Cooperative Institute for Research in Environmental Sciences

Abstracts: 14

NOAA-CIRES Joint Institute Research to Reconstruct Colorado Streamflow from Tree Rings

Connie Woodhouse, NOAA Paleoclimatology Program, National Climatic Data Center, Boulder, CO (connie.woodhouse@noaa.gov), Robert Webb, NOAA-CIRES, Boulder, CO (rwebb@cdc.noaa.gov) and Jeff Lukas, Institute of Arctic and Alpine Research, University of Colorado, Boulder (lukas@stripe.colorado.edu)

Colorado, along with much of the intermountain western United States, has experienced severe drought conditions in 2002. The drought, preceded by several years of below-average snowpack, has resulted in statewide projected water-year streamflows that are lower than any other year in the instrumental record. If the 2002 drought continues, it may exceed the overall severity of the 1950s drought, until now the drought of record, and the recurrence interval of this 2002 extreme drought will need to be established. The instrumental record is too short for this task, but extended records of streamflow, reconstructed from tree rings, will allow an assessment of the recurrence of this extreme drought in the context of 300 to 700 years of hydroclimatic variability.

In this study joint institute scientists have developed tree-ring reconstructions of annual streamflow for gages east and west of the Continental Divide to evaluate drought frequencies and intensities in the region. East of the Divide, a composite Front Range streamflow record for Middle Boulder, South Boulder, and Clear Creek is used (1685-1987), while west of the Divide, a reconstruction of the Blue River (1539-1999) is used. Drought is evaluated by ranking both records and examining the years in which flow falls below the 50th percentile. Duration, intensity and magnitude are calculated for droughts in both records, which are then compared to assess differences east and west of the Divide. Persistent droughts are shared in both watersheds during some periods (e.g., the late 1680s, early 1700s, and the late 1840s), while other episodes droughts are not (e.g., 1730s and 1750s droughts occurred only in the Blue River watershed, and the 1690s and 1880s primarily in the Front Range watershed). Distributions of single extreme years of drought and longer multiple-year droughts are inspected to assess how representative of drought characteristics the instrumental record period is, including 2002, in the context of past centuries. For single extreme years, reconstruction results suggest that streamflow for 2002 is one of the lowest 5 years in the 400-year Blue River, and likely the lowest year of flow in the 300-year Front Range streamflow record. Sequences of drought years are also examined to characterize different onset and ending patterns for droughts. In particular, the sequence of three years of somewhat below average conditions that lead to the 2002 drought is examined. No clear analogues of this pattern are evident, but sequences of a mix of slightly below average to moderate drought leading up to a severe drought year are evident in both reconstructions.