Connie Woodhouse samples an old Douglas-fir near Dillon Reservoir.

Project history

Surprisingly few tree-ring chronologies had been developed for the upper Colorado River basin since the pioneering collections of Edmund Schulman in the 1940s and Marv Stokes and Tom Harlan in the 1960s. The latter were used by Charles Stockton and Gordon Jacoby in their well-known reconstruction of streamflow for the Colorado River at Lees Ferry, which demonstrated that the mean flow of the river assumed in the 1922 Colorado River Compact had been based on a period of unusually high flow compared to the 450-year tree-ring record.

Fifteen new chronologies collected in the summer of 2000 by Connie Woodhouse, Jeff Lukas, and others confirmed the great potential for long tree-ring reconstructions of streamflow and other hydroclimatic variables in Colorado that had been suggested by the earlier collections. More chronologies were collected in 2001 and 2002, including a number on the Front Range. Incorporated into these chronologies were some of the oldest known pinyon pine (>870 years), Douglas-fir (>860 years), and ponderosa pine (>630 years) in Colorado. These chronologies complemented those collected by Connie Woodhouse and Peter Brown in southeastern Colorado in 1998-2000 for an NSF-funded project (ATM-9729571) reconstructing the climate of the western Great Plains.

In spring 2002, Connie Woodhouse and Robin Webb received funding from the NOAA Office of Global Programs for a multi-year project entitled Extended Hydroclimatic Records for the Upper Colorado River Basin, which proposed to use the new chronologies to develop reconstructions of hydrologic variables, principally streamflow, and to work with water resource managers to make these reconstructions more applicable to their operational and planning needs. The occurrence of a very severe drought event in 2002 spurred significant interest in the project, and by March 2003 a number of entities had become involved, including the Northern Colorado Water Conservancy District (NCWCD) and Denver Water. The scope of the project has expanded beyond the Upper Colorado basin to include Front Range watersheds, and future work will cover all major watersheds within Colorado.

In 2003 and again in 2004, additional funding was provided by NOAA/CIRES Western Water Assessment, a Regional Integrated Sciences and Assessment (RISA). Denver Water has also provided funding to update chronologies in western Colorado (2003) and the Front Range (2004).

This project has been supported by the work of past (Gary Bolton, Margot Kaye, Curtis Nepstad-Thornberry) and current (Henry Adams) assistants in the CU-INSTAAR Dendrochronology Lab. Mark Losleben (CU-INSTAAR Mountain Research Station) has also provided extensive field support.

Project objectives

• Work with water resource managers to identify the gages and hydrologic metrics most useful in water resource planning and management, as candidates for reconstruction.

• Develop reconstructions of annual streamflow, and associated products and analyses, for the gages of interest to water managers. Work with data users to ensure that the products and presentations are meaningful and useful.

• Develop an online dendrohydrologic reference web page and database that will contain reconstructions and gage data, as well as information on reconstruction methods and assessment of reconstruction quality, and caveats for use.

• Study the feasibility of developing reconstructions for other hydrologic metrics of interest (e.g., April 1 snow water equivalent, n-day low flow). Develop reconstructions of these as is possible.

• Use the reconstructions to evaluate hydroclimatic events (droughts and wet periods) and characteristics (distribution of mean, extremes, and change in variability) over the past 300 to 800 years in comparison with 20th and 21st century events and characteristics.

• Investigate linkages between drought in the upper Colorado and South Platte River basins and in other parts of the western U.S. to identify possible relationships between regional drought and atmospheric circulation.

• Use the reconstructions to assess the ability of ensembles of climate model runs to simulate statistical characteristics of the reconstructions.

Project Partners

The following agencies and entities have become involved in the project. Their involvement includes providing gaged streamflow or instrumental climate records to be reconstructed, and /or collaboration in the development of reconstruction techniques and data products.

A tree-ring reconstruction is a time series of tree ring-width data that have been calibrated with an instrumental or gaged record of a hydrologic or climatic variable (e.g. annual streamflow, precipitation, snow water equivalent, drought index). The reconstruction, based on a statistical model that describes the relationship between tree growth and the gage record, extends that record back hundreds of years into the past (see figure below).

Tree-ring reconstruction of annual streamflow for the Blue River at Green Mountain Reservoir, Colorado for the period 1540 to 1999. The green line shows annual values, and the black line is a 3-year running mean.

Once a gaged record of interest is selected for reconstruction, a set of tree-ring chronologies from the region near the gage is calibrated with the gage record to form a reconstruction model. A statistical technique called multiple linear regression is commonly used. The reconstruction is evaluated by comparing the observed gage values with the reconstructed values as assessing the amount of variance in the gage record that is explained by the reconstruction. The reconstruction model is then validated by (1) testing it on a portion of the gage data that was withheld from the calibration process, or (2) testing the ability of the chronologies used in the model to estimate streamflow in different subsets of the data, or in an iterative process using bootstrapped datasets, such as a linear neural network. For a much more detailed description of the reconstruction process and of how reconstructions are evaluated, see the Blue River Case Study.

The Blue River at Dillon Reservoir reconstruction model (green) and the gaged record (blue) over their common time period (1916-2002).

Water managers have long used instrumental records of climate and gaged records of streamflow to assess the natural variability of the system they are managing, determining the frequency of drought events of varying severity and establishing a drought of record to use as the worst-case scenario in contingency planning. However, instrumental and gaged records are usually only 30 to 100 years long and are unlikely to capture the full expression of potential natural variability. Tree-ring reconstructions, by providing a much longer window into the past (300-1000 years), more completely describe the potential natural variability of the system, including severe drought events. Many tree-ring reconstructions have indicated that droughts longer and more intense than those in the instrumental record have occurred in past centuries. This additional information on long-term hydroclimatic variability can guide water resource planning to better meet the challenges of potential future conditions. Water managers using tree-ring reconstructions will not be surprised by events, like the 2002 drought, that exceed the bounds of the operational experience of their system.

The gaged USGS streamflow record (light blue) for the Fraser River near Winter Park is compared with the undepleted flow record (dark blue) derived by Denver Water from the gaged record to account for diversions, which began in earnest in 1936. The gaged record poorly represents the trends in streamflow on short and long time scales.

Additional caveats for using reconstructions

• It is tempting to combine the reconstructed and gage records into a single record to which new gaged values (for 2003, 2004, etc.) could be easily appended. The problem with doing this is that the statistical properties of the reconstructed and gage streamflow series are not the same.Tree-ring chronologies do not explain all the variance in the gage record. Because of this, the estimates of flow produced by the reconstruction model do not represent the full range of values in the gage record, although dry extremes are typically more closely replicated than wet extremes. Consequently, the variance, standard deviation, range, and max/min values are different. Any attempt to blend the two records needs to acknowledge these important differences and the limitations they impose.
• Although tree-ring reconstructions can be used to guide expectations for the future, they cannot be used to predict future droughts. Furthermore, the variability in past climate may not be an analog to future variability, given the human impacts on climate in the last century. Future conditions will be influenced by climate forcings, including changes in land use and elevated inputs of greenhouse gases, as well as by natural climate variability. Future climate will reflect natural climate variabilty, including the long-term range of variability described in the tree-ring reconstructions, but superimposed over this natural variability will be the anthropogenic impacts on climate.