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.

Description

Thornthwaite moisture indices compare annual water surplus and deficit with potential evapotranspiration.

Thornthwaite moisture indices compare annual water surplus and deficit with potential evapotranspiration. Surplus raises the moisture index, while deficit reduces it with the model's specified 0.6 weighting.^[1]

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.

When to use Thornthwaite Moisture Index

Use this tool to 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.

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 Moisture Index 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 monthlyWaterSurplusesMm, monthlyWaterDeficitsMm, monthlyPotentialEtMm using the field table and calculation rules, then run the tool.
Read thornthwaiteHumidityIndex, thornthwaiteAridityIndex, thornthwaiteMoistureIndex. 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
monthlyWaterSurplusesMm	Monthly water surpluses mm	list of number	Required
monthlyWaterDeficitsMm	Monthly water deficits mm	list of number	Required
monthlyPotentialEtMm	Monthly potential et mm	list of number	Required

Supply exactly twelve aligned nonnegative monthly surpluses, deficits and potential evapotranspiration values in mm. A month cannot have both a positive surplus and a positive deficit, and annual PET must be positive. The tool totals each list and computes the humidity, aridity and signed moisture indices from those totals.

Paired lists must be nonempty and have equal lengths.

humidity = 100Σsurplus/ΣPET; aridity = 100Σdeficit/ΣPET; moisture = humidity − 0.6 aridity

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

Output field	Meaning	Value type
thornthwaiteHumidityIndex	Thornthwaite humidity index	number
thornthwaiteAridityIndex	Thornthwaite aridity index	number
thornthwaiteMoistureIndex	Thornthwaite moisture index	number

json

{
  "input": {
    "monthlyWaterSurplusesMm": [
      20,
      20,
      20,
      20,
      20,
      20,
      0,
      0,
      0,
      0,
      0,
      0
    ],
    "monthlyWaterDeficitsMm": [
      0,
      0,
      0,
      0,
      0,
      0,
      10,
      10,
      10,
      10,
      10,
      10
    ],
    "monthlyPotentialEtMm": [
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20,
      20
    ]
  },
  "output": {
    "thornthwaiteHumidityIndex": 50,
    "thornthwaiteAridityIndex": 25,
    "thornthwaiteMoistureIndex": 35
  }
}

Limitations and assumptions

Water surpluses and deficits must be calculated consistently before using this tool; it does not reconstruct soil storage or actual evapotranspiration. The indices are numerical summaries rather than automatic climate-class assignments, and they inherit the limitations of the underlying water balance and PET model.

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 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.

References

Thornthwaite Moisture Index — 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 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.