Drought Indices Track Moisture in Fuels That a Single Rainfall Cannot Wet
Daily weather explains fine fuels; seasons explain heavy ones. Drought indices accumulate the balance of evaporative demand against rainfall over weeks and months to estimate moisture in deep litter, duff and large woody fuels. The Keetch-Byram Drought Index runs from \( 0 \) to \( 800 \), in hundredths of an inch of soil-moisture deficit, dropping with rain above a small interception threshold and rising with daily maximum temperature, scaled by annual rainfall for the region. The Canadian system's Drought Code plays the same role with a nominal timelag near \( 52 \) days. High values do not make ignition more likely — fine fuels decide that — but they make the fire that starts burn deeper, resist containment, hold over and rekindle. The confusion this resolves is expecting a rain event to reset a drought index; a shower resets the fine fuels and barely moves the deep ones. After this Concept you can separate a landscape's short-term ignition risk from its seasonal capacity to sustain a long-burning fire.
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Daily weather explains fine fuels; seasons explain heavy ones. Drought indices accumulate the balance of evaporative demand against rainfall over weeks and months to estimate moisture in deep litter, duff and large woody fuels. The Keetch-Byram Drought Index runs from \( 0 \) to \( 800 \), in hundredths of an inch of soil-moisture deficit, dropping with rain above a small interception threshold and rising with daily maximum temperature, scaled by annual rainfall for the region. The Canadian system's Drought Code plays the same role with a nominal timelag near \( 52 \) days. High values do not make ignition more likely — fine fuels decide that — but they make the fire that starts burn deeper, resist containment, hold over and rekindle. The confusion this resolves is expecting a rain event to reset a drought index; a shower resets the fine fuels and barely moves the deep ones. After this Concept you can separate a landscape's short-term ignition risk from its seasonal capacity to sustain a long-burning fire.
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