| A
final word
In February 2007, a committee of scientists convened by the
National Research Council released a report called "Colorado
River Basin Water Management: Evaluating and Adjusting to Hydroclimatic
Variability". One of the main purposes of the report
was to review the available data describing the variability
of the flow of the Colorado River, including the gage-based
"natural flow" record and the tree-ring reconstructions
described in this Paleo Perspective.
The report's Summary conveys the critical role of the tree-ring
data in providing a more complete perspective of the Colorado
River and its long-term variability:
For many years, scientific understanding of Colorado River
flows was based primarily on gaged streamflow records that
covered several decades. Recent studies based on tree-ring
data, covering hundreds of years, have transformed the paradigm
governing understanding of the river’s long-term behavior
and mean flows. These studies affirm year-to-year variations
in the gaged records. They also demonstrate that the river’s
mean annual flow—over multi-decadal and centennial time
scales, as shown in multiple and independent reconstructions
of Colorado River flows—is itself subject to fluctuations.
Given both natural and human-induced climate changes, fluctuations
in Colorado River mean flows over long-range time scales are
likely to continue into the future. The paleoclimate record
reveals several past periods in which Colorado River flows
were considerably lower than flows reflected in the Lees Ferry
gaged record, and that were assumed in the 1922 Colorado River
Compact allocations (p.4).
Furthermore, the Summary points out, the tree-ring data have
shown that severe drought is a "normal" feature of
the Colorado River basin:
Multi-century, tree-ring based reconstructions of Colorado
River flow indicate that extended drought episodes are a recurrent
and integral feature of the basin’s climate. Moreover,
the range of natural variability present in the streamflow
reconstructions reveals greater hydrologic variability than
that reflected in the gaged record, particularly with regard
to drought. These reconstructions, along with temperature
trends and projections for the region, suggest that future
droughts will recur and that they may exceed the severity
of droughts of historical experience, such as the drought
of the late 1990s and early 2000s (p.4).
That last sentence is especially important, as it points the
importance of considering both past variability and current
and future trend with respect to drought. The phrase "temperature
trends and projections" refers to the general increase
in average temperature in the Colorado River basin since 1900--particularly
since 1970--and the consensus of climate models that future
climate in the basin will progressively warm, which would lead
to reduced snowpacks, earlier snowmelt, greater evapotranspiration,
and lower streamflows. The effects of this likely warming will
be superimposed on the natural variability described by the
tree-ring data, worsening the impacts of future drought.
The tree-ring reconstructions for Colorado River flow described
in these pages can play an important role in preparing for this
uncertain but very likely warmer future. Together with information
about climate change, the reconstructions can guide expectations
for future water yield from the Colorado River basin and help
provide a basis for sustainable water management.
Thanks for reading Colorado River Streamflow: A Paleo Perspective.
References
Credits
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