1. What river basins and streamflow gages were reconstructed?

    Streamflow reconstructions were completed for the following 8 rivers /gages, listed with their period of observed record.  See the Project Map for streamflow gaging sites, tree-ring sites, and climatic stations that were used in the project.

    A. Colorado at Lees Ferry 1906-1995
    B. Salt + Verde + Tonto 1914-2002
    C. Gila at head of Safford Valley 1915-2002
    D. Green at Green River, UT 1906-1995
    E. Colorado near Cisco, UT 1906-1995
    F. San Juan near Bluff, UT 1906-1995
    G. Salt + Tonto 1914-2002
    H. Verde 1914-2002

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  2. Why should there be a relationship between streamflow and tree growth in the arid and semi-arid western United States?

    Tree growth can be limited if there is not sufficient moisture in the soil. This can arise from either a lack of precipitation, or excessive evapotranspiration in warm, dry climates-- or both. Regionally, precipitation minus evaporation determines the amount of runoff available in rivers and streams. Hence both trees and streams can respond to weather and climate patterns that produce sustained drought condition. In the project study areas, the snow that falls in the winter prior to a tree's growing season is reflected in the tree's growth in spring and summer via soil moisture storage.

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  3. How good are the tree-ring based reconstructions of flow when compared to the gaged record?

    The reconstructions track the timing of high and low flow years quite well and do a good job of capturing the magnitudes of the flow, especially in the dry years. The reconstruction models explain 77.4 % of the variance for the Colorado at Lees Ferry (r = 0.88) and 57.8 % of the variance for the Salt-Verde-Tonto (r = 0.76). The magnitudes of extreme wet years are reconstructed more accurately in the large Colorado basin than in the Salt-Verde-Tonto basin, in part because of the latter smaller basin’s “flashier” streamflow.

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  4. Are the gaged and reconstructed records comparable in terms of mean?

    In both basins, the mean of the observed period is higher than the longterm reconstructed mean. This indicates that, in general, the 20th century has been wetter in both basins than in previous centuries. (The recent drought years of the late 1990s and early 2000’s were not included in this analysis, however, and their inclusion would lower the observed-record mean.)

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  5. What criteria were used to define extreme streamflow episodes in each basin?

    Extreme episodes were based on thresholds derived from quantiles of water-year annual discharge (compared to the mean).  High flow years (H) are those with flow > 0.75 quantile and Low flow years (L) are those with flow < 0.25 quantile. At a later stage of the analysis, thresholds of > 0.90 and < 0.10 were also examined in order to assess the most extreme years.

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  6. What types of synchronous extreme streamflow sceanrios have occurred in both the observed and reconstructed record and how frequently did each type occur?

    Four synchronous scenarios are possible when comparing the Upper Colorado and the Salt-Verde-Tonto River systems and are indicated by a two-letter code. (For consistency, the first letter will always refer to the Upper Colorado and the second to the Salt-Verde-Tonto):

    When the two basins are responding in the same direction:
    HH = High flow (H) in the Colorado at the same time as high flow (H) in the Salt-Verde-Tonto
    LL = Low flow (L) in the Colorado at the same time as low flow (L) in the Salt-Verde-Tonto

    When the two basins are responding in the opposite direction:
    HL = High flow (H) in the Colorado at the same time as low flow (L) in the Salt-Verde-Tonto
    LH = Low flow (L) in the Colorado at the same time as high flow (H) in the Salt-Verde-Tonto

     In both the observed and reconstructed records, HH and LL events were much more frequent than HL and HL events, especially in the long, 444-year reconstructed time series. In fact, no HL events at all occurred in the reconstructed record, and only 2 LH events occurred. In the observed record, only 3 HL events and no LH events occurred. In order to examine some LH-like scenarios in the observed record, the Colorado threshold was relaxed to < 0.50, yielding LH events. Due in part to the quantile method, the number of LL events tends to be counterbalanced by the number of HH events, but overall – in both the observed and reconstructed records – LL events are more frequent occurrences than HH events. A year-by-year listing of the HH, LL, HL and LH events, color coded by each year’s streamflow threshold (>0.90, > 0.75, < 0.25, < 0.10) is presented in Appendix 5 of the Final Report.

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  7. What are the implications of the project results?

    A widely held working hypothesis is that the Upper Colorado River Basin (UCRB) can serve as a buffer to compensate for extreme low flow in the Salt-Verde-Tonto Basin during drought periods. One reason this is plausible is because of the size of the UCRB basin and the overall magnitude of its discharge. It has also been assumed that streamflows in the two river systems are relatively independent of each other due to a difference in the climatic regimes influencing each basin.

    While the above presumptions sound logical, our analysis did not substantiate them. We found that the simple correlation between the annual flow values of the two 444-year reconstructed time series was r = .599 ( r     2 = 0.359), which is a statistically significant correlation, given the large sample size. Far more compelling with respect to extreme events, however, was the overwhelming dominance of HH and LL events in comparison to extremes where the basins responded in the opposite direction.

    These results suggest that annual streamflow variability in the Salt-Verde Basin – especially extreme streamflow – is not independent of annual streamflow variability in the Colorado Basin. Therefore it is not unlikely that severe drought in one basin will be accompanied by severe drought in the other basin, such as has occurred during the western drought scenario of the late 1990s and early 2000’s. It should be noted that the immense water supply of the large Upper Colorado basin may allow it to continue to serve as a buffer even when it is in a low flow scenario because a little flow can go a long way in a smaller watersheds such as the Salt-Verde-Tonto, however ever-increasing demand on the water supply of the UCRB by many stakeholders is likely to reduce the size of this buffer in the future.

