Archive of Spotlight Feature Articles

Tree-Ring Reconstructions of Streamflow Aid Drought Severity Assessment

Project Co-PIs Connie Woodhouse*, Robert S. Webb**, Research Assistant Jeffrey Lukas***

Severe drought conditions have existed throughout the West for the past few years. High demand levels in Colorado resulted in unprecedented impacts on their managed water supplies. Snowpack was extremely low, water year streamflow was the lowest on record for many gages, and it was the driest year on record (statewide Palmer Drought Hydrologic Index). This drought presented an opportunity to pursue collaborations with water managers interested in exploring the usefulness of tree-ring based streamflow reconstructions as an indicator of drought.

tree ring core samples show narrow ring in 2002, 1954 and 1956

These cores from a Douglas-fir tree in Eldorado Canyon, Colorado were mounted and sanded to reveal their annual ring structure under a microscope. The decades are marked and the 2002 ring is just visible at the right-hand end of the cores. It is a relatively narrow ring, but the 1954 and 1956 rings are narrower in this tree.

Centuries-long reconstructed streamflow records placed the 2002 drought into a long-term context. A broader question was if the 20th century record of flow was an adequate frame of reference for planning. We developed partnerships with water agencies including Denver Water, Colorado’s oldest and largest water provider serving over one million people, and the Northern Colorado Water Conservancy District (NCWCD) serving northeastern Colorado’s agricultural, municipal, and industrial needs. Both providers utilize water from the South Platte and Upper Colorado River basins, so our studies were focused in these areas.

Recent Results

This 650-year-old Douglas fir is one of 15 sampled in the study.

This 650 year-old Douglas-fir stands just east of Dillon Reservoir. It and 15 other very old trees were sampled to develop the Dillon tree-ring chronology used in reconstructing the annual flow of the Blue River, a tributary of the Colorado River. Dillon Reservoir is the largest reservoir in the Denver Water system.

In the spring of 2003, we recollected tree-ring samples from 12 sites in western Colorado to update tree-ring chronologies through 2002. Gage data from the Blue, Fraser and Williams Fork rivers were used to reconstruct annual headwaters flow for the Colorado River basin. When tree-ring chronologies were calibrated against gage data, the resulting model confirmed that trees duplicated the lows flows of 2002 very well. The full Blue River reconstruction indicated that while 2002 was a very low flow year, lower flows have occurred during the past 550 years (graph below, bottom). When 2002 was considered as part of a 3-year drought (2000-2002), the drought event appears even less unusual (graph below, top). In fact, the 3-year drought from 1952-1954 was more severe than for the 2000-2002 event. The gage record also indicates several 3-year periods in the 1950s had lower cumulative flows.

How are streamflow reconstructions being used in water resources management?

Streamflow reconstruction chart shows comparisons of annual values and three-year running sums.

These graphs depict reconstructed annual streamflows for the Blue River. The top graph shows the reconstruction as three-year running sums of annual flow. The bottom graph shows annual values. Red lines mark the 2002 value in each graph. As a single year, 2002 reconstructed flow was very low, but not unprecedented. It was one of the eight lowest flow years in this reconstruction. As a three-year drought, is it a much less rare event. In the 20th century, the 1950s were a more severe drought period. The most extreme 3-year low flow sum in this reconstruction occurred from 1845-1846. (larger image)

The streamflow reconstructions and information they provide have begun to be used in water resource planning and decision-making. The reconstructions are being used as input into water system models to assess the reliability of the water supply system under a broader range of conditions than afforded by the gage record alone. Analyses of streamflow reconstructions are now considered along with snowpack, runoff, and estimated diversions in setting water allocations and enacting conservation measures.

Continuing and future work

Jeff Lukas removes a quarter-inch core from a Douglas fir.

Jeff Lukas cores a Douglas-fir with an increment borer near Buena Vista, Colorado. Increment borers remove a quarter inch diameter cylinder of wood from the tree. Sap quickly fills the hole as the tree’s way of healing the injury.

We are continuing our current water provider partnerships to develop reconstructions that are applicable to water management and planning. Work includes tuning reconstructions to better duplicate extreme low flows, better quantify reconstruction uncertainties, and enable reconstructions of other metrics in addition to annual flow. We are updating and expanding Upper Colorado River basin reconstructions with University of Arizona and Montana State colleagues and are also developing new partnerships in the upper Rio Grande River basin.

The climate of the future will not be analogous to the that of the past because of the unprecedented effect of human activities, but the full range of natural climate variability is likely to underlie future climate. An understanding of the range and character of past streamflow variability along with climate modeling of future conditions can be useful in developing scenarios for future hydrologic variability.

This work is part of NOAA’s Regional Integrated Sciences and Assessments program being conducted between CIRES and CDC. Funding has been provided by the NOAA Office of Global Programs and Denver Water. For further information, see http://www.ncdc.noaa.gov/paleo/streamflow/.

* Paleoclimatology Branch of the National Climate Data Center; Institute of Arctic and Alpine Research and the Western Water Assessment of the Cooperative Institute for Research in Environmental Science (CIRES), University of Colorado
** NOAA/OAR Climate Diagnostics Center (CDC) and CIRES’ Western Water Assessment
*** Institute of Arctic and Alpine Research, University of Colorado

The Cooperative Institute for Research in Environmental Sciences (CIRES) is a cooperative institute between NOAA and the University of Colorado. The Institute collaborate in research programs that are aimed at understanding a variety of basic and applied problems associated with the physics and chemistry of the atmosphere, cryosphere, oceans and the solid earth, and brings together government and university researchers, post docs, and students from eight university departments and several NOAA laboratories in a wide-ranging array of scientific collaborations and interdisciplinary research.

[8/9/04]

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