Fig. 2

Trends in climate in northern Arizona, 1997–2017. a Palmer’s Drought Severity Index (PDSI; Palmer 1965; Alley 1984; Heim 2002) for the region containing our study area by year. This index ranges from − 10 to 10, with higher and lower values indicating wetter and drier conditions, respectively. Horizontal reference lines indicate thresholds for severe (–3.0; dashed line) and extreme (–4.0; solid line) drought, respectively (Palmer 1965). b Difference between annual precipitation (AP) during the study and the 1950 through 2016 mean at the National Weather Service (NWS) station located at Pulliam airport, Flagstaff, AZ, elevation 2136 m. Values above and below the horizontal reference line indicated years that were wetter or drier than average, respectively. c Difference between annual Cooling Degree Days (CDD) during the study and the 1950 through 2016 mean at the National Weather Service (NWS) station located at Pulliam airport. Values above and below the horizontal reference line indicated years that were warmer or cooler than average, respectively. All data sources are described in text