PROJECT OVERVIEW

Purpose:
 

The purpose of this project was to use long term records of tree-ring reconstructions of annual streamflow to analyze variations of low flow and high flow extremes in the water supply of the Upper Colorado and Salt-Verde River basins over the past several hundred years. 

 

The central question guiding the research was:  How frequently have extreme droughts or high flows occurred in both basins simultaneously in the past?  

 

Specifically the project goals were:

  • to determine how streamflow extremes in each basin have co-varied over time,

  • to assess the hydrometeorological and hydroclimatological causes of this past co-variation,

  • to provide probabilistic estimates of the likelihood of various scenarios of synchronous low-flow and high-flow extremes, and

  • to devise an assessment tool for implementing the project's results into operational water supply decision-making.

Summary of Results:

Tree-ring reconstructions of total annual  (water year) streamflow for gages in the Upper Colorado River Basin and Salt-Verde River Basin were computed and analyzed for the period 1521-1964.  These reconstructed flow series were used to identify years of extreme low flow (L) and high flow (H) discharge in each basin, based on 0.25 and 0.75 quantile thresholds, respectively.  

  • Synchronous extreme events in the same direction in both basins  (LL and HH events) were much more frequent than LH or HL events, which turned out to be extremely rare occurrences.  

  • Extreme synchronous low flow (LL) and high flow (HH) events tended to cluster in time. The longest period of consecutive LL years in the record was 3 years.

  • In terms of multi-year extremes, a scenario of 2 extreme years occurring anywhere within a 3-yr or 4-yr moving window was the most common.

  • The overall conclusion based on the long-term record is that severe droughts and low flow conditions in one basin are unlikely to be offset by abundant streamflow in the other basin.

It was also found that the recent drought in the Salt-Verde Basin ranks in severity with the drought of the 1950s, the previous extreme for low flow in the gaged record.  By extending the record back in time using tree-rings, it was found that 8 distinct periods prior to the start of the gaged record show long episodes (11-year long averages) of flow that are drier than the 1950s episode.  Hence the recent drought, while severe, does not appear to be unprecedented in the context of the long term record of the past 500 years.

 

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