#hydrology

Stormwater-runoff civil-engineering maths as an API, computed locally and deterministically. The rational endpoint computes the peak runoff from a catchment with the Rational Method, Q = C·i·A — in metric form Q(m³/s) = C·i·A/360 with rainfall intensity i in mm/h and area A in hectares, or in US form Q(cfs) = C·i·A with intensity in in/h and area in acres — where the runoff coefficient C is the fraction of rain that runs off (about 0.9 for paving and 0.2 for lawns). The time-of-concentration endpoint computes how long water takes to flow from the most remote point of the catchment to the outlet with the Kirpich formula, tc = 0.0195·L^0.77·S^(−0.385) minutes, from the flow-path length and slope; this sets the design-storm duration. The detention endpoint gives a first-order estimate of the detention-pond storage needed to throttle a peak inflow down to an allowable outflow over a storm duration, (Q_in − Q_out)·duration. Coefficients are dimensionless, intensities in mm/h or in/h, areas in ha or acres, lengths in m and flows in m³/s. Everything is computed locally and deterministically, so it is instant and private. Ideal for civil-engineering, drainage, urban-planning, landscape and flood-risk app developers, sewer-sizing and detention tools, and hydrology education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is stormwater runoff; for open-channel flow use a Manning API and for pipe friction a Darcy API.

api.oanor.com/runoff-api

Weir flow maths for open-channel discharge measurement as an API, computed locally and deterministically. The rectangular endpoint computes the flow over a rectangular sharp-crested weir, Q = (2/3)·Cd·b·√(2g)·H^1.5, from the crest width and the head of water above the crest — and solves the head back from a known discharge. The vnotch endpoint computes the flow over a triangular V-notch weir, Q = (8/15)·Cd·√(2g)·tan(θ/2)·H^2.5, from the notch angle and head, the most accurate weir for small flows because the discharge varies with the head to the power 2.5. The broadcrested endpoint computes the flow over a broad-crested weir, Q = Cd·(2/3)^1.5·√g·b·H^1.5 ≈ Cd·1.705·b·H^1.5, the rugged field structure used for river gauging. Each device carries its standard discharge coefficient (rectangular 0.62, V-notch 0.58, broad-crested 0.85) which you can override, and each solves either the discharge from a measured head or the head required for a target discharge. Everything is computed locally and deterministically, so it is instant and private. Ideal for hydrology, irrigation and civil-engineering tools, flow gauging in channels and treatment plants, stormwater and water-resource apps, and fluid-mechanics education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is weir overflow discharge; for uniform open-channel flow use a Manning API and for differential-pressure pipe metering use an orifice API.

api.oanor.com/weir-api

Global river-discharge and flood forecasting as an API, powered by the GloFAS (Global Flood Awareness System) model via Open-Meteo. For any coordinate on Earth, get a daily river-discharge forecast of up to 30 days — with the ensemble spread (mean, max and min across forecast members) so you can gauge uncertainty — plus up to 90 days of recent discharge history, and a quick current-situation summary with today's discharge and a 7-day outlook (peak day, max/min and rising/falling/stable trend). Discharge is reported in cubic metres per second. Ideal for flood early-warning and monitoring, insurance and reinsurance risk, agriculture and irrigation planning, hydropower, and environmental research. Data covers modelled rivers worldwide (none over open ocean). Open data via Open-Meteo / GloFAS.

api.oanor.com/flood-api

Live river and stream conditions from the USGS Water Services network. The catalogue of 16,900+ active US streamflow gauges is searchable by name, state or coordinate; the live endpoint returns the latest observation for any gauge: streamflow / discharge (in cubic feet per second and m³/s) and gauge height (in feet and metres), with the station name, observation time and coordinates. Find the gauges nearest any lat/lon. Ideal for flood monitoring, hydrology, whitewater/fishing & recreation apps, drought tracking and environmental dashboards.

api.oanor.com/streamflow-api