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

Global climate model (GCM) future projections indicate large temperature increases driven by anthropogenic forcings by the end of the 21^st century. Less certainty is found in regional GCM precipitation projections over parts of the western U.S., but the consensus points towards more drying and drought for the southwestern United States, including the southern parts of California and Nevada (CA-NV). It is commonly thought that increased temperatures will lead to increased atmospheric evaporative demand (E₀) and exacerbate precipitation-driven drought impacts. However, research has shown that when using a physically-based reference evapotranspiration (ET₀), a formulation of E₀, temperature is not always the dominant driver and wind speed, humidity, or solar radiation can have more of a control of the ET₀ values. This is particularly important in CA-NV, where over much of the region wind speed is the dominant of driver of daily ET₀ variability during the summer months, which coincides with the dry season, peak water and energy demand, and peak fire season. To fully understand long-term future changes in ET₀ it is therefore necessary to also examine changes in all of the drivers as a negative trend in wind speed or positive trend in humidity could offset some of the effects of large temperature increases. Here we examine projected changes in ET₀ and the four drivers over CA-NV through the end of the 21^st century based on the Localized Constructed Analogs (LOCA) statistically downscaled CMIP5 data. Annual and seasonal changes are examined for early-century (2006-2039), mid-century (2040-2069), and late-century (2070-2099). Changes in projected drought frequency and severity will be examined based on the Evaporative Demand Drought Index (EDDI). EDDI has been found to be strongly correlated to hydrologic and agricultural droughts in CA-NV as well as wildland fire dangers indices which reflect dead fuel moisture. EDDI is determined exclusively by atmospheric variables, as opposed to drought indices that use soil moisture or other metrics calculated by a land surface model that has its own set of parameterizations. We take advantage of this to compare drought projections from EDDI with those from the VIC land surface/hydrology model forced with the same LOCA meteorological fields, to better understand the interplay of meteorological forcing and land surface response in the incidence of droughts in the region. Results from the study will therefore provide a fuller glimpse into future potential drought and wildfire management challenges caused by climate change.