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 influence 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.

细小的地表垃圾燃料的燃料水分含量 (FMC) 在确定着火和蔓延方面起着决定性作用。了解决定垫料水分动态的物理过程对于准确预测垫料 FMC 是必要的。众所周知，蒸汽交换、沉淀和潜热传递是控制凋落物 FMC 变化的主要因素。然而，尽管假定土壤水分对凋落物补水很重要，但对土壤-凋落物界面的物理过程的研究有限。在这项研究中，我们在澳大利亚堪培拉的干湿场地进行了因子控制的田间试验，研究了土壤水分对凋落物 FMC 的影响。每个地点有两种处理方式：土壤和凋落物接触和不接触。垃圾 FMC，结合燃料湿度、温度和相对湿度传感器，自动连续测量地表和地下垃圾的温度和相对湿度。两种处理在干湿地表现出土壤水分对凋落物 FMC 的不同影响。在干燥场地，从土壤到凋落物的水分输送有限，几乎可以肯定是蒸汽通量占主导地位。在这里，土壤水分对凋落物 FMC 的影响是通过其对凋落物层局部湿度的影响。然而，在湿地，除了来自土壤的蒸汽通量外，还会发生毛细管流。毛细管流动响应土壤和凋落物之间的基质势梯度。测量结果证实，地下垃圾与土壤的耦合比地表垃圾更强烈，这与大气的耦合更强。对控制水分从土壤到凋落物输送的物理过程的更好理解应该有助于更准确地预测凋落物 FMC，尤其是在潮湿土壤条件下。需要进一步研究将土壤-凋落物相互作用纳入凋落物 FMC 模拟。