# Farm & Garden Calc – Hargreaves Reference Evapotranspiration (ET₀)

> Farm & Garden Calc – Hargreaves Reference Evapotranspiration (ET₀) is a paid API for AI agents from agri-calc.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-29).

Calculates reference evapotranspiration (ET₀) in mm/day and inches/day using the Hargreaves method from daily max/min temperatures, latitude, and day of year.

## Facts

- Endpoint: GET https://agri-calc.x402tookit.com/et0-hargreaves?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-29
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/farm-garden-calc-hargreaves-reference-evapotranspiration-et-3b4393b6
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_7j5mbQd0HihOF6ZCjhARM

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 farm-garden-calc-hargreaves-reference-evapotranspiration-et-3b4393b6
```

Example prompt: What's the Hargreaves reference ET₀ for today at latitude 38.5°N, day of year 195, with a max temperature of 34°C and a min temperature of 18°C?

## When to prefer this

Use this endpoint when you need a fast, formula-based ET₀ estimate using only temperature data and location — ideal when solar radiation data is unavailable. The Hargreaves method is well-suited for data-sparse environments. Prefer this over Penman-Monteith when you only have temperature and location as inputs. It is purpose-built for agricultural irrigation planning and water budget calculations at the field scale.

## Known failure modes

- Missing required query parameters (tmaxC, tminC, latitude, dayOfYear) results in a 400-level error
- Out-of-range latitude (outside -90 to 90) or dayOfYear (outside 1–366) may return an error or degraded result
- tminC greater than tmaxC may produce physically invalid results or a degraded flag
- Payment not provided or insufficient USDC results in 402 Payment Required

## How this service works

Hargreaves reference evapotranspiration (mm/day) from max and min temperature, latitude and day of year.

## Output

Returns ET₀ in mm/day and inches/day, mean air temperature, temperature range, and extraterrestrial radiation in both MJ/m²/day and mm/day equivalents. Also includes metadata fields such as source, asOf timestamp, summary, disclaimer, and degraded flag.

## 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": [
      "tmaxC",
      "tminC",
      "latitude",
      "dayOfYear"
     ],
     "properties": {
      "tmaxC": {
       "type": "number",
       "description": "Daily maximum air temperature, Celsius"
      },
      "tminC": {
       "type": "number",
       "description": "Daily minimum air temperature, Celsius"
      },
      "latitude": {
       "type": "number",
       "description": "Latitude in degrees, -90 to 90 (south negative)"
      },
      "dayOfYear": {
       "type": "number",
       "description": "Day of year, 1 to 366"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "asOf": {
       "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"
       ]
      },
      "meanTempC": {
       "type": [
        "null",
        "number"
       ]
      },
      "disclaimer": {
       "type": [
        "null",
        "string"
       ]
      },
      "notChecked": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "tempRangeC": {
       "type": [
        "null",
        "number"
       ]
      },
      "et0MmPerDay": {
       "type": [
        "null",
        "number"
       ]
      },
      "et0InchesPerDay": {
       "type": [
        "null",
        "number"
       ]
      },
      "extraterrestrialRadiationMmDay": {
       "type": [
        "null",
        "number"
     
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/farm-garden-calc-hargreaves-reference-evapotranspiration-et-3b4393b6/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from agri-calc.x402tookit.com](https://www.zero.xyz/host/agri-calc.x402tookit.com/llms.txt)