Using surface renewal analysis to develop a fire risk index

Plant water status is a major factor influencing burning potential of vegetation and a low-cost method to assess the water status can provide an index for fire potential. It can also reduce costs associated with travel to remote locations and can improve fire forecast models. Under severe water stress, plant stomata close and reduce the actual evapotranspiration rate (ETa) relative to the potential evapotranspiration rate (ETp). A fire potential index (I=1 - ETa/ETp) is a measure of vegetation burning potential. When there is no ET reducing water stress, I=0 and it increases as the ETa rate decreases relative to ETp. The difficulty in the application of a fire index is the cost and complexity to measure or estimate ETa and ETp. From solar radiation (Rs) measurements and site specific calibration, it is possible to estimate net radiation (Rn) and soil heat flux density (G). If a good estimate of sensible heat flux density is available, then latent heat flux density (LE) can be calculated as the residual of the energy balance equation (LE=Rn - G - H). The surface renewal (SR) method for estimating sensible heat flux from canopies provides a simple, portable, robust, and low-cost method to measure sensible heat flux density (H). High frequency temperature data are collected with fine-wire thermocouples. The data are analyzed with a structure function to identify average ramp characteristics (i.e., amplitude and duration) of the temperature traces during a sampling period. Then the amplitude and duration are used in a conservation of energy equation to estimate H. This method has been used over a wide variety of crops and natural vegetation with good results. Recently, the method was tested over grass in a wildfire-prone mixed oak-grassland region of the Sierra Nevada Mountain foothills in California. In this paper, the methodology to measure H with the SR method and the results of the fire index calculations will be reported.