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As noted by the Wyoming State Climatologist (Jan Curtis), June provided some break to the state's multi-year drought with cooler and wetter conditions, at least for many localities. However, hydrologic conditions remained severe (map of Wyoming river basins). End-of-June USGS reports indicated the lowest runoff (cumulative year or season to date) in 70+ years at 2 sites in the Tongue-Powder River Basin (Tongue Dayton and Powder Arvada). The low runoff was also reflected in the 7-day hydrologic unit graphic for June 28 (where the basin ranked in the 5th percentile or lower category, or severe hydrologic drought).
A detailed report of Wyoming's June climate is available at: |
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Paleoclimatic Perspective
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For much of the West, 2004 marked the start of the sixth year of generally dry conditions. In 2002, drought reached the most severe level in the last 100 years, statewide, for Wyoming, based on the Palmer Drought Index. Precipitation averaged across the state, plotted for consecutive overlapping 60-month intervals, indicated that July 1999-June 2004 was the driest such 5-year period in the state's 110-year instrumental record.
Paleoclimatic data, such as tree-rings, can be used to extend the climate record back several centuries beyond the 110-year instrumental record. Tree growth of moisture-sensitive Douglas-fir trees in north-central Colorado and southeastern Wyoming reflects variations in spring moisture. Two tree-ring chronologies from the upper North Platte River basin, one collected near Encampment and one near the Colorado/Wyoming border above the North Platte River, correlate well with Wyoming Division 10 (Upper Platte Basin) Palmer Drought Severity Index (PDSI) for June ( r = 0.445), 1900-2001. A comparison of the average of the two tree-ring chronologies vs. observed June PDSI for 1900-2001 indicates that tree growth often matched the year-to-year variations in PDSI, as well as the lower frequency variations. The tree-ring record reflects the major droughts of the 20th century, in the early 1900s, the 1950s, and the late 1980s. However the most severe drought in the instrumental record is the recent and ongoing drought, which is not reflected in this tree-ring record since it ends in 2001. |
| If considered as a proxy for drought, the full tree-ring record, extending back to 1486, allows a comparison of 20th century droughts with droughts in previous centuries. In the graph below right, the blue lines are the June PDSI for the instrumental record 1900-2004. The red lines are the reconstructed drought index based on tree-ring data for the period 1486-2001. Both sets of data have been standardized using the mean and standard deviation for the period of record common to both (i.e., 1900-2001) in order to facillitate direct comparison of the different types of data. Negative values are drier than the mean, while positive values are wetter than the mean. |
| When the record is smoothed with a 10-year moving average (the darker thick curves), the 1930s and 1950s periods of drought rank with several other periods of major drought in the last 500 years. Droughts in the late 16th century and the mid-17th century are comparable, and only the drought of the second half of the 19th century appears more persistent. Although shorter, the drought in the early 1700s appears as the most severe drought in this proxy record. |
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Resources: These tree-ring data are not yet published but similar proxy records of drought are available from the NOAA Paleoclimatology Program web site on North American Drought Variability at: |
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