Fine scale spatial variation in forest fuel moisture: Integrating the effects of terrain and forest structure with a distributed hydrology model

The moisture content of forest fuels is an important determinant of fire behavior and severity.  Thus, quantifying fuel moisture variability in space may be important for predictions of fire extent and effects.  The moisture content of fuel at any location is determined by local fluxes of water and energy.  These fluxes are influenced by the nature of the terrain (elevation, slope, aspect, landform shading) and forest structure (density, canopy cover).  Mountainous watersheds with high relief and forests with spatially variable structure are likely to exhibit heterogeneous patterns of fuel moistures.  I introduce a novel method for estimating temporal and spatial patterns in the fine scale (90m) variability of fuel moistures for large (200 - 500 km), forested mountain watersheds. Evapotranspiration calculations from a spatially explicit hydrology model integrate the effects of slope, aspect, terrain shading, temperature and precipitation gradients, and characteristics of forest structure on moisture flux.  Water deficit, the difference between evapotranspiration under saturated and estimated soil moisture conditions, is used as a proxy for fuel moisture in each pixel.  Late summer patterns of water deficit are strongly dependent on elevation and aspect.  Comparisons with remotely sensed methods of detecting water stress are made and the use of the technique in fire-climate modeling is discussed.