states held so far","cadence":"monthly","version":"1.0.0","released":"2026-09-01","changelog":[{"version":"1.0.0","date":"2026-09-01","summary":"First release. Hydrionis holds United States groundwater for the first time, completing the American water cycle alongside rainfall, river flow, flood peaks, runoff, snowpack and drought status.","additions":["Monthly mean depth to water for long-record United States observation wells."]}],"limitations":["Depth is measured downwards from the land surface at the well, so it rises as the water table falls. It is not a groundwater level, which is an elevation above a levelling datum and moves in the opposite direction, and the two must never be compared or combined.","Depth is referenced to the ground at each well, so it is specific to that well and must be read against that well’s own historic range rather than against another well’s.","A negative depth is ordinary at a flowing artesian well, where the pressure surface stands above the ground, and is not an error.","Coverage follows where the United States has historically drilled observation wells, and the per-state cap means the dataset is a national spread rather than the longest records available.","The collection is still filling. The publisher serves a fixed allowance each hour and the full selection needs more than one hour allows, so wells are gathered in tranches and most states are currently represented by their single longest record rather than by the twelve the cap permits."],"observationCount":43885,"seriesCount":103,"subjectCount":103,"from":1942,"to":2026,"sourceIds":["usgs-nwis"],"units":["m"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), United States Groundwater Monitoring Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/us-groundwater-depth/","downloads":[{"file":"/downloads/observations-groundwater-usgs-nwis.csv","format":"CSV","label":"groundwater observations from USGS (43,897 rows)"}],"apiPath":"/api/v1/datasets/us-groundwater-depth.json","lastFetched":"2026-09-01","fields":[{"id":"groundwater-depth-below-surface","name":"Depth to groundwater","definition":"Depth from the land surface down to the water table at a monitoring well, measured downwards.","unit":"m","nativeGeography":["borehole"],"comparabilityNotes":"Depth is measured from the land surface at the well, so it is specific to that well and must be read against that well’s own historic range. It must never be read against, or combined with, a groundwater level: level is an elevation above a levelling datum and rises as the water table rises, while depth is measured downwards from the ground and rises as the water table falls. The two move in opposite directions, and subtracting or comparing them directly inverts the result."}]}