1. April 28, 2008 Salt River Project, Phoenix, AZ
LTRR-SRP II
THE CURRENT DROUGHT IN CONTEXT: A TREE-RING BASED EVALUATION OF WATER SUPPLY VARIABILITY FOR THE SALT-VERDE RIVER BASIN
Dave Meko & Katie Hirschboeck University of Arizona - Laboratory of Tree-Ring Research
April 28, 2008 Salt River Project, Phoenix, AZ

2. MAIN OBJECTIVE
To update the tree-ring reconstructions of annual streamflow of the Salt-Verde-Tonto Basin through the period of the most recent drought and place it into a long-term, historical context linked to climatic variability

3. MAIN PROJECT ACTIVITIES
UPDATING TREE-RING CHRONOLOGIES – Field collections and laboratory analysis to develop chronologies in the Salt-Verde basin with data through growth year 2005
NEW STREAMFLOW RECONSTRUCTION – Analysis of the new tree-ring chronologies to place the most recent drought in a long-term context
EW-LW EVALUATION – Exploration of the seasonal precipitation signal in separate measurement of earlywood and latewood width measurements
CLIMATIC ANALYSES – Synoptic dendro-climatology studies of observed record to better interpret the reconstructed record

4. TREE-RING CHRONOLOGY UPDATING

5. Douglas-fir at Wahl Knoll site, White Mountains , AZ
Tree-Ring Collections
Douglas-fir at Wahl Knoll site, White Mountains , AZ 5

6. Tree-Ring Collections
Black River Pine Site
Robinson Mt Site
Collections at 14 Sites in Fall 2005
Species:
Douglas-fir
ponderosa pine
pinyon pine
Some re-collections, some new collections
Cores only 6

7. Tree Ring Widths – the Basic Data
Site 2 - Black River Pine Core 13B
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
HH Years
LL Year
Link to previous LTRR-SRP I study on joint drought (LL HH) in Salt-Verde and Upper Colorado Basins
Narrow rings in dry years, wide rings in wet years 7

8. STREAMFLOW RECONSTRUCTION PROCESS

9. Overview of the Reconstruction Process
Tree Ring Network
Statistical Calibration: regression
Reconstruction Models
Time Series of Reconstructed Streamflow
Observed Streamflow

10. Three Different Models Used
(based on different sub-periods)
Tree-ring sites not in model
Tree-ring sites in model *
Key to dots
Tree-ring sites have variable time coverage
Uniform time coverage required for a model
Sub-period reconstructions ultimately blended into final time series of reconstructed streamflow 10

11. RESULTS OF THE NEW RECONSTRUCTION

12. Annual Reconstructed Flows, 1330-2005
Plotted as % of normal* *normal = defined as median of observed flows
Median baseline
5-yr dry spells
2002 and 1996 have the lowest reconstructed annual flows in the entire record (28% and 30% of normal* respectively)
Maximum number of consecutive years below normal = 5 (in 1590s and 1660s)
Longest stretch of consecutive years below normal in recent interval of is 4 years (in 1950s)
Normal as defined here is 895 kaf
Tally of runs depends on choice of threshold for normal. Medians of observed and reconstructed flows differ for common period : kaf for observed flows and 1059 kaf for reconstructed flows. If use the higher median as threshold for normal, the longest reconstructed run below normal is 9 years. This occurred twice – and Even with the relaxed threshold, the reconstructed flows do not contain any runs longer than 4 years in the period 12

13. “Missing” Rings (locally absent on tree where cored)
Close up of cores from two different trees at Site 10, located near Flagstaff:
// = false ring bands
This core is missing the year (27 of the 30 trees at this site had no 2002 ring)
This core has a very narrow 2002 “micro-ring” (only 3 trees at this site had a 2002 ring) 13

14. Missing-Ring Percentage Through Time
How unusual is such a high % of missing rings?
2002
2002 was unprecedented for frequency of missing rings 14

15. Variations in Time-Averaged Flows
Plotted as % of normal* *normal =median of all 6-year running means
Baseline
14 distinct prior occurrences of flow as low as average
1- 3 occurrences in each century
Most severe conditions at ~1590 and ~1670 15

