@article {bnh-8355, title = {Representing vapour and capillary rise from the soil improves a leaf litter moisture model}, journal = {Journal of Hydrology}, volume = {612}, year = {2022}, month = {06/2022}, abstract = {

Litter moisture content plays a critical role in fire danger rating systems and forest ecosystems. Soil moisture content has been proposed to affect litter moisture due to vapour flux and capillary rise from the soil. However, few models consider soil water content when predicting litter moisture, and to our knowledge, no model includes capillary fluxes. In this study, we represented soil moisture coupling in a physics-based litter moisture prediction model by describing the vapour and capillary fluxes from the soil to litter. We aimed to evaluate if litter moisture predictions can be improved by incorporating the hydrological process at the soil-litter interface and explored the possible role of soil moisture in litter moisture simulations. Three model versions were compared against observations at a dry and wet experimental site in Australia: the original physics-based model, a model version extended with soil vapour flux only, and a version that incorporates both soil vapour flux and capillary rise. The simulation results suggest that soil moisture considerably influences litter moisture through soil vapour flux and capillary rise, which can lead to more than 10\% of oven-dried weight, particularly under wet soil conditions. The corresponding model showed the best performance in comparison with field observations. The contribution of upward soil moisture fluxes was small after long dry and warm periods but noticeable during more moist periods. Further research is needed to evaluate the revised model for a broader range of weather, soil and litter conditions.

}, keywords = {capillary rise, fuel moisture content, leaf litter, Soil moisture, vapour flux}, doi = {https://doi.org/10.1016/j.jhydrol.2022.128087}, url = {sciencedirect.com/science/article/abs/pii/S002216942200662X}, author = {Li Zhao and Marta Yebra and Albert van Dijk and Geoffrey J. Cary} } @article {bnh-8100, title = {Continental-scale prediction of live fuel moisture content using soil moisture information}, journal = {Agricultural and Forest Meteorology}, volume = {307}, year = {2021}, month = {06/2021}, abstract = {

Live fuel moisture content (LFMC) is a key factor that determines the flammability of vegetation in ecosystems. Soil moisture (SM) is one of the variables that is known to influence plant water use. The present study analyses the LFMC-SM relationship over Australia using gridded, remote sensing-based LFMC and land surface model-based SM products. A lag-correlation analysis conducted over 60 selected sites shows that the strength of the relationship between LFMC and SM varies from site to site and, in general, is moderately strong (median lag-correlation of ~0.5). However, the strength of the relationship changes with vegetation type and also with soil profile depth. At all the sites, SM is found to be a leading indicator of LFMC. The lag also varies with the location and is found to range from days to months. Based on the location-based correlation analysis, we identify the 0-35 cm SM profile (SM0-35cm) to be the best predictor of LFMC. We developed a simple model to predict daily LFMC, where it is hypothesised that daily variations in LFMC from its annual cycle can be predicted using daily deviations from the annual cycle in SM0-35cm. The annual cycles of LFMC and SM0-35cm are modelled using Fourier cosine series. The averaged (over 60 sites) correlation obtained for the validation period is 0.74 when a time-lag of 14 days is assumed at all locations. When the model is applied nationally at a 5 km grid, the normalised root mean squared error for the validation period is found to be less than 25\% in general. The results from the present study highlight a modelling strategy that can be used to address a critical gap in the forecast of spatially and temporally continuous LFMC at regional scales in advance for operational fire management applications.

}, keywords = {AFMS, Fire, JASMIN, JULES, live fuel moisture, Soil moisture}, doi = {https://doi.org/10.1016/j.agrformet.2021.108503}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0168192321001866}, author = {Vinod Kumar and Imtiaz Dharssi and Marta Yebra and Paul Fox-Hughes} } @article {bnh-7761, title = {The influence of soil moisture on surface and sub-surface litter fuel moisture simulation at five Australian sites}, journal = {Agricultural and Forest Meteorology}, volume = {298-299}, year = {2021}, month = {03/2021}, abstract = {

