Table Of ContentGILA RIVER STREAMFLOW RECONSTRUCTION
By
David M. Meko
and
Donald A. Graybill
Laboratory ofTree-Ring Research
The University ofArizona
Tucson, Arizona
A
Report Submitted to the U.S, Bureau ofLand Management
28 April 1993
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PREFACE
The Laboratory of Tree-Ring Research was contacted by the U.S. Bureau of Land
Management(BLM) in the fall of 1991 to investigate the potential ofextending streamflow records
A
on the Gila River using tree-ring data. dendrohydrology study of the upper Gila River was
BLM
initiated in March, 1992, as Project No. 1422A040C20001, "Gila River Streamflow
Reconstruction". The Principal Investigator on the project was David Meko. The study used tree-
ring chronologies previously developed by Donald A. Graybill.
We thank Mike McQueen, ofthe SafTord Office ofthe BLM, for initiating the project and
providing guidance and suggestions throughout the course of the work, and Gary Funkhouser and
Wanmei Ni for their help in computer processing ofdata.
TABLEOF CONTENTS
EXECUTIVE SUMMARY
iii
LIST OF APPENDIXES x
LIST OF FIGURES
xi
LIST OF TABLES
xii
INTRODUCTION
1. 1
STUDY AREA
2. 1
DATA
3. 2
METHODS
4. 4
RESULTS
5. 5
CALBRATION-VERIFICATIONOF RECONSTRUCTION MODEL
5.1 5
DECADAL AND MULTIDECADAL TRENDS
5.2 8
RETURN PERIODS OF LOW ANNUAL FLOWS
5.3 10
FLOOD FREQUENCY
5.4 11
CONCLUSIONS AND RECOMMENDATIONS
5.5 13
REFERENCES
6. 14
m
EXECUTIVE SUMMARY
Streamflow in the upper Gila River Basin is strongly influenced by climate
variations at time scales of decades and longer. The climate influence in the 20th
century is clearly demonstrated by large fluctuations in streamflow averaged over
several years, and by multi-decadal trends in streamflow. For the Gila River at the
head of Safford Valley, Arizona, a distinct climate signature in the streamflow
record is a shift fi-om high flow in the 1910s to low flow in the 1950s, and again to
high flow in the 1980s. Imbedded within the large changes in annual streamflow
are changes in flood fi*equency. Large peak annual flows on the Gila River were
much more common after 1973 than in the period 1918-1972.
A
critical question for water-resource management is whether the gaged
record is a representative sample of natural variations in annual streamflow and
peak flow. This question can be answered by extending the gaged record
A
statistically using tree-ring analysis. tree-ring study was commissioned with the
following objectives:
IV
Reconstruct annual flow series for the Gila and San Francisco Rivers for the
1
.
last 300 to 700 years.
2. Establish the return periods for extremely low annual streamflow.
3. Provide an analysis of the variability of historic mean annual streamflow.
4. Analyze the gaged and reconstructed streamflow record for decadal and
multi-decadal trends in mean annual flow.
5. Provide an analysis of return periods for peak streamflow events for the
period of gaged record, and extend this analysis with tree rings, if feasible,
to cover earlier periods.
A
network ofnine tree-ring chronologies, each comprising 10 or more trees,
was developed for the study. The tree-ring data were applied in regression models
to derive statistical estimates of log-transformed annual flow for the period 1663-
1985. Reconstructions ofannual flow (1663-1985) were generated for three gages:
the Gila River at the head of Safford Valley, the Gila River near Clifton, and the
San Francisco River at Clifton. The regression equations explain more than 60
V
percent of streamflow variance in the 1916-1985 calibration period. The
reconstruction was found to closely track observed calibration-period flow
variations at interannual to multi-decadal time scales.
The reconstruction for the gage on the Gila River at the head of Safford
Valley was analyzed statistically for a long-term (323-year) perspective on flow
variations. Findings of the study are summarized below:
1. Low flows are more frequent in the modem period (1916-1985) than in the
long-term period (1663-1985). The difference in frequencies results largely
from a period of recurrent low-flow years in the 1950s. The probability of
annual flow less than a specified value over the range 80,000 acre-ft to
modem
150,000 acre-ft is greater in the period than for the long-term period.
The modem period does not, however, include the minimum reconstmcted
annual flow. The long-term minimum annual flow (49,055 acre-ft) is
considerably lower than the modem minimum (73,050 acre-ft observed,
74,399 acre-ft reconstructed).
VI
2. The reconstruction indicates that mean annual streamflow computed for a70-
year base period highly variable. Reconstructed 70-year means vary by
is
more than 100,000 acre-ft. The extremes are 378,890 acre-ft for 1865-1934
and 269,627 acre-ft for 1713-1782. The modem reconstmcted mean is
slightly lower than the long-term reconstructed mean, but is extremely
sensitive to shifting the averaging period by a just a few years.
3. Trends and large multi-decadal fluctuations in annual flow are characteristic
of the long-term record, but have increased in magnitude since about 1905.
A
cluster of high-flow years in the early 1900s and a cluster of low flow
years in the 1950s impart a downward trend to smoothed time series of
annual flow. This trend is more prominent than any trend before the 20th
century. The clusters ofhigh-flow and low-flow years produce the long-term
extremes in 10-year average flow. The lowest reconstructed ten-year mean
(171,596 acre-ft) was for 1947-1956. This value was about 25,000 acre-ft
Vll
lower than the next lowest mean, which occurred in the 1600s. The highest
ten-year mean (788,043 acre-ft) was for 1906-1915. This value more than
300,000 acre-ft higher than the second-highest ten-year mean, which
occurred in the late 1700s.
5. The 100-year flood for the Gila River at the head of Safford Valley was
computed from gaged data, 1914-1988, as 88,500 cfs. Peak annual flow is
significantly correlated with total annual flow. The relationship between
peak flow and ring-width indices of drought-sensitive trees is too weak,
however, for accurate reconstruction of flood history before the period of
gaged flow.
Vlll
6. The accuracy of annual flow reconstructions presented here is moderately
high compared with accuracy for similar reconstructions in other basins in
the western United States. Accuracy possibly could be improved by
expanding the tree-ring coverage in the watershed. Alternative information
on flood frequency might be available using paleoclimate methods beyond
—
the scope ofthe current study for example, examination ofaccurately dated
flood scars in riparian trees.
IX