16. Variations in Length of Intervals Between High Flow / Wet Years
1950 & narrow rings
1952 wide ring
High flows and large floods can occur during periods of drought and low flows
Wide rings can occur within otherwise narrow-ring sequences

17. Floods / High Flows & Reconstructed Flows
High flow / flood “wet years” are tracked reasonably well by Verde River tree-ring reconstruction
Verde River Basin Comparison: Observed, Reconstructed, & Instantaneous Peak Flows

18. Length of Intervals Between Wet Years Based on Observed Flows, 1914-2007
Recent gap = 9 yr
“Wet Year” = flow above 75th percentile
Plotted series is for Salt+Verde+Tonto
Circles mark flows above 75th percentile
Interval or gap is the number of consecutive year between wet years (0 if wet years adjacent)
Wet year threshold 1504 kaf
23 of the 94 years classified as wet
Interval longer in 1950s than during recent drought period
If not for mildly wet 1952, the earlier interval would have been 25 years
Median interval is 2 years in the observed record 18

19. Length of Intervals Between Wet Years Based on Reconstructed Flows, 1330-2005
Recent gap
12 yr
“Wet Year” = flow above 75th percentile
Longest interval = 22 years ( )
Recent interval = 12 years ( )
1950s interval = 12 years ( )
10 intervals ≥ 12 years
Median interval is 3 years
Salt+Verde+Tonto reconstruction, as in previous slide observed flows
Reconstruction does not perfectly classify wet years of the observed record. For example, 1993 was a wet year in the observed, but not in the tree-ring record. This is why the “recent” interval is shorter for the observed flows than for the reconstruction
Other large intervals are
20-yr gap ending in 1585
16-yr gaps ending in 1676, 1742 19

20. Upper figure: same as we had in presentation to GM
LL: cdf’s of reconstructed flows for entire reconstruction and for the period of the reconstruction that overlaps observed flows.
LR: cdf of gap, or interval, between wet years, again for both the full-length reconstruction and the part that overlaps observed flows
LL:shows that 0.75 quantile is slightly lower for the than for The difference is a bit over 100,000 acre-ft. This is consistent with the long-term record being a bit drier than the instrumental period. Note that the long-term 0.75 quantile is the threshold (horizontal red line) in the top plot.
LR: This plot summarizes the time, or gap, between wet years. If two consecutive years are above the 0.75 quantile, the “gap” is defined as zero. The median gap for both the long-term and the instrumental period is 3 years. The 0.9 quantiles are also equal, at 9 years. The difference comes in the extremes. The record contains a 22-year gap, around the start of the 15th century. In contrast, the longest gap in the part of the reconstruction is 12 years ( and )
LR:

21. EARLYWOOD-LATEWOOD EVALUATION

22. Earlywood / Latewood Evaluation
Ring width can be partitioned into parts formed early and late in the growth year
Studies have shown some success at inferring summer rainfall variations from latewood width 22

23. Latewood Precipitation Signal – Wahl Knoll Example
Signal Strength by Seasonal Grouping
Prism Precipitation
Wahl Knoll
Latewood Index
Input-Output
Model
Signal confined mainly to summer
Maximum of ~30% of variance of July or July+August precipitation explained by modeling
Wahl Knoll gave some of the more promising results
We modeled PRISM precipitation against latewood width indices at four sites
Because summer rainfall is inherently spotty, a relatively dense tree-ring network would be needed to capture the regional signal for monsoon rainfall, especially with the single-site signal being relatively weak

24. Testing for Latewood Signal of Summer Rainfall
Total width had signal for annual precipitation, but no signal for summer precipitation
Latewood width had a weak but significant signal for summer precipitation
SUMMARY: Results encouraging, but summer precipitation signal in partial ring widths is too weak to expect useful reconstruction of summer monsoon variability from this limited site coverage 24