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  8. Is the observed frequency of occurrence of HH and LL years greater than can be expected by chance? What’s the probability of different lengths of LL and HH sequences ?

    The probability of the observed numbers of joint HH and LL years is less than one in a billion if the extreme streamflow in the two basins were indeed independent of one another!

    For different lengths of LL and HH sequences, there is > 10% chance that a single year could be an extreme LL year or an extreme HH year. There is also a 10% chance that 2 LL years (or HH years) could occur in any given 4-year period and a > 5 % chance that 2 years out of a moving 3-year window will be extreme LL years (or HH years).

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  9. What are the implications of the observed tendency for extreme years to occur in sequences or clusters?

    It is possible that if the number of wet extreme years is about equal to the number of dry extreme years, that the two extremes could “cancel each other” on a year-to-year basis such that there would be little long-term stress on water supply operations. However, because of the clustering tendency of extreme events, it is more probable that episodes of sustained drought or sustained high flow will persist, placing more of a burden on water systems management and operations. In other words, reservoir storage can buffer water supplies in the Upper Colorado River Basin and Salt-Verde-Tonto region, but supplies will be increasingly strained as droughts extend over multi-year periods.

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  10. How can the probability of joint multi-year drought occurrence in the two basins be assessed?

    The results of the Monte Carlo analysis are summarized in this Table, which lists for each length of moving average the number of LL events in the reconstructed flows and threshold number of LL events with empirical non-exceedance probability 0.50, 0.95, 0.99 and 0.999 by chance. For example, for the 7-year moving average, the Lees Ferry reconstruction and SVT were simultaneously below their drought thresholds in 62 years, which is far more than expected by chance: the Monte Carlo tally indicates the probability is of fewer than 50 LL events if the flows on the Colorado and the SVT are unrelated. The expected number of chance events, given by the probability point is only 27. In summary, none of the observed LL counts (column labeled ) has a greater than 0.001 probability of occurring by chance alone. Since there were only 1000 simulations done for the analysis, it is also true no simulation had a count of LL events as large as for the reconstructed series themselves.

    The results strongly support a linkage in multi-year drought occurrence in the two basins.

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  11. How have the numbers of LL and HH years varied from century-to-century?

     APPENDIX  5 (pdf)  display a century-by-century record of extreme streamflow events in the Upper Colorado and the Salt-Verde-Tonto River basins. The 19th century experienced more LL years than any other in the record (22 events) and the 18th and 20th centuries (prior to 1965) experienced the most HH years (14 events). 
     
     

    Century

    		 # LL Years # HH Years
      1520-1599 11 11
      1600s 11 8
      1700s 13 14
      1800s 22 11
      1900 -1964 8 14

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  12. What atmospheric circulation patterns lead to the LL, HH, LH and HL scenarios?

    Seasonal composites of upper level (500 mb geopotential height) circulation anomalies for each type of extreme event scenario (based on the observed record) are shown in these two FIGURES. The characteristic circulation pattern for LL events is higher-than-normal upper level pressure over the west in early winter (Oct -Dec) and over the North Pacific ocean storm track region in mid- to late winter (Jan - Mar). The inverse of this pattern leads to HH events.  LH and HL scenarios arise when the Pacific storm track appears to shift to an anomalous poleward (HL) or equatorward (LH) location.

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  13. Are there any linkages of these anomaly patterns to climate-scale driving mechanisms such as sea surface temperature anomalies, El Niño, La Niña, etc.?

    Preliminary examination of El Niño, La Niña influences and ocean indices such as the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO) suggest linkage to some – but not all LL years in the observed record (see FIGURES). A University of Arizona M.S. thesis is in preparation to examine this in more detail.

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  14. Is the recent drought in the Salt-Verde Basin unprecedented in severity?

    The annual flows of the basin decreased beginning in water-year 1994 and culminated in single-year flows for 2000 and 2002 lower than any previously experienced in the observed record (FIGURE, top left). The 5-year, 11-year and 15-year running means reached their recent low points in the periods ending with water year 2004. As a 5-year running mean, the recent drought is about as severe as the lowest-flow period in the 1950s. The same is true of the 11-year running mean, suggesting that the period commencing with the decline in water year 1994 and continuing through water year 2004 ranks with the driest conditions in the entire gaged record. When the running mean is extended to 15 years (FIGURE, lower right), the recent drought no longer ranks among the most severe; the reason for this is the wet sequence of years in the early 1990s enters the moving average. But up to an averaging period of 11 years, it appears the recent drought is at least comparable in severity to any earlier drought in the gaged record.

    The tree-ring reconstruction for the Salt+Verde+Tonto (SVT) ends in 1988, and so does not cover the recent drought. Nevertheless that reconstruction does sample the 1950s drought, and because the 1950s drought was characterized by flow departure of roughly the same magnitude as the recent drought, we can use the lowest reconstructed flows of the 1950s to indirectly evaluate the relative severity of the recent drought in the context of the reconstruction to A.D. 1199.

    A plot of 11-year running means of the SVT reconstruction with the baseline marked as the low point in the 1950s suggests that the current drought was exceeded in severity several times in the past 800 years (FIGURE). Eight distinct periods before the start of the gaged record show lower 11-year mean flow than the lowest reconstructed value of the 1950s. The most severe of the tree-ring droughts was in the late 1500s, during the well-documented “mega-drought” of North America, when 11-year average flow is reconstructed about 100 cfs below the lowest flows of the 1950s.

    BOTTOM LINE:  The recent drought, while severe, does not appear to be unprecedented when viewed in a multi-century context.

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