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UBC Theses and Dissertations

Spatial patterns of humidity, fuel moisture, and fire danger across a forested landscape van der Kamp, Derek W.


Spatial variability in fuel moisture driven by changes in microclimate is an important bottom-up factor determining spatial wildfire behaviour, as fuel moisture impacts fire intensity, severity, and spread probability. However, few studies have examined how landscape scale patterns in near-surface microclimates impact fuel moisture patterns. This study quantified patterns of near-surface atmospheric conditions within a heterogeneous forested landscape, and determined how those patterns impact the spatial variability of fuel moisture and fire danger across the landscape. Observations across a forested landscape demonstrated that, in general, spatial variability in near-surface relative humidity and temperature was highest during dry, clear-sky conditions. However, daytime relative humidity was an exception, being relatively homogenous across the landscape and only weakly related to weather conditions. Canopy cover and above-canopy radiation load predicted a significant portion of the spatial patterns in relative humidity and temperature. Changes in canopy cover had the largest impact on near-surface conditions. Open sites saw higher relative humidity, on average, due to nocturnal longwave cooling. A novel fuel moisture model was presented that predicted between 76% and 93% of the variance in observations from independent sites or time periods, which is an improvement on a more complex model currently used operationally. This model was combined with meteorological observations to quantify spatial patterns in fuel moisture and potential fire danger across the landscape. Daytime fuel moisture and potential fire danger exhibited low spatial variability, regardless of weather conditions, and only 1-hour fuel moisture was related to canopy cover or radiation load. Fuel moisture and potential fire danger were more variable at night and that variability increased during cool, moist periods with low wind speeds. Patterns in fuel moisture and potential fire danger were dominated by differences in nocturnal longwave cooling due to changes in canopy cover. Open sites had lower daily mean potential fire danger. When fire danger was extrapolated over a larger study region, daytime conditions remained homogenous. Moreover, radiation load and canopy cover did not have a large enough direct influence on daytime fuel moisture to generate patches within the landscape that remain significantly wetter than the surrounding landscape.

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