The University of Arizona Desert Laboratory Past Research Highlights

Recent and ongoing droughts emphasize the importance of understanding precipitation variability in semi-arid regions of western North America. In two recent studies Desert Laboratory researchers Stephen Gray and Julio Betancourt and their close collaborators Steve Jackson, University of Wyoming and Chris Fastie, Middlebury College used ancient tree rings to examine the timing and duration of severe dry spells and extended wet periods in two such areas, the Bighorn and Uinta Basins. The Bighorn Basin represents 30% of the Yellowstone River's contribution to Upper Missouri River flows, while the Uinta Basin contributes about 10% of the inflow of the Colorado River into Lake Powell. Both the Bighorn and Uinta Basins are managed on the basis of gaged streamflow since 1900. Questions remain about whether or not the instrumental record adequately represents the long-term risk to water resources from sustained drought.

Published in this month's Journal of Climate, a tree-ring reconstruction of precipitation from A.D. 1260 to 1998 for the Bighorn Basin of northern Wyoming/southern Montana shows that regional hydroclimatic variability was dampened in the instrumental era (post-1900) compared to the previous 640 years. Both single-year and decadal-scale droughts were more severe before 1900, and multi-year droughts tended to last much longer. This reconstruction also shows that regional droughts of magnitude and duration similar to the current (1999-2004) drought have been common occurrences for at least the last seven centuries.

Plot of reconstructed annual precipitation for the Bighorn Basin region of northern Wyoming and southern Montana (1260-1998 AD). The graphs also shows a 10-yr running average of precipitation values with drought periods highlighted in red.

Results from a similar study in the Uinta Basin of northeastern Utah (September issue Journal of the American Water Resources Association) demonstrate that single-year dry events in this region also tended to be more severe than those after 1900, and multi-year droughts were longer and more severe prior to 1900. In particular, dry events in the late 13th, 16th and 18th Centuries surpass the magnitude and duration of droughts seen in the Uinta Basin after 1900. The last four decades of the 20th Century represent one of the wettest periods since A.D. 1226.

Reconstructed precipitation for the Uinta Basin of northeastern Utah (1226-2001 AD). Drought periods are shown in red. Wet decades in the late 20th Century are shown in blue.

Given the overall wetness and lack of extreme droughts in the twentieth century, water-planning based on the instrumental record clearly underestimates the risk of drought in both the Bighorn and Uinta Basins. These studies show instead that mitigation and planning efforts must consider a wider range of climatic scenarios, including droughts of different lengths, magnitudes, and intensities than those presented by the instrumental record. Managers should also consider numerous combinations of wet/dry events in their decision-making. Furthermore, planning efforts could focus more on the potential for long-duration dry events rather than average values for water availability.

As a new addition to the Desert Laboratory staff, National Research Council Postdoctoral Research Associate Stephen Gray will continue to focus on causes and consequences of droughts in the West. Funding for Steve's fellowship comes from the Geographic and Monitoring Program of the USGS-Mapping Division, the Earth Surficial Processes Research Institute of the USGS-Geologic Division, and the National Research Program of the USGS-Water Resources Division.