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Historical Perspectives on Colorado Drought ng tree-r l Tobie 3: Occurrences of wet/dry decodes from 1500-1995 based on growth Index at Colorado data points In Figure 5. Wet Decade Very Dry Decade Total Events - ~ .. 3 20's, 60's, go's 2 OO's, 70's 5 - - 3 20'5,40'5,60's 2 30's, 70's 5 - - 2 10'$, 50's 2 10's.30's 4 - - 2 20'5, 30's 2 50'5,60'5 4 - - - 2 0'5, 20's 2 30's, 50'5 4 - - - 2 0 22 1500's 6oo's 7oo's 800', 1900's I ghl I Totals From this historical perspective it appears that the current Drought of 1999.2002 likely has been exceeded in duration, intensity and coverage by historical droughts of the past. Consider the occurrence of coincident droughts such as the one apparent in the yellow bar on the graph 10 the right. The eastern plains of Colorado. southwestern New Mexico and Baja. Mexico all experienced a mulli- decade drought simultaneously. Winter and spring drought conditions appear to have been particularly severe in the Southwestern U.S. and northwestern Mexico, where this drought appeared to have lasted several decades (1625-1655). In other areas, drought conditions were milder, suggesting drought impacts may have been tempered by seasonal variations. The non-traditional database of historic tree-ring analyses reported by Woodhouse, 2001, provides new opportunities for an in-depth look at the past that could open doors into the future of Colorado drought occurrence, while linking the infonnation to state stream now and water supply. In the graph tree rin derived Palmer Or Severity Indices wer calculated from tree across Southwester New Mexico, easter Colorado and Baja Mexico, What Is drought: nut UVUryunu agl'Bus? Drought has many different meanings. According to the Glossary of Meteorology. r' edition (American Meteorological Society 2000), drought is defined as "a period of abnonnally dry weather sufficiently long enough to cause a serious hydrological imbalance," the definition may sound like a simple textbook characterization, with the following qualification Whilcthis continues The solid yellow bar indicates a coincide severe drought in al three locations in th mid.l500s Drought is a relative tenn; therefore any discussion in terms of precipitation deficit must refer 10 the particular precipitation-related activity that is under discussion. For example, there may be a shortage of precipitation during the growing season resulting in crop damage (agricultural drought). or during the winter runoff and percolation season affecting water supplies (hydrological drought). Clearly, there is no singular expression of the meaning of the term drought. Not only docs the meaning vary with the application context, but it is also subject to recycled content paper HDR Engineering, Inc. Printed on 30% post-consumer