A TREE‐RING RECONSTRUCTION OF STREAMFLOW FOR THE COLORADO FRONT RANGE1

ABSTRACT: Water resource planning is based primarily on 20th century instrumental records of climate and streamflow. These records are limited in length to approximately 100 years, in the best cases, and can reflect only a portion of the range of natural variability. The instrumental record neither can be used to gage the unusualness of 20th Century extreme low flow events, nor does it allow the detection of low‐frequency variability that may underlie short‐term variations in flow. In this study, tree rings are used to reconstruct mean annual streamflow for Middle Boulder Creek in the Colorado Front Range, a semi‐arid region of rapid growth and development. The reconstruction is based on a stepwise regression equation that accounts for 70 percent of the variance in the instrumental record, and extends from 1703–1987. The reconstruction suggests that the instrumental record of streamflow for Middle Boulder Creek is not representative of flow in past centuries and that several low flow events in the 19th century were more persistent than any in the 20th century. The 1840s to early 1850s period of low flow is a particularly notable event and may have coincided with a period of low flow in the Upper Colorado River Basin.     

EVALUATING RELIABILITY OF STORAGE SCHEMES WITH DENDROHYDROLOGY AND THE HURST PHENOMENON1

ABSTRACT: The mean annual flow at a damsite during a water project yield study of several decades may differ considerably from the mean flow of several hundred years. The frequency of the most severe droughts of record may be much different than apparent from the historical record as well. Dendrohydrology and the Hurst Phenomenon provide means to evaluate the validity of the study period for project reliability analysis. The most severe hydrologic drought (1928–1934) affecting the watersheds of the Sacramento River and tributaries in Northern California during the 75-year period 1906–1980 was also the worst drought in 421 years (1560–1980). In contrast, the most severe drought (1945–1951) in the Santa Ynez River watershed in Santa Barbara County, California, during the 62-year period 1918–1979 was the ninth worst drought in 443 years (1537–1979). Thus, in one case the drought risk indicated by the dendrohydrologic time series would actually be less; in the other, actual risk would be greater than perceived from the historical record. In the absence of a dendrohydrologic prehistoric time series, Hurst Phenomena would have provided clues to this outcome. Hurstian plotting of the accumulated deviation from the mean for long time series facilitates observation of the wet-dry regime of the examples and identification of characteristics that should be accounted for in water development planning.     

THE TREE-RING RECORD OF SEVERE SUSTAINED DROUGHT1

ABSTRACT: Frequent and persistent droughts exacerbate the problems caused by the inherent scarcity of water in the semiarid to arid parts of the southwestern United States. The occurrence of drought is driven by climatic variability, which for years before about the beginning of the 20th century in the Southwest must be inferred from proxy records. As part of a multidisciplinary study of the potential hydrologic impact of severe sustained drought on the Colorado River, the physical basis and limitations of tree rings as indicators of severe sustained drought are reviewed, and tree-ring data are analyzed to delineate a “worst-case” drought scenario for the Upper Colorado River Basin (UCRB). Runs analysis of a 121-site tree-ring network, 1600–1962, identifies a four-year drought in the 1660s as the longest-duration large-scale drought in the Southwest in the recent tree-ring record. Longer tree-ring records suggest a much longer and more severe drought in 1579–1598. The regression estimate of the mean annual Colorado River flow for this period is 10.95 million acre-feet, or 81 percent of the long-term mean. The estimated flows for the 1500s should be used with caution in impact studies because sample size is small and some reconstructed values are extrapolations.     

A TECHNIQUE TO RECONSTRUCT RIVER DISCHARGE HISTORY FROM TREE-RINGS1

ABSTRACT: Cores were obtained from several tree species located both on a river floodplain and a nearby terrace. A ratio of annual tree growth on floodplains to terrace growth was developed and shown to be related to the annual river discharge. Growth ratios from the time prior to written records can therefore be used to reconstruct river discharge and infer past unrecorded flood frequency. Oak and basswood ratios yielded the best models for discharge reconstruction, whereas those of elm and birch were less useful. This method permits reconstruction of river discharge from an assemblage of growth cores obtained within a relatively small area.     

EXTENSION OF RIVER FLOW RECORDS IN ARGENTINA FROM LONG TREE-RING CHRONOLOGIES1

ABSTRACT: Tree-ring chronologies were derived from two coniferous species growing in the foothills of the Patagonian Andes of Argentina, using crossdating and standardization techniques developed by the Laboratory of Tree-Ring Research. Seven of these chronologies were selected, along with data from two long reliable gaging stations to reconstruct annual stream-flow for two major rivers extending back to the year 1601. Calibration between tree-ring and river flow data was established using the multivariate technique of canonical analysis. Correlations of 0.73 were obtained between gaged and reconstructed flow for both rivers. These Andean conifers contain good climatic and hydrological information and tree-ring chronology and calibration techniques are capable of extracting much of this information.