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| Abundant rain and snow fell over Utah during October, bringing considerable improvement to drought in the state. However, long-term conditions (36 to 48 to 60 months) remained dry. |
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Paleoclimatic Perspective
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Streamflow is a good indicator of moisture conditions for a region because it integrates across the basin the rainfall from storms, meltwater from spring snowmelt, and groundwater that feeds the streams. The level of the stream reflects hydrologic conditions, with low flows indicative of drought. On a short-term basis, daily streamflow measurements are useful for monitoring floods from heavy rainstorms, but streamflow averaged over longer time periods, such as a month or year, is more useful for monitoring drought.
Recent low flows on the Green River in Utah reflect the ongoing drought conditions in the intermountain basin and central Rockies of the western U.S. In the graph below left, the blue lines are a record of water year (October 1 to September 30) streamflow at the gage on the Green River at Green River, Utah, for the period 1906-2003. The light blue line shows annual values, while the heavy dark blue line shows the record smoothed with a 9-weight filter. This record, obtained from the USGS, has not had the effects of diversions for irrigation, reservoirs, and other factors removed, so it is not an exact reflection of natural conditions. The natural flow record is available only through 1995, but the two records are closely correlated ( r = 0.959). In the USGS gage record, water year flow has been below average since 1999. Flow values are not yet available for 2004, but estimates based on snowpack in the late spring predicted a flow of 41% of average (based on 1971-2000), so 2004 will make six years of drought, according to this streamflow record. The 2004 estimated value is marked with an "x" on the graph. Despite the severity of the current drought, the late 1980s to the early 1990s was a period of more severe and persistent low flow, with eight years of below average flow, from 1987-1994. |
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The water year precipitation record for southwest Wyoming, which is the headwaters of the Green River, for the period 1895-2004 is shown in the graph below right. The last 6 years (last 5 water years) have been extremely dry in this basin. The late 1980s were also very dry. The precipitation record for the Uinta Basin in northeast to east central Utah, which is where the Green River streamflow gage is located, shows a similar recent dry pattern. |
| The graph above left also shows a tree-ring record that corresponds well to the Green River flow record (annual values in light red, smoothed values in thick red line). The tree ring record extends from 1402-1999. This record is the average of five tree-ring chronologies that are highly correlated with the natural streamflow record for Green River. Together, they are a proxy record of past streamflow. The five chronologies are from northeastern Colorado and south central Wyoming, and tree growth at these sites is influenced by the same storms and moisture which affect Green River streamflow. The correlation between the 5-chronology average and Green River streamflow is 0.745, indicating a high degree of shared variance. The match between the two records is also visible in the graph. |
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| If the tree-ring series is considered a long-term proxy of streamflow, an assessment of periods of persistent low flow and drought can be made over the past 600 years. The proxy record displays a wide range of variability in drought. There are short, severe episodes around 1845, 1685, and 1653. Other periods are characterized by more persistent drought, such as the 1460s and 1580s, which is a period corresponding to widespread drought conditions throughout western North America. The period of low growth/drought in the second half of the 19th century is particularly notable, and for several reasons. It appears to be the most persistent period of drought, on average, in the entire record. In addition, it is followed by a period of high growth/wet conditions in the early 20th century, the length of which appears to be unmatched in the rest of the record. The persistence of these wet conditions is also well reflected in the Green River gage record, which indicates even more extreme values. |
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Resources: Water resource data, including the streamflow for the Green River at Green River, Utah shown above, can be obtained from the U.S. Geological Survey web site NWISWeb Data for the Nation: The five tree ring chronologies used as a proxy record for Green River streamflow are from sites named Land's End, Pumphouse, Red Canyon, Trail Gulch, and Encampment. These data are available from the World Data Center for Paleoclimatology, International Tree-Ring Data Bank: The data for the Land's End and Pumphouse chronologies can be found by using the Tree-Ring Search Engine. Red Canyon, Trail Gulch, and Encampment can be found by selecting Download Recent Tree-Ring Data, and are the files named CO583.crn, CO548.crn, and CO585.crn. |
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