Keetch-Byram Drought Index
Also known as: KBDI, drought severity index
The Keetch-Byram Drought Index (KBDI) is a cumulative drought severity index used in fire danger rating systems to track long-term soil moisture depletion and drying trends. Developed in 1968 by Keetch and Byram, KBDI integrates daily temperature, precipitation, and prior drought state to produce a continuous index ranging from 0 (no drought, moist soil) to 800 (severe drought, very dry soil). KBDI is widely used in fire danger prediction and fire behavior modeling because soil moisture is a major driver of fuel drying and flammability.
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When to use it
Use KBDI in fire danger prediction and fire behavior modeling, especially for rating long-term drought severity and deep-fuel drying. It is most effective in predicting fire season onset and intensity trends. Less useful for moment-to-moment fire behavior prediction (use fuel moisture and weather indices instead). Widely used operationally across North America for fire danger watches and advisories.
Strengths & limitations
- Cumulative index that captures seasonal drought trends and long-term soil drying
- Requires only common weather data (temperature, precipitation) available from networks or forecasts
- Operational standard: integrated into National Fire Danger Rating System (NFDRS) and fire incident command systems
- Lagged response to drought reflects moisture dynamics in deep soils and dead fuels
- Dimensionless, standardized scale (0–800) enables comparison across regions
- Strong empirical correlation with fire occurrence and burn area in many regions
- Regional parameterization needed; coefficients vary by climate and soil type
- Does not capture spatial heterogeneity in soil type or topography that affects soil moisture
- Assumes a single homogeneous soil reservoir; ignores layering and lateral water movement
- Vegetation type differences in rooting depth and water uptake are not represented
- KBDI does not directly measure fuel moisture; requires separate models to translate to live and dead fuel moisture
Frequently asked
What is the difference between KBDI and fuel moisture?
KBDI is a long-term drought severity index tracking cumulative soil water depletion over weeks to months. Fuel moisture (1-hour, 10-hour, 100-hour timelag fuels) varies on hourly to daily timescales and is estimated from current weather (humidity, temperature). KBDI influences fuel moisture but does not equal it; high KBDI creates conditions for low fuel moisture, but the actual moisture depends on recent weather.
Why does KBDI have a maximum of 800?
The maximum of 800 is conventional, chosen to represent a severely desiccated soil state that rarely occurs in practice. Values above 700–750 are rare except in extreme multiyear droughts. The 0–800 scale provides good granularity for fire danger ratings.
How does precipitation affect KBDI?
Only infiltrating rainfall reduces KBDI. Light showers (< 0.1 inches) evaporate quickly and do not reduce KBDI; substantial rain (> 0.2 inches) begins recharging the soil and reducing KBDI. The amount of KBDI reduction is proportional to rainfall depth. This lag in response reflects the time soil water takes to permeate downward.
Can I use KBDI from one state or region in another?
No; KBDI parameters (drying rates, recharging coefficients) are calibrated for regional soil and climate conditions. Using KBDI from one region in another without recalibration produces biased predictions. Each region should develop or validate its own KBDI parameterization.
Sources
How to cite this page
ScholarGate. (2026, June 3). Keetch-Byram Drought Index. ScholarGate. https://scholargate.app/en/forestry/keetch-byram-drought-index
Which method?
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