Establishing Relationships between Drought Indices and Wildfire Danger Outputs: A Test Case for the California-Nevada Drought Early Warning System

Abstract

Relationships between drought indices and fire danger outputs are examined to (1) incorporate fire risk information into the National Integrated Drought Information System California–Nevada Drought Early Warning System and (2) provide a baseline analysis for application of drought indices into a fire risk management framework. We analyzed four drought indices that incorporate precipitation and evaporative demand (E0) and three fire indices that reflect fuel moisture and potential fire intensity. Seasonally averaged fire danger outputs were most strongly correlated to multi-scalar drought indices that use E0 (the Evaporative Demand Drought Index (EDDI) and the Standardized Precipitation Evapotranspiration Index (SPEI)) at approximately annual time scales that reflect buildup of antecedent drought conditions. Results indicate that EDDI and SPEI can inform seasonal fire potential outlooks at the beginning of summer. An E0 decomposition case study of conditions prior to the Tubbs Fire in Northern California indicate high E0 (97th percentile) driven predominantly by low humidity signaled increased fire potential several days before the start of the fire. Initial use of EDDI by fire management groups during summer and fall 2018 highlights several value-added applications, including seasonal fire potential outlooks, funding fire severity level requests, and assessing set-up conditions prior to large, explosive fire cases.

Projected Changes in Reference Evapotranspiration in California and Nevada: Implications for Drought and Wildland Fire Danger

“…and destructive wildfires both historically and recently (McEvoy et al, 2019). These regions also provide surface water to large populations and/or major regional agricultural areas, making them ideal for studying long-term hydrologic drought.…”

“…Evaporative demand ( E 0 )—the upper limit of actual evapotranspiration ( ET ) that could occur given unlimited surface water supply (Hobbins et al, 2017)—has a strong connection to drought and wildfire potential in the western United States (e.g., Abatzoglou & Kolden, 2013; Abatzoglou & Williams, 2016; Littell et al, 2016; McEvoy et al, 2016) and globally (e.g., Dai, 2011; Vicente‐Serrano et al, 2010). In California and Nevada, elevated E 0 contributed to the 2012–2016 drought's severity (Hobbins et al, 2016; McEvoy et al, 2016; Shukla et al, 2015; Williams et al, 2015) and to wildfire potential (Brown et al, 2020; McEvoy et al, 2019; Nauslar et al, 2019). Although E 0 is sometimes calculated from temperature alone, a physically based E 0 formulation is critical to obtaining realistic estimates that include not only temperature changes but also the wind speed, humidity, and incoming shortwave radiation components that drive land surface‐atmosphere interactions and drying (Hidalgo et al, 2005; Hobbins et al, 2017).…”

“…Extreme EDDI values (greater than 95th percentile) have been found to occur simultaneously with start dates of recent large and destructive wildfires in California (Brown et al, 2020; McEvoy et al, 2019; Nauslar et al, 2019) and has been put into use by fire management agencies (McEvoy et al, 2019). In addition to the aforementioned case studies, McEvoy et al (2019) compared EDDI to seasonally averaged fire danger indices (ERC, 100‐, and 1,000‐hour fuel moisture) across California and Nevada for 1979–2015, and results show EDDI strongly reflects the buildup of antecedent drought conditions also found in dead fuel moisture. The multi‐scalar nature of EDDI (Hobbins et al, 2016) and ability to decompose into individual weather drivers is appealing to fire managers who found added value when used in combination with traditional fire danger metrics (McEvoy et al, 2019).…”

“…In addition to the aforementioned case studies, McEvoy et al (2019) compared EDDI to seasonally averaged fire danger indices (ERC, 100‐, and 1,000‐hour fuel moisture) across California and Nevada for 1979–2015, and results show EDDI strongly reflects the buildup of antecedent drought conditions also found in dead fuel moisture. The multi‐scalar nature of EDDI (Hobbins et al, 2016) and ability to decompose into individual weather drivers is appealing to fire managers who found added value when used in combination with traditional fire danger metrics (McEvoy et al, 2019). Abatzoglou and Kolden (2013) found ET 0 to have good interannual relationships to fire season total burned area in California.…”

Deciphering Active Wildfires in the Southwestern USA Using Topological Data Analysis

“…Previous studies have shown that wildfires occur more frequently and become more severe in hot and dry weather [7]. Thus, heat and drought indices have served as a proxy for predicting the intensity and frequency of wildfires [8][9][10]. However, accurately predicting wildfires is still difficult.…”