Use the tabs in the calculator to compute several common drought/aridity metrics from precipitation, potential evapotranspiration (PET), temperature, and/or historical precipitation statistics.
Drought / Aridity Index Formulas
The calculator above provides several commonly used drought/aridity metrics. The formulas used are shown below.
\begin{aligned}
AI &= \frac{P}{PET}\\
IdM &= \frac{kP}{T+10}\\
SPI &\approx \frac{P-\mu}{\sigma}\\
KBDI_t &= KBDI_{t-1}-100R_{\text{net}}+dQ
\end{aligned}Variables:
- P is precipitation for the selected period (any depth unit, but keep units consistent); PET is potential evapotranspiration for the same period.
- T is mean air temperature for the selected period (in °C in the IdM formula); k is a period factor (k=1 annual, k=4 seasonal, k=12 monthly).
- μ is the historical mean precipitation and σ is the historical standard deviation (same timescale and units as P). KBDI is a 0–800 index (hundredths of an inch of soil moisture deficit); Rnet is effective rain in inches (max(0, R − 0.20 in)); dQ is the daily drought factor term in KBDI units.
To calculate one of these indices, choose the appropriate tab in the calculator, enter the required inputs for the same location and time period/timescale, and click Calculate. The calculator will also provide a commonly used classification for the computed value.
What is a Drought Index?
A drought index is a quantitative indicator used to describe anomalously dry (or wet) conditions over a chosen time period and region. Different indices use different inputs and have different interpretations—for example, SPI standardizes precipitation anomalies, SPEI standardizes a precipitation minus PET water balance, and PDSI uses a simplified soil‑water balance model driven by precipitation and temperature. The U.S. Drought Monitor is not a single numerical index; it is a weekly composite map/classification that synthesizes multiple indicators and expert assessment.
How to Calculate Drought Index?
The following steps outline how to calculate the selected drought/aridity index using the calculator above.
- Select the metric you want to compute: AI, IdM, SPI (z‑score approximation), or KBDI (daily).
- Enter the required inputs for the same location and time period/timescale, and select the appropriate units.
- For SPI, make sure the historical mean and standard deviation are for the same timescale and are in the same units as the current accumulation.
- Click Calculate to compute the value and view the associated classification.
- Use Reset to clear the fields for the current tab (or switch tabs to calculate a different index).
Example Problem :
Use the following variables as an example problem to test your knowledge (AI example: AI = P / PET).
Total precipitation for the period (P) = 400 mm
Potential evapotranspiration for the same period (PET) = 1000 mm
