Accurate Quantification of Seasonal Rainfall and Associated Climate–Wildfire Relationships

Abstract

Wildfires are often governed by rapid changes in seasonal rainfall. Therefore, measuring seasonal rainfall on a temporally finescale should facilitate the prediction of wildfire regimes. To explore this hypothesis, daily rainfall data over a 58-yr period in south-central Florida were transformed into cumulative rainfall anomalies (CRAs). This transformation allowed precise estimation of onset dates and durations of the dry and wet seasons, as well as a number of other variables characterizing seasonal rainfall. These variables were compared with parameters that describe ENSO and a wildfire regime in the region (at the Avon Park Air Force Range). Onset dates and durations were found to be highly variable among years, with standard deviations ranging from 27 to 41 days. Rainfall during the two seasons was distinctive, with the dry season having half as much as the wet season despite being nearly 2 times as long. The precise quantification of seasonal rainfall led to strong statistical models describing linkages between climate and wildfires: a multiple-regression technique relating the area burned with the seasonal rainfall characteristics had an R 2 adj of 0.61, and a similar analysis examining the number of wildfires had an R 2 adj of 0.56. Moreover, the CRA approach was effective in outlining how seasonal rainfall was associated with ENSO, particularly during the strongest and most unusual events (e.g., El Niñ o of 1997/98). Overall, the results presented here show that using CRAs helped to define the linkages among seasonality, ENSO, and wildfires in south-central Florida, and they suggest that this approach can be used in other fire-prone ecosystems.

Dendrochronological Field Methods for Fire History in Pine Ecosystems of the Southeastern Coastal Plain

“…Long-term averages depicted in Figure 3 indicate a pronounced wet season that typically is initiated in May/June and extends through September, and a longer dry season typically extends from October/November through April. Rainfall in the dry season has been influenced by ENSO (Slocum et al 2010a). Transitions typically occur in May (dry to wet) and October (wet to dry).…”

“…Transitions typically occur in May (dry to wet) and October (wet to dry). Thunderstorms that produce lightning are frequent from May to August, but are rare during the transition from wet to dry season and thereafter (Figure 3) , Slocum et al 2010a). …”

“…There is little lightning during most of the earlywood growth period before the onset of the wet season. During the final period of earlywood growth, when the season's first lightning occurs and becomes frequent, the landscape is at its driest and has the greatest potential for fires to spread and burn large areas (Slocum et al 2010a & b).…”

“…Lightning is at peak levels of frequency. The combination of these two elements of dryness and lightning make this period the most likely time for lightning-ignited fires (Slocum et al 2010a & b).…”

“…Because of this it is possible to use our fire scar position system to infer whether past fires occurred during the climatically-driven fire season when lightning-caused ignitions are frequent (our Transition designation), or during times when lightning is absent or very infrequent and any ignitions would more likely have been human-caused (the Early, Late and Dormant designations). We also can relate the seasonal occurrence of historical fires to the wet/dry season seasonal cycles that exert strong influence on likelihood of fire spread in savanna landscapes of central and southern Florida (e.g., Beckage et al 2003, 2010a.…”