{"uid":"cap_vmmyCt23Pn6CsruiUxVCg","slug":"hydrology-open-channel-calc-time-of-concentration-b2290029","name":"Hydrology & Open-Channel Calc – Time of Concentration","description":"Time of concentration by the Kirpich formula and the NRCS lag equation, with the limits of each stated.","url":"https://hydrology-calc.x402tookit.com/time-of-concentration?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":["flowPathLength","slope"],"properties":{"slope":{"type":"number","description":"Average slope of the flow path, dimensionless (m/m or ft/ft), for example 0.02"},"units":{"type":"string","description":"si (default) or us"},"method":{"type":"string","description":"both (default), kirpich or nrcs-lag"},"curveNumber":{"type":"number","description":"NRCS runoff curve number 30 to 100 (required for nrcs-lag and both)"},"flowPathLength":{"type":"number","description":"Longest flow path length in metres (units si) or feet (units us)"}}}},"additionalProperties":false},"output":{"type":"object","required":["type"],"properties":{"type":{"type":"string"},"example":{"type":"object","properties":{"asOf":{"type":["null","string"]},"slope":{"type":["null","number"]},"units":{"type":["null","string"]},"source":{"type":["null","string"]},"kirpich":{"type":["null","object"],"properties":{"formula":{"type":["null","string"]},"tcHours":{"type":["null","number"]},"tcMinutes":{"type":["null","number"]}}},"nrcsLag":{"type":["null","object"],"properties":{"tcHours":{"type":["null","number"]},"lagHours":{"type":["null","number"]},"tcMinutes":{"type":["null","number"]},"curveNumber":{"type":["null","number"]},"maxRetentionInches":{"type":["null","number"]},"watershedSlopePercent":{"type":["null","number"]}}},"reasons":{"type":["array","null"],"items":{"type":"string"}},"summary":{"type":["null","string"]},"verdict":{"type":["null","string"]},"degraded":{"type":["boolean","null"]},"disclaimer":{"type":["null","string"]},"notChecked":{"type":["array","null"],"items":{"type":"string"}},"flowPathLength":{"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_lQ7eYftgkhzsCo79EXNLQ","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 time of concentration using the Kirpich formula and/or the NRCS lag equation for a watershed flow path, with method-specific applicability limits stated.","exampleAgentPrompt":"Calculate the time of concentration for a watershed with a flow path length of 500 metres and an average slope of 0.02, using both the Kirpich formula and the NRCS lag equation with a curve number of 75, and show me results in SI units.","exampleUseCases":[{"title":"Stormwater detention pond sizing","prompt":"I need the time of concentration for a 1,200-metre flow path with a 0.015 slope and a runoff curve number of 80 — use both Kirpich and NRCS lag in SI units so I can pick the governing Tc for my detention pond design."},{"title":"US customary small watershed Tc","prompt":"Can you compute the time of concentration using the Kirpich method only for a watershed whose longest flow path is 3,500 feet and average slope is 0.03? Give me the answer in US customary units."},{"title":"Comparing Tc methods for a low-CN basin","prompt":"What does the NRCS lag equation give for time of concentration on a flow path of 800 metres, 1% slope, and a curve number of 55 in SI units? I also want the Kirpich result so I can see how they compare."}],"resultDescription":"Returns a JSON object containing Kirpich Tc (in hours and minutes, plus the formula string), NRCS lag Tc and lag time (hours and minutes), NRCS max retention, watershed slope percent, curve number, a plain-language summary, disclaimer, applicability reasons or warnings, and flags for degraded or not-checked conditions. Both methods can be returned simultaneously or individually.","failureModes":["Missing required flowPathLength or slope parameters returns a validation error","curveNumber not provided when method is nrcs-lag or both causes an error or degraded result","slope or flowPathLength outside method applicability limits triggers warnings or reasons flags in the response","Invalid units string (not 'si' or 'us') may cause unexpected behavior","Very low or very high curve numbers (outside 30–100) will be rejected or flagged","Network or service unavailability returns a non-200 status"],"whenToPreferThis":"Choose this endpoint when you need hydrologic time of concentration specifically via the Kirpich formula or the NRCS lag equation — the two most widely used empirical Tc methods in stormwater engineering. It is ideal when you want both methods compared side by side, or when you need applicability limits and disclaimers stated automatically. Prefer it over general-purpose computation tools when working on watershed hydrology, rational method inputs, or NRCS TR-55 workflows.","instructions":null,"reviewSummary":null,"reviewSummaryHighlights":null,"reviewSummaryConcerns":null,"reviewSummaryGeneratedAt":null,"activationCount":0,"lastUsedAt":null,"lastSuccessfullyRanAt":null,"lastHealthCheckAt":"2026-11-02T18:33:05.285Z","isFirstParty":false,"canonicalSlug":"hydrology-open-channel-calc-time-of-concentration-b2290029"}