Climate Variability and Change in Western U.S. Rangelands

We examine variability and change components of precipitation and minimum and maximum daily temperatures, and the derived variables potential evapotranspiration (PET) and the Palmer Drought Severity Index (PDSI), over rangelands in the region 30-50N, 100- 125W. We focus on areas administered by the U.S. Bureau of Land Management (BLM) and Bureau of Indian Affairs (BIA), with a view toward understanding how future climate variations may affect ecosystems, and ultimately, grazing on these lands. Based on an analysis of the annual precipitation cycle we adopt a three-season partition for the year, classifying land areas by season of maximum precipitation; this yields a coherent subregional map. Masking with a combined BLM/BIA footprint, we find that in all subregions both tmin and tmax have increased in response to anthropogenic forcing, the rate being generally greater for tmax. Significant precipitation trends are not detected, whereas PET exhibits significant upward trends in all regions. While PET-normalized precipitation, as well as PDSI, do not exhibit significant trends individually (by variable and region), the fact that most trend downward nevertheless suggests a systematic drying. We conclude that temperature constitutes the principal detectable control on hydroclimatic changes in rangelands within the study area. Although ecosystem responses may be complex, future temperature increases are expected generally to reduce soil water availability. The unforced component of variability is
investigated with respect to several key climate indices on both interannual and decadal time scales.