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

Abstract

Recent high impact wildfires and droughts in California and Nevada have been linked to extremes in the Evaporative Demand Drought Index (EDDI) and Standardized Precipitation Evapotranspiration Index (SPEI), respectively. Both indices are dependent on reference evapotranspiration (ET 0). Future changes in ET 0 for California and Nevada are examined, calculated from global climate model simulations downscaled by Localized Constructed Analogs (LOCA). ET 0 increases of 13-18% at seasonal timescales are projected by late century (2070-2099), with greatest relative increases in winter and spring. Seasonal ET 0 increases are most strongly driven by warmer temperatures, with increasing specific humidity having a smaller, but noteworthy, counter tendency. Extreme (95th percentile) EDDI values on the 2-week timescale have coincided with recent large wildfires in the area. Two-week EDDI extremes are projected to increase by 6-10 times during summer and 4-6 times during autumn by the end of the century. On multiyear timescales, the occurrence of extreme droughts based on 3-year SPEI below the historical fifth percentile, similar to that experienced during the 2012-2016 drought across the region, is projected to increase 3-15 times by late century. Positive trends in extreme multiyear droughts will further increase seasonal fire potential through degraded forests and increased fuel loads and flammability. Understanding how these drought metrics change on various climate timescales at the local level can provide fundamental information to support the development of long-term adaptation strategies for wildland fire and water resource management. Plain Language Summary Since the start of the 21st century, California and Nevada have observed extreme wildland fires and droughts that have caused devastating impacts to ecosystems and society. A common feature of these events has been very high atmospheric evaporative demand-the "thirst" of the atmosphere-which has largely been driven by increased air temperatures caused by anthropogenic climate change. This study examines projected changes in evaporative demand, which of the input variables are causing those changes and how the frequency of extreme wildfire potential and multiyear droughts will change. Evaporative demand is found to increase during all seasons, and increased temperatures drive most of that change. The likelihood of extreme wildfire potential based on 2-week periods of elevated evaporative demand during summer and autumn increases substantially. A climatic water balance based on precipitation and evaporative demand indicates extreme 3-year droughts that hold potential to deplete regional-scale water supply also become much more likely. Future adaptation planning efforts for wildfire management agencies, forest management, and water resource managers should account for a greater likelihood of more extreme events.

Comment on hess-2021-472

“…The increase in temperature and precipitation results in an increase in ET (62%), consistent with the findings of other recent studies (e.g. McEvoy et al, 2020). Nevertheless, the larger amount of precipitation associated with the EoC is enough to offset higher ET demand and recharge groundwater and surface water, which experience an increase of 4% and 19% respectively.…”

A Shorter, Sharper Rainy Season Amplifies California Wildfire Risk

“…Moreover: the expansion of fire season into historically unusual calendar months has begun to severely tax wildland firefighting resources, as many relevant state and federal agencies maintain only skeleton staffing levels during "non-peak" fire season months. And climate model projections strongly suggest that these challenges will only increase as the climate warms further in the coming decades as California's autumn fire season becomes warmer and drier still (Goss et al, 2020, McEvoy et al, 2020, Williams et al, 2019, Westerling, 2018). Yet the news is not all bad.…”

“…The historically unprecedented level of vegetation dryness during 2020 (Higuera & Abatzoglou, 2021) and other recent autumns in California (Figure 2) has co‐occurred with both record warm temperatures and anomalously low precipitation, and a rapidly growing body of evidence suggests this is not a coincidence. Not only have extreme autumn “fire weather” conditions more than doubled since the late 1970s in California (Goss et al., 2020), but these increases have been primarily attributed to increased vegetation flammability (rather than to shifts in the offshore wind patterns that often accelerate these fires [McEvoy et al., 2020]). The combined effects of warming summer and autumn temperatures during fire season (Williams et al., 2019), as well as decreases in autumn precipitation (e.g., [Luković et al., 2021] and others), have yielded an increasingly pronounced risk of dangerous peak season fires.…”

Hydrological Intensification Will Increase the Complexity of Water Resource Management

“…Actual evapotranspiration is also likely to increase as ETo rises, at least initially in places not water‐limited. Increases in actual and reference/potential evapotranspiration rates have been observed (Greve et al., 2014; Greve & Seneviratne, 2015; Huntington et al., 2018; Milly & Dunne, 2016) and are projected to continue with climate change (Ficklin et al., 2019; Lavers et al., 2015; McEvoy et al., 2020). However, eventually, plants close their stomata directly in response to the increases in vapor pressure deficit (VPD) that accompany increasing ETo (Grossiord et al., 2020), which decouple actual evapotranspiration from its reference rate (Novick et al., 2016).…”