Background

Below are resources that provide useful background information on the physical basis of tree-ring reconstructions of streamflow, how the reconstructions are generated, and how the reconstructions are used in water management. Note that other climate and hydrologic variables besides annual streamflow have been reconstructed using the same or similar methods, and the information in these resources can be applied to those reconstructions as well.

Tree-ring reconstructions of streamflow and climate and their application to water management - An annotated PowerPoint presentation. This presentation adds much more detail to the topics introduced in the top 10 list below:

    • How tree-ring reconstructions of streamflow are developed—from the sampling of trees in the field to statistical modeling
    • What the reconstructions tell us about past hydrologic variability
    • How the reconstructions are being used in the planning and management of water resources.

The presentation can be downloaded in three sections:

    1) What is the value of tree-ring reconstructions?

    2) How tree rings record climate information

    3) Building the tree-ring chronology

    4) Generating the reconstruction of streamflow

    5) Uncertainty in the reconstructions 

    6) What the reconstructions can tell us about past drought

    7) How the reconstructions are being applied to water management 

    8) Why the reconstructions are relevant in a changing climate

    9) Summary and the TreeFlow web resource

(This presentation has extensive notes on each slide, so we recommend viewing it in "Normal" view with the notes window at bottom enlarged, or in "Notes Page" view. If you would like to use any of the slides or graphics in your own presentations, please contact Connie Woodhouse.)

Reconstruction Case Study - Blue River, CO

The Case Study covers, in greater detail than the presentation, the statistical procedures involved in generating a reconstruction, and analyzing the reconstruction data.



The top 10 things to know about tree-ring reconstructions of streamflow

This list covers the basics of reconstructions of streamflow and their application.

1) A reconstruction is a plausible estimate of past streamflow.

    A tree-ring reconstruction is an estimate of streamflow for some past period using trees that have been proven to be good estimators of observed streamflow over the period of the gaged record. A statistical model is developed which captures the relationship between tree growth and the gage record during their period of overlap. Then, this model is applied to the tree-ring data for the period prior to the gage record.

2) The science behind streamflow reconstructions has a long history.

    The first studies quantitatively relating tree-growth to streamflow in the western US were done in 1930s. The first modern tree-ring reconstructions of climate and streamflow (using computers and multiple linear regression techniques) were developed in the 1960s and 1970s. Techniques for calibrating and validating reconstruction models have been progressively refined since then.

3) Tree growth in the western US is often closely associated with moisture variability, leading to high-quality streamflow reconstructions.

    In semi-arid climates, the same climate factors, primarily precipitation and evapotranspiration, control both the growth of moisture-limited trees and the amount of runoff. Several widespread conifer species (ponderosa pine, pinyon pine, Douglas-fir) are particularly responsive to the variability of moisture from year to year—a sensitivity that is even greater when they grow on dry, rocky sites. Since the trees that are most sensitive to moisture are not those growing directly in river beds, but those on steep slopes in the surrounding watersheds, the relationship between tree growth and streamflow is not direct. Instead, tree growth and streamflow are robustly linked by the regional climate that influences both.

4) With extensive field collections, the already-strong moisture signal in the trees is enhanced through replication.

    At each site, multiple trees are sampled (usually 20-40) to maximize the common climate signal. Each growth ring is “cross-dated”, assigning it to the exact year. Then measured ring-widths from multiple trees are averaged into one site “chronology”. Multiple tree-ring chronologies (when available, though very few reconstructions are based on just one chronology) from the region are used to reconstruct streamflows for a particular stream gage.

5) The reconstruction process is based on the assumptions that the relationship between tree growth and streamflow over the gage period also existed in past centuries, and that the tree-ring data that best-fit the gaged flows will also do a good job of estimating earlier flows.

    One of several statistical methods, typically based on multiple linear regression, is used to determine the subset of tree-ring chronologies that best estimates the gaged streamflow, resulting in a regression equation (the reconstruction model). The skill of the model is evaluated using independent data or on subsets of the calibration data. The model is then applied to the full tree-ring record, generating the streamflow reconstruction extending back hundreds of years.

6) The tree-ring models generally do a very good job of estimating streamflow, but there is always some uncertainty around the reconstructed flows.

    Generally, streamflow reconstructions in the western US explain 50-80% of the variance (R2) in the gaged record. They also capture the important features, particularly droughts, of the gaged record. But trees are imperfect recorders of streamflow; that is, they can never explain 100% of the variance in the actual flow. We can assess the statistical uncertainty in the model using the errors (reconstructed flows minus gaged flows) to generate confidence intervals. Confidence intervals can be constructed, based on the error in the reconstruction, to allow an assessment of the probability of any given year’s value falling outside these confidence bands. There is also additional, unquantified, uncertainty related to the choices made in data treatment and modeling approaches, which can all affect the final result.

7) By providing a longer window into the past, the tree-ring reconstructions describe the natural variability of climate more completely than gaged records.

    The tree rings clearly show that the hydrologic variability of the 20th century does not simply repeat itself. Reconstructions indicate longer and more severe droughts than those in the gaged record—and many some cases, longer and more pronounced wet periods, too. They also demonstrate that the mean annual streamflow has changed on century time scales.

8) Reconstructions of past variability are still relevant in a changing climate.

    Even as human activities exert a stronger influence on climate, this influence will still be superimposed on natural variability. There is no reason to think that the range of natural hydrologic variability, particularly periods of drought, documented in the past will not be repeated in the future. The difference will be that events such as drought will likely occur under warmer conditions. Using the reconstructed flows, rather than just the gaged record, as the frame of reference for planning can lead to fewer “surprises” as we head into a climatically uncertain future. Although projected changes in precipitation are still uncertain in many area, especially in mountain watersheds, projections for temperature are robust. Adding warming to the range of natural flow variability seen in the reconstructions can provide useful scenarios for the future.

9) The reconstructions can be applied to water management in different ways, depending on the needs and capabilities of the data user.

    These applications fall into three general categories:

    • As informal, qualitative guidance for water managers, stakeholders and decisionmakers. 
    • For quantitative assessments of long-term hydrologic variability. For example, assessing the frequency of reconstructed droughts of a given duration and/or severity.
    • As direct inputs into hydrologic models of a water system. This allows water managers to model system performance under the tree-ring reconstructed hydrology, as they would do with the gaged hydrology. This typically requires additional processing of the reconstruction (annual values) to ingest it into the system model, which may have monthly, weekly, or daily time steps.

10) Many different water utilities and agencies across the West have used, or are using, streamflow reconstructions as decision support tools.

    A partial list: 
    • Colorado Water Conservation Board
    • Denver Water
    • Northern Colorado Water Conservancy District
    • City of Boulder, CO
    • Wyoming Water Development Commission
    • Salt River Project (Phoenix area)
    • City of Phoenix 
    • City of Chandler, AZ
    • New Mexico Interstate Stream Commission
    • City of Santa Fe, NM
    • California Department of Water Resources
    • U.S. Bureau of Reclamation - Lower Colorado Region