U.S. Geological Survey. 2002. Precipitation history of the Colorado Plateau Region, 1900-2000. USGS Fact Sheet 119-02.
RELEVANT TO: |
AQUATIC/RIPARIAN SYSTEMS FORESTS/GRASSLANDS/SHRUBLANDS SPECIES/INVASIVE SPECIES LIVESTOCK GRAZING |
DESCRIPTION
Variation in Colorado Region precipitation from 1900-2000 is examined, revealing two multi-decade wetter periods (1905-1941 and 1978-1998) and one drier period (1942-1977). Episodes of unusually wet or dry climate arise from global-scale fluctuations of sea-surface temperature (SST), atmospheric pressure, and atmospheric circulation patterns.
Shorter-term precipitation variation within these multi-decade variations, on the order of 4-7 years, Region is related in the Colorado Plateau to the El Niño Southern Oscillation Index (SOI). Lower precipitation is associated with a weak El Niño during the cool and warm seasons, and with strong La Niña episodes during cool seasons. Strong El Niño episodes are associated with increased variability of warm season precipitation or the frequency of cool season above normal precipitation.
The longer-term precipitation variation is largely in phase with the Pacific Decadal Oscillation (PDO), a phenomenon of the northern Pacific Ocean. Changes in SST and atmospheric pressure can trigger sharp transitions from one climate regime to another.
MAJOR FINDINGS
"Of particular interest is the downward shift in the PDO beginning in 1999 with concomitant decreased precipitation that continued through the winter of 2002. The weather, SST, and surface-pressure patterns of the past three years suggest to climatologists that the transition to another regime is presently underway..., signaling the end of the warm PDO phase and the wet climate of 1978-1998." (p. 4)
"Recent trends in Colorado Plateau precipitation and the PDO suggest that climate of the region may become drier for the next 2-3 decades in a pattern that could resemble the drought of 1942-1977... Water resources were heavily affected during the early part of the 1942-1977 drought [citation]; the population of the region has increased fourfold since the mid-1950s, substantially increasing the demand for water in a region without abundant supplies and creating the possibility of severe or catastrophic consequences if such a drought were repeated." (p. 4)
"Precipitation along with other climate variables affects the...magnitude of natural disturbances to the ecosystem [citation] as well as the recovery rates from natural and human disturbances. For example, the results of studies of floral and faunal population dynamics and the affects [sic] of grazing are dependent on the prevailing climate. Inferences and projections based on these studies may not be valid or may need adjustment or reappraisal if applied during a different climate regime."(p. 4)
"A change to drier climate - particularly reduced winter rainfall - would have a variety of related effects on Plateau ecosystems. These may include reduced groundwater recharge, lower baseflow in perennial streams, increased frequency of dust storms and strong winds, weakening of biological soil crusts, reduction of plant cover and possible changes in species composition...and increased frequency of forest and range fires." (p. 1)
QUESTIONS RAISED FOR THE THREE FORESTS
1. Does the Forest agree with the USGS that "Environmental organizations, ranchers, and agencies responsible for resource management in the...Federal lands of the region have a need to understand long-term climate and how climate may change in the near future"? (p.4) If not, on what basis does the Forest disagree?
2. To what degree are Forest expectations of vegetation and damage recovery based on responses recorded during the wetter period of 1978 to 1998?
3. Does the Forest have drought management plans based on predictions of global warming and decades-long drought?
RELEVANCE TO FOREST MANAGEMENT
1. Precaution would have each Forest basing its upcoming Forest Plan permitted activities on the understanding of the U.S. Geological Service that the "...climate of the region may become drier for the next 2-3 decades in a pattern that could resemble the drought of 1942-1977."
2. Using "potential vegetation" rather than actual vegetation as a basis for determining capability/suitability of livestock grazing is not defensible in the face of short-term and long-term variability of precipitation.
3. Drought management plans are essential to avoid degradation of the Forest ecosystems.