UNEP Aridity Index P over PET

Compute annual UNEP P/PET from matched monthly normals and show seasonal nonnegative climate deficits; no fixed aridity classification or inference of actual crop water stress.

Description

The UNEP-style aridity ratio compares annual precipitation with annual potential evapotranspiration.

The UNEP-style aridity ratio compares annual precipitation with annual potential evapotranspiration. Monthly differences provide a separate view of seasonal atmospheric water-demand shortfalls.^[1]

Compute annual UNEP P/PET from matched monthly normals and show seasonal nonnegative climate deficits; no fixed aridity classification or inference of actual crop water stress.

When to use UNEP Aridity Index P over PET

Use this tool to compute annual UNEP P/PET from matched monthly normals and show seasonal nonnegative climate deficits; no fixed aridity classification or inference of actual crop water stress.

Prepare the climate observations and independently chosen model parameters. Confirm units, daily coverage, chronological order and the intended seasonal window.

How to use UNEP Aridity Index P over PET and understand its results

Prepare the climate observations and independently chosen model parameters. Confirm units, daily coverage, chronological order and the intended seasonal window.
Enter monthlyPrecipitationMm, monthlyPotentialEtMm using the field table and calculation rules, then run the tool.
Read annualUnepAridityRatio, monthlyWaterDeficitsMm. 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
monthlyPrecipitationMm	Monthly precipitation mm	list of number	Required
monthlyPotentialEtMm	Monthly potential et mm	list of number	Required

Supply exactly twelve aligned nonnegative monthly precipitation and PET values in mm. Annual PET must be positive. The tool divides annual totals and returns nonnegative deficits for each month; wetter months do not contribute negative deficits.

Paired lists must be nonempty and have equal lengths.

aridity ratio = Σ precipitation/Σ PET; monthly deficit = max(0,PET−precipitation)

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

Output field	Meaning	Value type
annualUnepAridityRatio	Annual unep aridity ratio	number
monthlyWaterDeficitsMm	Monthly water deficits mm	list of number

json

{
  "input": {
    "monthlyPrecipitationMm": [
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25
    ],
    "monthlyPotentialEtMm": [
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50
    ]
  },
  "output": {
    "annualUnepAridityRatio": 0.5,
    "monthlyWaterDeficitsMm": [
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25,
      25
    ]
  }
}

Limitations and assumptions

No category boundaries are built in. These climate deficits omit soil storage and are not actual crop stress or irrigation demand. PET and precipitation must represent matching periods.

Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. 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 De Martonne Aridity Index: Compute annual De Martonne P/(T+10) from twelve monthly normals, with explicitly positive shifted temperature; not a calibrated drought category or validity at cold singular temperatures. 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

UNEP Aridity Index P over PET — tool.io

Similar or alternative tools

De Martonne Aridity Index
Compute annual De Martonne P/(T+10) from twelve monthly normals, with explicitly positive shifted temperature; not a calibrated drought category or validity at cold singular temperatures.
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.