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* Metric Name: Mean Percent FRI Departure, Since 1908  

* Tier: 3  

* Data Vintage: 2022. Includes disturbances through the end of 2022.  

* Unit Of Measure: Percent  

* Metric Definition and Relevance: This metric, mean percent FRID, is a measure of the extent to which contemporary fires (i.e., since 1908) are burning at frequencies similar to the frequencies that occurred prior to Euro-American settlement, with the mean reference FRI as the basis for comparison. Mean PFRID is a metric of fire return interval departure (FRID) and measures the departure of current FRI from mean reference FRI in percent.  

* Creation Method: The current FRI is calculated by dividing the number of years in the fire record (e.g., 2019-1908=112 years inclusive) by the number of fires occurring between 1908 and the current year in a given polygon plus one (CurrentFRI = Number of years/Number of fires +1). The mean reference FRI is an approximation of how often, on average, a given PFR likely burned in the three or four centuries prior to significant Euro-American settlement. This measure does not return to zero when a fire occurs, unlike FRID values used in some other analyses (e.g., NPS FRID Index). Instead, the following formulas are used to calculate Mean PFRID:

When current FRI is longer than reference FRI (the common condition in most
coniferous PFRs) the formula is:

 _[1-(MeanRefFRI/CurrentFRI)]*100_

When current FRI is shorter than reference FRI (common in some shrub dominated
PFRs, and areas in the Wildland Urban Interface) the formula is:

_-{[1-(CurrentFRI/MeanRefFRI)]}*100_

For areas dominated by PFRs with a mean reference FRI greater than 112 years,
and that have not burned in the period of historical record considered in this
analysis (i.e., since 1908), the FRID is assumed to equal zero.

  

* Credits: Fire History (2022), CAL FIRE \--Department of Forestry and Fire Protection Existing Vegetation (CALVEG 2011), USDA Forest Service, Region 5, MARS Team \--Region 5 - Pacific Southwest Region