Using Tree
Rings to
Reconstruct
Past Streamflow
Connie A. Woodhouse
Department of Geography
and Regional
Development, University
of Arizona
Assistance from many including Jeff Lukas, Robin Webb, Brad Udall, David Meko,
Stephen Gray, and funding from NOAA, Denver Water and the National Science
Foundation.
Overview
• Why look to the past?
• How are tree rings used to
reconstruct records of past
streamflow?
• How has San Juan River
streamflow varied over past
centuries?
• How relevant is information from
the past to planning for the future?
http://www.nps.gov/grsa/resources/photos.htm
5 year average flows
lowest
highest
1999-2003
1916-1920
1974-1978
1919-1923
1950-1954
1917-1921
How often do droughts, such
as 1999-2003 occur?
Have more severe and/or
persistent droughts occurred?
What about that wet period in
the 1910s and 1920s?
Reconstructing
Streamflow from Tree
Rings
How is tree growth
related to climate?
Variations in annual ring
widths reflect the conditions
that influence tree growth.
Climate is often the primary
influence on growth, and
because of this, ring width
can be used as a proxy for
past climate.
Moisture-sensitive trees in
Southwestern Colorado
Moisture-stressed trees closely track variations in
precipitation
Western CO Annual Precip vs. Pinyon ring width (WIL731)
25
0.8
0.6
0.5
15
0.4
10
0.3
0.2
5
0.1
0
1900
0
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
Ring widths from a single tree near Grand Junction are plotted with
annual precipitation in the Colorado River basin. (r = 0.69).
Ring Width (mm)
Precipitation (in.)
0.7
20
How can tree rings be used to reconstruct streamflow?
Ring widths and
streamflow both integrate
the effects of precipitation
and evapotranspiration,
as mediated by the soil,
over the course of the
water year.
How do we sample trees to get climate information?
An increment borer is used
to sample cores from
about 20 trees at a site
1900
1910
1920
1930
Cores mounted and sanded, then dated,
measured, and averaged into site treering chronologies
Tree-ring collections from
moisture-sensitive sites
BLUFF
x
Collections by INSTAAR Dendrochronology Lab
Tree-ring chronologies are calibrated with the gage
record to generate a tree-ring model of reconstructed
flow values.
San Juan River Gaged and Reconstructed
Streamflow, 1906-1995
The tree-ring model explains 73% of the variance in the gage record; 27% of
the variance is not explained, representing the uncertainty in the
reconstruction model.
San Juan River Reconstruction, 1569-1997
San Juan gage record
How representative
is the 20th century
gage record
compare to the full
reconstruction?
The reconstruction contains a richer variety of sequences wet, dry,
and average years, and shows the variable distribution of extreme
low flow years over the past four centuries
What does the reconstruction tell us about past drought?
The 20th-21st century (red) contains a subset of conditions possible over the
longer reconstruction time period (orange).
Periods of wet and dry conditions
averaged over 20 years
San Juan River Reconstruction, 1569-1997, 20-year running mean
3 4
2
2
Driest and wettest non-overlapping
20-year averages
5
1
3
1
5
4
DRIEST
WETTEST
1886-1905
1905-1924
1622-1641
1602-1621
1863-1882
1825-1844
1953-1972
1849-1868
1803-1822
1978-1997
But will the climate
of the future look
anything like the
past?
Temperatures in the
San Juan region have
warmed over the past
decades, and are
projected to continue
warming.
Precipitation trends are
not evident, and projects
are less certain as well.
WestMap: http://www.cefa.dri.edu/Westmap/westmappass.php
How can tree-ring data be
useful in planning for the
future?
• Tree-ring reconstructions of
hydrology can provide a long-term
context from which to evaluate
shorter instrumental records
• These data contain valuable
information about the range of
natural hydrologic variability
beyond that contained in
instrumental records.
• The past will not be an analogue
for the future, but the extended
records of past hydrology provide a
baseline for planning which also
must consider a warmer future.
http://www.media.utah.edu/UHE/Pictures/; http://www.oars.com/utah/sanjuan4day.html