Controlled field experiment clarifies the influence of soil moisture on litter moisture content

Fuel moisture content (FMC) of fine surface litter fuel plays a decisive role in determining fire ignition and spread. Understanding physical processes that determine litter moisture dynamics is necessary to accurately predict litter FMC. It is known that vapour exchange, precipitation and latent heat transfer are dominant factors governing changes in litter FMC. However, limited research has been undertaken on physical processes at the soil-litter interface, despite the presumed importance of soil moisture in hydrating litter. In this study, we investigated the influence of soil moisture on litter FMC in factorial controlled field experiments at a dry and a wet site in Canberra, Australia. Each site had two treatments: with and without contact between soil and litter. Litter FMC, temperature and relative humidity were automatically and continuously measured in both surface and subsurface litter with a combination of fuel moisture, temperature and relative humidity sensors. The two treatments showed a different influence of soil moisture on litter FMC at the dry and wet site. There was limited moisture transport from soil to litter at the dry site, almost certainly dominated by vapour flux. Here, the in-fluence of soil moisture on litter FMC is through its influence on local humidity in the litter layer. However, at the wet site capillary flow occurred in addition to vapour flux from the soil. The capillary flow responded to the matric potential gradient between soil and litter. The measurements confirmed that subsurface litter was more strongly coupled to the soil than the surface litter, which was more strongly coupled with the atmosphere. The improved understanding of physical processes governing water transport from soil to litter should help predict litter FMC more accurately, especially under wet soil conditions. Further research is needed to incorporate soil-litter interactions in litter FMC simulations.