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Calcimator

Drought Index Calculator

Estimate a Palmer Drought Severity Index (PDSI) category from temperature, precipitation, and soil moisture data. Includes water balance analysis and impact assessment.

Inputs

°F
mm
mm
°F
mm

Results

PDSI Estimate

-3.1

Drought Category

Severe Drought

Precip Departure-40 mm
Precip Ratio (actual/normal)0.5
Potential ET22.8 mm
Water Deficit0 mm
Vegetation ImpactMajor crop losses, high wildfire risk
Water Supply OutlookCritical — mandatory restrictions likely
Normal Pet20.4
How to Use This Calculator
  1. Enter Average Temperature (°C or °F) and Total Precipitation for the period being assessed.
  2. Input Normal Precipitation and Normal Temperature for the same period at the same location (long-term averages).
  3. Set Soil Moisture Capacity (mm) — the water-holding capacity of the local soil type.
  4. Review PDSI Estimate and Drought Category: 0 to -1 = abnormally dry, -2 = moderate, -3 = severe, -4+ = extreme drought.
  5. Use Precip Departure and Precip Ratio to contextualize how far current conditions deviate from normal.

How the result changes with Average Temperature

Average TemperaturePDSI EstimateDrought Category
-4.5-2.5Moderate Drought
9.25-2.5Moderate Drought
26-2.8Moderate Drought
40-4.9Extreme Drought

What each input means

Average Temperature
Mean temperature for the period in °C. Higher temperatures increase evapotranspiration demand.
Total Precipitation
Total observed precipitation for the period in millimeters.
Normal Precipitation
Climatological normal (30-year average) precipitation for this period in mm.
Normal Temperature
Climatological normal (30-year average) temperature for this period in °C.
Soil Moisture Capacity
Available water capacity of the soil in mm. Sandy soils ~100 mm, loam ~200 mm, clay ~300 mm.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Average Temperature = 28, Total Precipitation = 40, Normal Precipitation = 80, Normal Temperature = 24 = 5 input(s) provided
  2. Calculate PDSI Estimate
    PDSI Estimate
    -3.1 = -3.1
  3. Calculate Drought Category
    Drought Category
    Severe Drought = Severe Drought
  4. Calculate Precip Departure
    Precip Departure
    -40 = -40
  5. Calculate Precip Ratio
    Precip Ratio
    0.5 = 0.5

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