Fuel moisture content (FMC) of the litter is a critical factor determining fire ignition and spread and is an important input for most fire behaviour prediction models. Several models, ranging from empirical regression functions to physics-based models, have been developed to forecast litter FMC. Soil moisture below the litter layer has been shown to influence litter FMC, but few models explicitly consider its effect. This study aimed to evaluate how soil moisture content may affect litter FMC by coupling soil moisture as a boundary condition to the physics-based {\textquoteleft}Koba{\textquoteright} model, which simulates radiation, energy and moisture fluxes in the surface and subsurface litter layer. The coupled model was tested at five sites in Victoria, Australia, where litter FMC values were recorded continuously during 2014-2015 using calibrated fuel stick sensors. Two versions of the model were compared against the observations: an uncoupled model and a model version accounting for the vapour flux from soil to litter. The simulation results show that the influence of soil moisture depends on environmental conditions. Soil moisture appeared to have a minor influence on FMC when both soil and litter are dry, but a stronger influence when the soil is relatively wet. Correspondingly, the coupled model explained observed FMC better than the uncoupled model for the two wetter study sites. As expected, the subsurface litter in contact with the soil appeared more sensitive to soil moisture conditions than the surface litter. The influence of soil vapour flux on litter FMC can be considerable during the transition from wet to dry litter and soil conditions. This has implications for hazard reduction burning, which is typically planned to take advantage of transitional fuel moisture conditions. Further research is needed to understand the influence of the structure and thickness of litter on the importance of soil vapour flux.

}, keywords = {Coupled model, Dead fuel, forecast, fuel moisture content, Soil moisture}, doi = {https://doi.org/10.1016/j.agrformet.2020.108282}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0168192320303841?dgcid=coauthor}, author = {Li Zhao and Marta Yebra and Albert van Dijk and Geoffrey J. Cary and Stuart Matthews and Gary J. Sheridan} } @article {bnh-7470, title = {Exploring the soil moisture-live fuel moisture relationship}, number = {624}, year = {2020}, month = {10/2020}, institution = {Bushfire and Natural Hazards CRC}, address = {MELBOURNE}, abstract = {

Live fuel moisture content is a key factor that determines the flammability of vegetation in ecosystems. Prediction of live fuel moisture content is inherently a very difficult problem since it is modulated by the complex physiological, phenological and ecological processes characteristic of the plant species. Soil moisture is one of the key variables that is known to influence plant water use. Recently, a new live fuel moisture content near-real-time product has been developed for Australia using a radiative transfer model inversion technique on the MODerate Resolution Imaging Spectroradiometer reflectance data. This live fuel moisture content product forms the basis of the Australian Flammability Monitoring System. At the same time, an advanced soil moisture analysis system has been developed by the Bureau of Meteorology recently, called the Joint United Kingdom Land Environment Simulator based Soil Moisture Information (JASMIN). JASMIN can estimate soil moisture at 5 km resolution on a daily timestep for the whole of Australia.

The present study brings together the above two products and explores the live fuel moisture content{\textendash}soil moisture relationship on a national scale. This study will report the preliminary work carried out in understanding live fuel moisture content{\textendash}soil moisture relationship and suggests an approach that may be constructive in advancing the ability to predict live fuel moisture content reliably to support fire management. A preliminary analysis is being conducted over 60 selected locations where JASMIN is found to have good skill. These 60 sites together represent a range of land cover types and climate zones typical of the Australian landscape. All the possible soil moisture profiles that can be derived from the four JASMIN soil layers are used for the analysis. Lag-correlation analysis shows that the strength of the relationship between live fuel moisture content and soil moisture varies from site to site and in general, is moderately strong (median lag-correlation of ~0.5). However, the strength of the relationship varies with vegetation type and also with soil profile depth. At all the sites, soil moisture is found to be an (important) leading indicator of live fuel moisture content. The lag also varies with the location and is found to range from days to months. Except for the forested sites, the top two soil layers exhibit a higher correlation with live fuel moisture content compared to the deeper layers. We develop a simple model to predict live fuel moisture content. The model is found to have good skill with an average R2 of 0.64 over the 60 sites. The normalized root mean square error for the model prediction is found to be less than 25\% in general.

}, keywords = {live fuel moisture, Soil moisture}, issn = {624}, author = {Vinod Kumar and Imtiaz Dharssi and Marta Yebra and Paul Fox-Hughes} }