# Hydrology & Open-Channel Calc – Rational Method Peak Runoff

> Hydrology & Open-Channel Calc – Rational Method Peak Runoff 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 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.

## Facts

- Endpoint: GET https://hydrology-calc.x402tookit.com/rational-method?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-rational-method-peak-runoff-029e5577
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_g-fmlmzhlXFF90Eazn4P0

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-rational-method-peak-runoff-029e5577
```

Example prompt: 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.

## When to prefer this

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.

## Known failure modes

- 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

## How this service works

Peak runoff Q = C i A by the rational method, from a runoff coefficient, surface type or weighted sub-areas, SI or US.

## Output

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.

## 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": [
      "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"
       ]
 
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/hydrology-open-channel-calc-rational-method-peak-runoff-029e5577/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)