25. THE CLIMATIC CONTEXT OF RECENT DROUGHTS & HIGH FLOW EPISODES

26. The “Big Picture” Global Climate Context
NH Temperature Data from NASA/GISS; data are departures from mean based on GHCN met stations
Upper Colorado Flows natural flows for Lees Ferry from USBR
Salt-Verde-Tonto Flows from USGS
Horizontal lines for flows are at medians
Recent drought: mean NH temperatures near record highs
1950s drought: mean NH temperatures near middle of long- term warming trend
Wet late 1970s to early 1980s: mean NH temperatures higher
Wet period : mean NH temperatures low
(not shown here) severe tree-ring drought of : mean NH temperatures very low
set(gcf,’Position’,[ ]); 26

27. Link to LTRR-SRP- I Project
Low Flow
Years in Both SVT & UCRB
Updated LL and HH years exhibit anomaly patterns similar to those of the earlier study
700 mb composites of new LL and HH years for Dec-Feb
High Flow
Years in Both SVT & UCRB

28. Floods / High Flows & Reconstructed Flows
High flow / flood “wet years” are tracked reasonably well by Verde River tree-ring reconstruction
Verde River Basin Comparison: Observed, Reconstructed, & Instantaneous Peak Flows

29. Analyzing the reconstruction synoptically:
Verde River Basin Instantaneous Peak Flows Classified by Synoptic Type
Water Year
Synoptic Convective Tropical Storm
Jan 1993
Mar 1938
Feb 1995
Dec /Jan 2005
Winters of
Dec /Jan 1952
Feb 1927
TS Norma Sep 1970
1950s convective dominance

30. Analyzing the reconstruction synoptically:
Synoptic Convective Tropical Storm
Analyzing the reconstruction synoptically:
Both reconstructed & observed annual flows track the magnitude of the instantaneous peaks best during SYNOPTIC (winter) events

31. Synoptic Circulation Patterns for SVT
Verde Basin study: tree-ring record is a good indicator of winter storm track activity
1950s Drought Low flow years in SVT Dec – Feb
1950s pattern vs. “Recent Drought” pattern
Recent High Flow Year pattern
“Recent Drought” Low flow years in SVT Dec – Feb
“Recent” High flow years in SVT Dec – Feb

32. Synoptic Atmospheric Circulation Patterns Linked to Dry and Wet Intervals
Water Year
Atmospheric Circulation Anomaly Patterns (700 mb) associated with Dry and Wet Intervals in the Verde Basin
“Current” Drought
1950s+ Drought
1996 & 2002
1993

33. Importance of BLOCKING circulation anomaly patterns:
Blocking leads to the PERSISTENCE of circulation features that produce EXTREMES
drought /

34. Are streamflow variations cyclic?
Spectrum, Reconstruction
ENSO footprint?
Is streamflow cyclic? A spectral analysis of the reconstructed flows for the Salt-Verde-Tonto shows no evidence of strong periodicity (Figure ??). A weakly significant spectral peak occurs at about 5.7 years, but is more broadly consistent with generally high variance between 4 and 8 years than with a true cycle at 5.7 years. About 1/3 the total variance occurs in the 4-8 year band. The high variance in this wavelength band is consistent with ENSO influence on climate.
Spectrum of reconstructed annual SVT flow, Spectral estimation by smoothed periodogram method (Bloomfield 1976) using Daniell filter spans { }. Red noise (autoregressive) null continuum after Wilks (1995) .

35. SUMMARY & CONCLUSIONS

36. Reconstruction Model Summary
Ring widths of the new collections have a strong annual runoff signal
Subset models blended together yield a streamflow reconstruction covering
The reconstruction explains % of the variance of the annual flows

37. Extreme Single-Year Summary
The reconstructed 1996 value was the 2nd lowest reconstructed flow since 1330
The reconstructed 2002 value was the LOWEST reconstructed flow since 1330
From tree’s perspective 2002 was a year like no other: 60% of 300+ cores were missing the 2002 ring!

38. CONCLUSIONS
1) Single-year intensity: drought in recent years unsurpassed in long-term tree-ring record (i.e., 1996, 2002)
2) Multi-year intensity: 14 distinct prior occurrences of flow as low as average

39. CONCLUSIONS
3) Several intervals between “drought relieving” wet years were longer than any observed in the instrumental record
4) Winter storm track position key factor in drought signature (1950s vs. recent drought)

40. Final Report will be posted at: