Palmer Drought Severity Index (PDSI)
Also known as: PDSI, Palmer Index, Palmer Drought Index, Self-Calibrating PDSI (sc-PDSI)
The Palmer Drought Severity Index (PDSI), developed by Wayne Palmer in 1965, was the first comprehensive water-balance drought index and remains a benchmark in drought monitoring. Rather than tracking precipitation alone, the PDSI runs a two-layer soil-moisture accounting that balances precipitation against evapotranspiration, runoff, and recharge to gauge whether the moisture supply is abnormally short for the prevailing conditions. It compares actual precipitation to the 'climatically appropriate for existing conditions' (CAFEC) precipitation, converts the departure into a standardized moisture anomaly, and accumulates it over time so that the index reflects the persistence and severity of drought, typically on a scale from about −4 (extreme drought) to +4 (extreme wetness). Because Palmer's original empirical constants were calibrated to particular U.S. regions and limited its spatial comparability, Wells, Goddard, and Hayes introduced the self-calibrating PDSI (sc-PDSI) in 2004, which derives those constants from local data and makes the index far more consistent across climates.
Key highlights
- Based on a physical soil-moisture water balance, so it represents accumulated moisture deficit and persistence rather than instantaneous precipitation.
- Provides a long-established, widely understood drought scale used operationally and for multi-decadal drought climatology.
- Conditions drought on existing moisture and climate via the CAFEC benchmark, accounting for both supply and demand.
- The self-calibrating variant makes the index spatially comparable by deriving its constants from local data.
Intuition
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How it works
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When to use it
Use the PDSI, and especially its self-calibrating form, when you need a water-balance-based measure of meteorological and soil-moisture drought that reflects accumulated deficits and persistence rather than instantaneous rainfall — for long-term drought monitoring, drought climatology, and studies of agricultural and hydrological drought where soil moisture matters. It is well suited to regional and continental monitoring on a monthly time step and to historical reconstruction, and it pairs naturally with precipitation-based indices for cross-comparison. It requires monthly precipitation and temperature plus an estimate of soil available water capacity. The index is less appropriate when a fixed, single time scale is too coarse for the question (its effective memory is roughly seasonal to annual, and it cannot be tuned the way multiscalar indices can), in regions where snow and frozen-soil processes dominate (which the simple model handles poorly), or when temperature-based potential evapotranspiration misrepresents demand. In those cases the multiscalar SPI or SPEI is often a better choice.
Strengths & limitations
- Based on a physical soil-moisture water balance, so it represents accumulated moisture deficit and persistence rather than instantaneous precipitation.
- Provides a long-established, widely understood drought scale used operationally and for multi-decadal drought climatology.
- Conditions drought on existing moisture and climate via the CAFEC benchmark, accounting for both supply and demand.
- The self-calibrating variant makes the index spatially comparable by deriving its constants from local data.
- The original PDSI's empirical constants were calibrated to specific U.S. regions, undermining comparability across different climates.
- It operates at a single, fixed effective time scale (roughly seasonal to annual) and cannot be adjusted to short or long horizons like multiscalar indices.
- Its simple two-layer soil model and reliance on temperature-based potential evapotranspiration handle snow, frozen soil, and energy-limited conditions poorly.
- There is autocorrelation and lag in the accumulated index, and the established-versus-tentative backtracking rules complicate near-real-time interpretation.
Common pitfalls
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Applications
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Frequently asked
What does a PDSI value mean?
The PDSI is a dimensionless index centered on zero, where zero is normal moisture for the location and season. Negative values indicate dryness and positive values wetness, with conventional categories of roughly −2 to −3 for moderate to severe drought and at or below −4 for extreme drought, and the mirror-image categories for wet spells. Because the index accumulates with memory, a given value reflects the cumulative state of a drought or wet spell rather than a single month's weather, so it changes gradually and persists until sustained moisture recovery occurs.
Why was the self-calibrating PDSI (sc-PDSI) developed?
Palmer fixed the constants governing the index's accumulation and standardization using data from a few U.S. regions, so the same numerical value does not represent equally rare conditions everywhere — extreme drought as measured by the original PDSI occurs more often in some climates than in others. Wells, Goddard, and Hayes's sc-PDSI replaces these fixed constants with values computed from each location's own record and rescales the index so that its extremes have consistent frequency across sites. The result keeps Palmer's water-balance logic but makes PDSI values genuinely comparable from place to place, which is essential for spatial and global analyses.
How does the PDSI compare with the SPI and SPEI?
All three are widely used drought indices, but they differ in formulation and flexibility. The PDSI is a water-balance index with a single, fixed effective time scale and built-in soil-moisture memory, making it good for accumulated, persistent drought but inflexible in horizon. The SPI is a simple, multiscalar standardization of precipitation that can be computed at many time scales but ignores temperature. The SPEI adds atmospheric demand to the SPI's multiscalar framework via the precipitation-minus-PET balance. In practice analysts often use the sc-PDSI alongside SPI and SPEI, exploiting the PDSI's soil-moisture realism and the standardized indices' tunable time scales.
Sources
- 1.Palmer, W. C. (1965). Meteorological Drought. Research Paper No. 45, U.S. Department of Commerce, Weather Bureau, Washington, DC, 58 p.
- 2.Wells, N., Goddard, S., & Hayes, M. J. (2004). A Self-Calibrating Palmer Drought Severity Index. Journal of Climate, 17(12), 2335-2351.
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Cite this page
ScholarGate. (2026, June 23). Palmer Drought Severity Index. ScholarGate. https://scholargate.app/disaster-studies/palmer-drought-severity-index