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Paleoclimatic Perspective
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Pre-Instrumental Perspective Bighorn Basin, Wyoming and Montana Despite near-average precipitation over the past 12 months, Wyoming and Montana have been mired in long-term drought since mid-1999. The Bighorn Basin in north-central Wyoming and extreme south-central Montana is representative of conditions in both states, with precipitation in the corresponding climate division (Wyoming Division 4) since July 2000 totaling 84% of the 60-month normal (based on 1950-2000) (also see graph to left). |
| The graph to the right (annual values in light blue, 5-year weighted average in dark blue) shows the average annual (July-June) precipitation, 1896-2005, for Wyoming Division 4. Several severe multi-year drought events can be seen in this record: around 1900; the mid-1930s Dust Bowl; the 1950s; and the last six years (2000-2005), all of which have been below the long-term average. |
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Also shown on this graph is a 739-year tree-ring reconstruction (1260-1998) of Bighorn Basin annual precipitation (annual values in light red; 5-year smoothed values in dark red), developed by Gray et al. (2004). This reconstruction is based on four long tree-ring chronologies (one Douglas-fir, three limber pine) from the Bighorn Basin, and was calibrated on an instrumental precipitation record (1907-1996) averaged from five long-term weather stations in the Bighorn Basin, four of which are within Wyoming Division 4: Buffalo Bill Dam, WY; Lovell, WY; Powell Field Station, WY; Worland, WY; and Bridger, MT. The reconstruction was calibrated on a 13-month "annual" period (June-June), but it correlates well with the Wyoming Division 4 annual (July-June) precipitation. Over their common period (1896-1996) the correlation is 0.602, indicating a high degree of shared variance. The precipitation units shown are standardized for comparison; negative values indicate below-average precipitation, and positive values indicate above-average precipitation.
The tree-ring reconstruction can put the precipitation variability of the last century in north-central Wyoming into a much longer perspective. First, the most severe single-year droughts of the 20th century (e.g., 1934, 1956) were probably matched or exceeded on numerous occasions in the previous 650 years. The reconstruction also shows several extended dry periods much longer than those in the instrumental record, including events in the late 1200s and the late 1500s. The latter event has been found, from other tree-ring reconstructions, to have extended through much of North America (Stahle et al. 2000). These events, were they to recur, would likely pose a greater challenge to human activities and ecosystems in the Bighorn Basin than even the severe droughts of the 1930s, 1950s, and 2000-2005. |
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NCDC /
Clim. Monitoring /
Climate-2005 /
Jun /
U.S. Regional Drought /
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