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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