Although streamflow forecasts in the intermountain west have become increasingly sophisticated in recent years, they are still largely based on instrumental records of flow, most of which are less than 100 years in length. In Colorado, Front Range water managers, many of whom tap into water supplies in the South Platte River and Upper Colorado River (through transbasin diversions), have based drought planning on the premise that the most severe drought in the instrumental record, typically in the 1950s, represents a likely worst-case scenario. This approach presumes that the 20th century records of flow adequately represent the full range of natural variability. However, extended records of streamflow, based on tree-ring reconstructions of several hundred years and longer, indicate that the 20th century records of flow often contain only a subset of the possible range of variability. This point became clear to water managers in 2002, when water-year streamflow values for many gages in Colorado, including the Upper Colorado and South Platte watersheds, were the lowest on record. Because of this, a number of Front Range water managers have begun to consider tree-ring reconstructions of streamflow in their planning for future droughts. In this presentation, we document our experience with several Front Range water resource managers and their use of tree-ring based reconstructions of streamflow.

We have generated tree-ring reconstructions of annual streamflow for gages identified as critical for water resource management by the two primary water providers for the Front Range metropolitan area and northeastern Colorado agricultural areas. The reconstructions have provided a basis for our work with managers to determine how paleoclimate records can best be utilized in planning. Since the reconstructions of streamflow demonstrate a broader range of natural variability, using the reconstructed streamflow series in water supply models allows an assessment of system performance over a broader set of drought event characteristics. Our collaborative effort has addressed water management challenges and concern that include model requirements, specification of error, and tailoring reconstructions to improve annual hydrographs. Ongoing work has begun to clarify the representativeness of 20th century droughts and the additional stresses on water supplies that may come from considering a broader range of climatic variability.