Predicting live herbaceous moisture from soil moisture and a drought index in the mountainous areas

Plant moisture content plays an important role in determining the availability of natural fuels for wildfires in Mediterranean ecosystems. While dead fuel moisture variation mainly depends exclusively on the weather conditions, live herbaceous moisture content is relatively more difficult to predict due to its seasonality in response to plants' physiological and phenological processes such as spring flushing and fall curing as well as soil water availability. Efforts have been made using some kind of indices that employ routinely measured meteorological parameters to model live plant moisture content, such as the Keetch-Byram drought index (Keetch and Byram 1968), the cumulative water balance index (Dennison and Roberts 2003), etc.

Our study aims to develop a model to accurately predict live herbaceous moisture content annual and seasonally variability in southern California areas where large wildfires often occur when the fuels are dry and the Santa Ana winds are strong. Our results indicate a strong correlation between live fuel moisture of the new growth Adenostoma Fasciculatum (Chamise) and soil moisture of 40-100cm depth layer that lagged by around 20-40 days depending on station elevations. Soil moisture in combination with Keetch-Byram drought index has shown to improve the prediction of the live fuel moisture annual and seasonal variations over a 10-year period for over 20 stations. The average coefficient of determination of the individual station models is as high as 0.72. The improved live fuel moisture model can work with other components of fuel moisture to help monitor fuel inflammability over southern California regions for fire danger assessment.