# Hydro Context > Apply US hydrology domain knowledge — units, formulas, and conventions — when reading or writing hydrological code Use when this capability is needed. - Skill: `tomevault-io/hydro-context` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/hydro-context` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/hydro-context/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/hydro-context --- You are assisting with US hydrology / water resources engineering code. Apply the following domain knowledge automatically without asking the user to re-explain it. ## Units - Discharge (streamflow): cubic feet per second (cfs) in US datasets; m³/s (cms) internationally - Stage (water level): feet in USGS data; metres in international datasets - Always check that input/output units are consistent; flag mismatches as a comment or warning ## Key formulas **Rating curve (stage-discharge):** `Q = a × (H – H₀)^b` - H = stage, H₀ = zero-flow stage offset, a and b are fitted constants - Typical b range: 1.5–3.0 for natural channels; fit in log-log space **Manning's equation (open-channel flow):** `Q = (1/n) × A × R^(2/3) × S^(1/2)` - n = Manning's roughness (0.025–0.05 for natural channels) - R = hydraulic radius, S = channel slope (dimensionless, m/m or ft/ft) **Log-Pearson Type III** is the standard US flood frequency distribution (USGS Bulletin 17C). - Work in log₁₀ space: fit the mean, standard deviation, and skewness of log₁₀(Q) **Weibull plotting position:** `P(X ≤ x) = m / (n + 1)`, return period `T = (n + 1) / m` - m = rank (1 = smallest), n = total number of data points ## Flood frequency terminology - **Return period** (recurrence interval): average years between events of that magnitude or greater - **Q2**: 2-year flood (50% annual exceedance probability) ≈ bankfull flow in many streams - **Q10**: 10-year flood (10% AEP) - **Q100**: 100-year flood (1% AEP) — US regulatory floodplain standard - **AEP**: Annual Exceedance Probability = 1 / T ## Data quality conventions - USGS missing-data sentinel: -999999 — always filter these before any calculation - Negative discharge values are physically impossible — treat as errors - **Water year**: Oct 1 – Sep 30 (US convention); water year N starts Oct 1 of year N-1 - Minimum record length for reliable flood frequency: ≥ 10 years (30+ preferred for Q100) --- > Source: [lorenliu13/claude-code-for-hydrology](https://github.com/lorenliu13/claude-code-for-hydrology) — distributed by [TomeVault](https://tomevault.io).