• Although a series of wet winter storms brought relief elsewhere in the West, Arizona, Utah and parts of Nevada remain stuck in the fifth year of a record-setting drought, one that climate watchers say shows no signs of abating. Some of its most serious consequences are readily visible in the blackened skies near Tucson and other wildfire locations this week.
• "The worst case has happened," said Mark Svoboda, a climatologist at the National Drought Mitigation Center. "El Niņo failed to give us much oomph, and now many areas have slid back and gotten worse. We won't be talking about this drought going away until this time next year."
• What increasingly troubles many water experts is the accumulating effects of such a long dry period. The rain that doesn't fall adds up over time. Since 1996, Phoenix has built a "rain deficit," the difference between what should fall and what does, of more than 13 inches, or nearly two years' worth of precipitation. In Flagstaff, the deficit totals more than two years' worth.
  • Such shortfalls rob the soil of moisture, weakening trees, shrubs and other vegetation. That, in turn, ruins wildlife habitat and livestock feed, heightens fire danger and invites disease and pests such as the bark beetles.
• Storage reservoirs have fallen behind as well, as runoff-starved rivers deliver less water. Over the past four years, the Colorado River has fallen behind by 8.3 million acre-feet of water, or about the amount used in a year by Arizona, Nevada and California.
  • Those three states have drawn on storage in Lakes Powell and Mead, which have both fallen to near-record low levels.
• Conditions around the region:
  • Arizona's forests are suffering catastrophic losses from six years of below-average precipitation. Trees are like kiln-dried lumber, pushing wildfire danger to extreme levels again. Bark beetles have ravaged millions of drought-weakened ponderosa and piņon trees, leaving behind even more unpredictable wildfire fuel.
  • Nearly all of Utah is suffering from extreme drought conditions, according to the mitigation center. Numerous communities along the populous Wasatch Front have imposed outdoor water restrictions, and some areas in the southern third of the state expect shortages before summer's end.
  • Southern Nevada officials want to impose mandatory conservation measures after Lake Mead, the area's chief source of water, fell below 1,145 feet in elevation. That level also triggers restrictions along the Colorado River.

Tree-rings provide a means for developing long-duration climate proxies that can overcome this problem which is inherent in the instrumental record. Tree-rings yield continuous, exactly-dated proxies of climate that are usually highly replicable. When properly analyzed, tree-rings also provide records of both high- and low-frequency climate variability.

The top graph to the right shows reconstructed Palmer Drought Severity Index (PDSI) values for the Uinta Basin region (Utah Climate Division 6) of northeastern Utah from 1405-2000 AD. High positive PDSI values indicate wet conditions while negative values represent droughts. Estimates for PDSI values prior to the instrumental period (1900-Present) were derived from the measurement of pinyon pine (Pinus edulis) tree-rings at four sites in the Uinta Basin (see map bottom right). Samples from 144 trees (both living and dead) are included in the reconstruction.

Comparison of the instrumental and estimated values shows that the reconstruction provides a good estimate of both single-year drought events and multi-year wet/dry trends. Droughts in the 1950s, 1930s and around the beginning of the 20th century are well represented. While these 20th century dry events were quite severe, at least two droughts prior to the instrumental period (e.g. mid 1400s and about 1735-1755) likely equaled or exceeded their duration. Several pre-instrumental droughts, particularly those in the 1500s through mid 1600s, were of greater magnitude or severity than any dry events after the early 1900s. Overall, this reconstruction suggests that severe, long-duration (longer than 10 years) droughts are a common feature of northeastern Utah's climate.

Gray, S.T. 2003. Long-term climate variability and its implications for ecosystems and natural resource management in the Central Rocky Mountains. Ph.D. dissertation. University of Wyoming.

Gray, S.T., J.L. Betancourt, C.L. Fastie, and S.T. Jackson. 2003. Patterns and sources of multidecadal oscillations in drought-sensitive tree-ring records from the central and southern Rocky Mountains. Geophysical Research Letters, 10:1029/2002GL01654.