High resolution spatial and temporal variability of fine dead fuel moisture content in complex terrain

Abstract

The moisture content of fine dead fuel plays an important role in forest fire behaviour, affecting the probability of ignition at the fire front, the probability of night-time extinguishment, and the availability of the depth of fine fuel for burning. A recent review of dead fuel moisture research by Matthews (2014) concluded that one of the key research and modeling needs is the capacity to represent the complexity of vegetation structure and topography and forecast fuel moisture content across the landscape. Fine fuel moisture content varies at a range of spatial scales due to many factors, however in complex, steep and dissected landscapes, topographic aspect can play a significant role in small-scale (ie. scales in the order of 10’s ha) variability. Experimental sites for monitoring microclimate variables and moisture content in litter and in near-surface soils were established at a control site and on four contrasting aspects (north, south, east and west) in southeast Australia. At each of the four microclimate sites sensors are arranged to measure the soil moisture (2 replicates), fine dead surface fuel moisture at 2.5cm depth (12 replicates), precipitation throughfall (3 replicates), radiation (3 replicates), and screen level relative humidity, air temperature, leaf wetness, and wind speed (1 replicate of each). Temperature and relative humidity are also measured within the dead fine surface fuel using Ibutton’s (4 replicates). All measurements are logged continuously at 15 min intervals. The moisture content of the fine dead surface fuel is estimated using high-replication of low-cost continuous soil moisture sensors placed at the centre of a 5cm deep sample of fine dead surface fuel, referred to here as “dead fine fuel packs” . The dead fine fuel packs were constructed from fuels collected from the area surrounding the microclimate site. The initial results show the moisture regime on the forest floor was highly sensitive to the incoming shortwave radiation, which was up to 6 times higher in the north-facing (equatorial) slopes due to slope orientation and the sparse vegetation compared to vegetation on the south-facing (polar facing) slopes. Differences in shortwave radiation resulted in peak temperatures within the litter that were up to 2 times higher on the equatorial-facing site than those on the polar-facing site. For instance, on a day in November 2013 with maximum open air temperature of 35o C, the temperatures within the litter layer at the north-facing and south-facing sites were 54o C and 32o C, respectively, despite air temperature at the two sites differing by less than 2o C. The minimum gravimetric water content in the litter layer on the same day was 21% on the equatorial-facing slope and 85% on the polar-facing slope. The experimental data is being used to calibrate and test models of fuel moisture spatial variability against an independently collected gravimetric fuel moisture dataset.