{"uid":"cap_g-fmlmzhlXFF90Eazn4P0","slug":"hydrology-open-channel-calc-rational-method-peak-runoff-029e5577","name":"Hydrology & Open-Channel Calc – Rational Method Peak Runoff","description":"Peak runoff Q = C i A by the rational method, from a runoff coefficient, surface type or weighted sub-areas, SI or US.","url":"https://hydrology-calc.x402tookit.com/rational-method?utm_source=zero.xyz","method":"GET","headers":{},"bodySchema":{"type":"object","$schema":"https://json-schema.org/draft/2020-12/schema","required":["input"],"properties":{"input":{"type":"object","required":["type","method"],"properties":{"type":{"type":"string","const":"http"},"method":{"enum":["GET"],"type":"string"},"queryParams":{"type":"object","required":["rainfallIntensity","area"],"properties":{"c":{"type":"number","description":"Runoff coefficient 0 to 1 (send one of c, surface or subareas)"},"area":{"type":"number","description":"Catchment area in hectares (units si) or acres (units us)"},"units":{"type":"string","description":"si (mm/h, hectares, m3/s; default) or us (in/h, acres, ft3/s)"},"surface":{"type":"string","description":"Surface type for a typical C range (midpoint used): pavement, roof, brick, gravel, lawn-sandy-flat, lawn-sandy-average, lawn-sandy-steep, lawn-heavy-flat, ... (see reasons)"},"subareas":{"type":"string","description":"Weighted C from pairs c:area separated by semicolons, e.g. 0.9:2;0.3:8 (areas in the same unit as area)"},"rainfallIntensity":{"type":"number","description":"Design rainfall intensity i in mm/h (units si) or in/h (units us), for the time of concentration"}}}},"additionalProperties":false},"output":{"type":"object","required":["type"],"properties":{"type":{"type":"string"},"example":{"type":"object","properties":{"area":{"type":["null","number"]},"asOf":{"type":["null","string"]},"flow":{"type":["null","number"]},"units":{"type":["null","string"]},"source":{"type":["null","string"]},"reasons":{"type":["array","null"],"items":{"type":"string"}},"summary":{"type":["null","string"]},"verdict":{"type":["null","string"]},"degraded":{"type":["boolean","null"]},"flowUnit":{"type":["null","string"]},"disclaimer":{"type":["null","string"]},"notChecked":{"type":["array","null"],"items":{"type":"string"}},"coefficientRange":{},"rainfallIntensity":{"type":["null","number"]},"runoffCoefficient":{"type":["null","number"]},"flowLitersPerSecond":{"type":["null","number"]},"runoffVolumePerHour":{"type":["null","number"]}}}}}}},"responseSchema":null,"example":null,"exampleRequest":null,"tags":["x402"],"displayCostAmount":"0.02","displayCostAsset":"USDC","priceDynamic":false,"priceHint":null,"priceStatus":"priced","priceSource":"probe","requiresHandshake":false,"reviewCount":0,"rating":{"score":"0.00","successRate":"0.00","reviews":0,"stars":null,"state":"unrated"},"availabilityStatus":"unknown","priceObserved":null,"sessionDeposit":null,"pricing":{"kind":"static","summary":"$0.02/call","primary":{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"},"accepted":[{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"}]},"paymentMethods":[{"uid":"pm_IgtWSHtGDVIZ6ctqTO039","protocol":"x402","methodType":"crypto","chain":"base","mode":"charge","costAmount":"0.02","costPer":"request","priority":0,"asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","unit":"request","depositMicros":null,"planRef":null}],"brandName":null,"brandSlug":null,"brandBaseUrl":null,"brandDocsUrl":null,"whatItDoes":"Calculates peak stormwater runoff flow Q = C × i × A using the rational method, supporting direct runoff coefficient, surface type lookup, or weighted sub-areas in SI or US units.","exampleAgentPrompt":"Calculate the peak runoff using the rational method for a 5-hectare catchment with a runoff coefficient of 0.7 and a design rainfall intensity of 80 mm/h in SI units.","exampleUseCases":[{"title":"Parking lot stormwater drainage design","prompt":"I'm designing drainage for a 2-acre asphalt parking lot. Can you calculate the peak runoff using the rational method with a rainfall intensity of 3.5 in/h in US units?"},{"title":"Mixed land-use watershed peak flow","prompt":"I have a watershed with two sub-areas: 0.9 runoff coefficient over 2 hectares and 0.3 over 8 hectares. The design rainfall intensity is 60 mm/h. What's the peak discharge using the rational method in SI units?"},{"title":"Residential roof drain sizing","prompt":"What's the peak runoff flow from a 0.5-hectare residential roof with a 100 mm/h design storm? Use SI units and the roof surface type for the runoff coefficient."}],"resultDescription":"Returns the calculated peak runoff flow Q (in m³/s or ft³/s depending on unit system), the effective runoff coefficient used, rainfall intensity, catchment area, flow in liters per second, unit labels, a human-readable summary, coefficient range for the selected surface type, and a disclaimer about the rational method's applicability limits.","failureModes":["Missing required parameters (rainfallIntensity or area) returns an error","Conflicting inputs (c, surface, and subareas all provided) may cause ambiguous results","Runoff coefficient outside 0–1 range is rejected","Unknown surface type string returns an error or falls back","Very large or zero area values may produce unrealistic results without explicit warnings","Unit system mismatch (e.g., acres with SI intensity) produces incorrect output if not specified carefully"],"whenToPreferThis":"Use this endpoint when you need a fast, standards-based rational method peak runoff calculation for small catchments (typically <80 ha or 200 acres), especially when working with mixed land-use sub-areas or need surface-type lookup for C values. Prefer it over manual calculation or spreadsheets for automated pipelines, and when both SI and US unit support is needed in a single API.","instructions":null,"reviewSummary":null,"reviewSummaryHighlights":null,"reviewSummaryConcerns":null,"reviewSummaryGeneratedAt":null,"activationCount":0,"lastUsedAt":null,"lastSuccessfullyRanAt":null,"lastHealthCheckAt":"2026-11-02T18:33:03.525Z","isFirstParty":false,"canonicalSlug":"hydrology-open-channel-calc-rational-method-peak-runoff-029e5577"}