Thornthwaite PET Monthly

Evaluate basic monthly Thornthwaite PET from twelve temperature/daylight/month-length normals with explicit heat index and high-temperature calibration-domain refusal; no measured ET or soil budget.

Description

Thornthwaite potential evapotranspiration estimates atmospheric water demand from monthly mean temperature and daylight.

Thornthwaite potential evapotranspiration estimates atmospheric water demand from monthly mean temperature and daylight. Its annual heat index links each month's estimate to the complete supplied year's temperatures.^[1]

Evaluate basic monthly Thornthwaite PET from twelve temperature/daylight/month-length normals with explicit heat index and high-temperature calibration-domain refusal; no measured ET or soil budget.

When to use Thornthwaite PET Monthly

Use this tool to evaluate basic monthly Thornthwaite PET from twelve temperature/daylight/month-length normals with explicit heat index and high-temperature calibration-domain refusal; no measured ET or soil budget.

Prepare the climate observations and any independently chosen thresholds or model parameters. Confirm units, daily coverage, time order and the intended seasonal window.

How to use Thornthwaite PET Monthly and understand its results

Prepare the climate observations and any independently chosen thresholds or model parameters. Confirm units, daily coverage, time order and the intended seasonal window.
Enter monthlyMeanTemperaturesC, monthlyDaylightHours, monthlyDayCounts using the field table and calculation rules, then run the tool.
Read annualThornthwaiteHeatIndex, thornthwaiteExponent, monthlyThornthwaitePetMm. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
monthlyMeanTemperaturesC	Monthly mean temperatures c	list of number	Required
monthlyDaylightHours	Monthly daylight hours	list of number	Required
monthlyDayCounts	Monthly day counts	list of number	Required

Supply exactly twelve monthly mean temperatures in °C, twelve daylight durations from zero to 24 hours and twelve integer month lengths from 28 to 31 days. Temperatures above 26.5 °C are rejected because the implemented equation omits high-temperature corrections. The tool computes I and a, then monthly PET in mm using the displayed expression for positive temperature and positive I; other months return zero. It returns the annual heat index and exponent alongside the monthly estimates.

Paired lists must be nonempty and have equal lengths.

I = Σ[max(0,T_month)/5]^1.514; a = 6.75×10⁻⁷I³ − 7.71×10⁻⁵I² + 0.01792I + 0.49239; PET = 16(10T/I)^a(daylight/12)(days/30)

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
annualThornthwaiteHeatIndex	Annual thornthwaite heat index	number
thornthwaiteExponent	Thornthwaite exponent	number
monthlyThornthwaitePetMm	Monthly thornthwaite pet mm	list of number

json

{
  "input": {
    "monthlyMeanTemperaturesC": [
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20
    ],
    "monthlyDaylightHours": [
      12,
      12,
      12,
      12,
      12,
      12,
      12,
      12,
      12,
      12,
      12,
      12
    ],
    "monthlyDayCounts": [
      30,
      30,
      30,
      30,
      30,
      30,
      30,
      30,
      30,
      30,
      30,
      30
    ]
  },
  "output": {
    "annualThornthwaiteHeatIndex": 97.88137756782629,
    "thornthwaiteExponent": 2.1407477862616355,
    "monthlyThornthwaitePetMm": [
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049,
      73.86829548795049
    ]
  }
}

Limitations and assumptions

This temperature-based approximation excludes wind, humidity and radiation measurements. All-cold years validly return zero PET. Month lengths are checked for their numeric range, not against a particular calendar. Do not extrapolate this implementation above its temperature bound or treat PET as actual water use.

Numeric inputs and returned numeric values must be finite; an unrepresentable returned value produces an error. Numeric lists are limited to 100,000 entries; the rules above may require smaller limits. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Water Balance Surplus Deficit Classifier: Conserve sequential single-bucket soil water with precipitation, PET, actual evaporation, overflow surplus and unmet-demand deficit; no groundwater recharge/irrigation guarantee and all bucket processes are declared simplifications. For a complementary task, use Thornthwaite Moisture Index: Calculate named Thornthwaite Ih-0.6 Ia from caller precomputed monthly soil-water surplus/deficit and PET; not precipitation-minus-PET substituted for a full soil water budget.

References

Thornthwaite PET Monthly — tool.io

Similar or alternative tools

Water Balance Surplus Deficit Classifier
Conserve sequential single-bucket soil water with precipitation, PET, actual evaporation, overflow surplus and unmet-demand deficit; no groundwater recharge/irrigation guarantee and all bucket processes are declared simplifications.
Thornthwaite Moisture Index
Calculate named Thornthwaite Ih-0.6 Ia from caller precomputed monthly soil-water surplus/deficit and PET; not precipitation-minus-PET substituted for a full soil water budget.