Modeling seasonal changes in live fuel moisture and equivalent water thickness using a cumulative water balance index

Live fuel moisture, an important determinant of fire danger in Mediterranean ecosystems, exhibits seasonal changes in response to soil water availability. Both drought stress indices based on meteorological data and remote sensing indices based on vegetation water absorption can be used to monitor live fuel moisture. In this study, a cumulative water balance index (CWBI) for a time series spanning 1994-1997 and 1999-2001 was compared to field measured live fuel moisture and to equivalent water thickness (EWT) calculated from remote sensing data. A sigmoidal function was used to model the relationships between CWBI, live fuel moisture, and EWT. Both live fuel moisture and EWT reach minima at large CWBI deficits. Minimum and maximum live fuel moisture, minimum and maximum EWT, and the modeled inflection points of both live fuel moisture and EWT were found to vary with vegetation type. Modeled minimum and maximum EWT were also found to vary with vegetation biomass. Spatial variation in modeled EWT inflection points may be due to vegetation type and to local variation in soil moisture. Based on their temporal and spatial attributes, CWBI and EWT offer complimentary methods for monitoring live fuel moisture for fire danger assessment.