Modelling the Combustion of Mediterranean Shrubs with FDS and WFDS: Sensitivity to Fuel Moisture Content

• Below 50% FMC, both models accurately predict experimental heat release and mass loss. • FDS accurately reproduces the ignition threshold (85%). • WFDS tends to overestimate combustion at moisture contents above 50%. Understanding how live vegetation burns is essential for predicting the behaviour of fires in WUI areas, where homes and vegetation coexist. Fuel moisture content plays a major role in determining ignition and fire spread. However, its influence remains difficult to represent accurately in numerical models. This study evaluates and compares the performance of two CFD models, Fire Dynamics Simulator ( FDS ) and the Wildland-Urban Interface Fire Dynamics Simulator ( WFDS ), in reproducing the combustion of Cistus monspeliensis shrubs ignited by a fire spreading across a wood wool litter on a laboratory scale. Controlled burning experiments conducted under a large-scale heat release ( LSHR ) calorimeter were used as reference data. Simulations were performed for foliar moisture contents ranging from 10 % to 100 %. For foliar moisture contents below 50 %, both FDS and WFDS accurately reproduced the experimental heat release rates and mass loss. WFDS overestimated combustion intensity at leaf moisture contents between 55 % and 85 %, while FDS underestimated the burning duration. Experimentally, the shrubs did not ignite for moisture contents greater than 85 %. FDS correctly reproduces this trend, whereas WFDS still predicts shrub combustion even at high moisture contents. In summary, the FDS model produced results that were closer to the experimental observations, capturing the ignition threshold and influence of the foliar moisture content. However, it does not predict accurately the propagation threshold.