# Hydrology & Open-Channel Calc – Time of Concentration

> Hydrology & Open-Channel Calc – Time of Concentration is a paid API for AI agents from hydrology-calc.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-11-02).

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.

## Facts

- Endpoint: GET https://hydrology-calc.x402tookit.com/time-of-concentration?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-11-02
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/hydrology-open-channel-calc-time-of-concentration-b2290029
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_vmmyCt23Pn6CsruiUxVCg

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability hydrology-open-channel-calc-time-of-concentration-b2290029
```

Example prompt: 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.

## When to prefer this

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.

## Known failure modes

- 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

## How this service works

Time of concentration by the Kirpich formula and the NRCS lag equation, with the limits of each stated.

## Output

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.

## Request schema (JSON Schema)

```json
{
 "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": {
         
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/hydrology-open-channel-calc-time-of-concentration-b2290029/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from hydrology-calc.x402tookit.com](https://www.zero.xyz/host/hydrology-calc.x402tookit.com/llms.txt)