Using the Paleoclimatic Data Record to Understand Historical Precipitation Patterns

Transcription

1 Using the Paleoclimatic Data Record to Understand Historical Precipitation Patterns Connie Woodhouse School of Geography and Development University of Arizona Workshop on Drought Vulnerability and Tools for Improving Water Resilience October 19, 2016 Long Beach CA Acknowledgements: Research and photo contributions from Dave Meko, Erica Bigio, Brewster Malevich, & Chris Zemp. Funded is from CADWR

2 Why consider the past when planning for the future?

3 Most instrumental records extend 100 years or so, at best. These records document extreme dry years and persistent drought. Sacramento 4 Rivers, Are these droughts representative of the past, and a good indication of what we can expect in the future?

4 Extended records from tree-ring data can document droughts over past centuries and place the gage record into a long-term context Reconstructed Sacramento River flow Reconstructed Sacramento River flow Sacramento River reconstruction (smoothed) and the gage record,

5 Extending the instrumental records of climate using tree rings

6 How is tree growth related to climate?

7 What trees are the best recorders of precipitation and drought? Moisture-sensitive tree species growing on open, well drained sites In southern California, these include: bigcone Douglas-fir ponderosa pine Jeffrey pine foxtail pine blue oak

8 Moisture-stressed trees closely track variations in precipitation Ring widths from a single tree near Bear Valley (County) with water year precipitation in the Russian River Valley.

9 Trees are sampled with increment borer. Cores are taken from ~20 trees per site. Cross sections from dead trees are also used.

10 Tree rings are dated and measured, and ring width series are compiled into site tree-ring chronologies

11 Reconstruction process in a nutshell Tree Rings Observed Streamflow Statistical Calibration Reconstruction Model Time Series of Reconstructed Streamflow

12 This project: Reconstructions of streamflow and annual precipitation for southern California (and the upper Colorado River basin)

13 Today: Results for a preliminary reconstruction of San Gabriel Dam water year (October-September) precipitation

14 Comparison of observed and reconstructed San Gabriel Dam precipitation, The tree-ring model explains ~79% of the variance in the water year precipitation record except for the last 12 years (65%).

15 Statistical comparison of observed and reconstructed San Gabriel Dam precipitation Units are inches

16 Statistical comparison of observed and reconstructed San Gabriel Dam precipitation Units are inches

17 San Gabriel Dam reconstructed precipitation, Blue line = annual values Gray lines = 50% confidence intervals Less certainty in the reconstruction for for now.

18 San Gabriel Dam reconstructed precipitation, , smoothed with a 20-year running mean with observed precipitation

19 How do the driest intervals in the period of the modern record rank in the context of the past 6 centuries? Driest non-overlapping and 20-year average periods Values are in percentile (0.50 = median)

20 What is the distribution of driest non-overlapping 5-yr 10-yr and 20-yr average periods since 1400? Values are in percentile (0.50 = median)

21 How Runs below is the median frequency and duration of droughts different over full and modern periods? *Defined as a single or consecutive series of years below long-term median f

22 How are droughts > 3 years in length distributed over the past six centuries? Gray horizontal line marks the 20 th percentile

23 How does the distribution of droughts > 3 years in the San Gabriel Dam record compare to the Sacramento and San Joaquin River basin droughts?

24 How does the distribution of droughts > 3 years in the San Gabriel Dam record compare to the Sacramento and San Joaquin River basin droughts?

25 ..and to Colorado River droughts?

26 ..and to Colorado River droughts?

27 Summary San Gabriel Dam precipitation reconstruction shows The instrumental period contains severe droughts in the context of the past 6 centuries; not always the driest, but within the top nine. This instrumental period does not capture longer duration droughts (6 and 7 years) that are evident in the full reconstruction The most notable series of dry year runs is the back-to-back set of 7 dry years, and during which precipitation averaged at or below the 30 th percentile. Runs of relatively extreme dry years appear to cluster in the 19 th century Comparison of San Gabriel reconstructed precipitation with Sacramento and San Joaquin flow reconstructions shows overlapping periods of region-wide droughts do occur. Concurrent periods of drought are evident in southern and northern California and the upper Colorado River basin --- these are less common, but they do occur.

28 Are droughts of the past relevant to the future? The climate of the past will not be replicated in the future, but the range of natural climate variability is likely to continue, underlying human-induced changes to climate. We should anticipate droughts similar to those of the past centuries, with the added effects of climate change.