streamflow) or climatic index series (AMO, PDO, etc.) can be used to calculate the occurrence and return probabilities of climatic episodes. This information can then be mapped to decisions about annual water resource operations or facilities planning.

By synchronizing ecological disturbances and recruitment pulses, D2M variability also plays a key role in structuring woodland and forest communities in western watersheds. In the event of longer, hotter growing seasons, D2M variability will still determine the timing and pace of ecosystem changes. Oscillations between warm-dry and warm-wet regimes will continue to produce uncommonly large disturbances followed by accelerated regeneration and succession. Such large-scale vegetation changes will have complex hydrological effects and will add further uncertainty to water resource availability. A principal challenge for land managers in the 21st century will be to manage for disturbance and succession in purposeful and systematic ways that promote asynchrony and patchiness at local to regional scales while still preserving goods and services that ecosystems provide.

Julio L. Betancourt is project chief, National Research Program, Water Resources Division, U.S. Geological Survey and adjunct professor in the Departments of Geoscience and Geography at the University of Arizona. The focus of his research is ecosystem and watershed responses to climate variability on different temporal and spatial scales. Dr. Betancourt received his Ph.D. in geosciences from the University of Arizona, Tucson.

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