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Last modified
8/11/2009 11:42:28 AM
Creation date
9/30/2006 10:18:42 PM
Metadata
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Publications
Year
2000
Title
A History of Drought in Colorado: Lessons Learned and What Lies Ahead
CWCB Section
Water Conservation & Drought Planning
Author
Colorado State University
Description
Lessons learned and what lies ahead
Publications - Doc Type
Tech Report
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<br />"Water in the Balance" <br /> <br />Analysis of Historical Dry and Wet Periods in Colorado <br /> <br />Now that we have an understanding of how we define and <br />assess droughts, how precipitation falls in different parts <br />of Colorado, and how Coloradans use that precipitation to <br /> <br />supply water for different sorts of demands, we can look at <br />the historical dry and wet periods in Colorado with a better <br />understanding of the significance of those periods, and <br />how similar dry or wet periods would impact Colorado <br />today. <br /> <br />The historical record of precipitation, snowpack, and <br />streamflow data were analyzed for each river basin in <br />Colorado. The periods of above or below average precipi- <br />tation were identified both for individual basins and for a <br />majority of the state as a whole. The table on the following <br />page shows the periods during which at least 60% of <br />Colorado was wet or dry. as determined by the Standard- <br />ized Precipitation Index (SPI) values for 24-month periods. <br /> <br />The analysis performed in this study revealed several <br />important facts about dry and wet periods in Colorado: <br /> <br />1) Drought is a very frequent visitor to Colorado <br /> <br />Single season droughts with precipitation of75% or <br />Jess or average for one io ihree monins in a row occur <br />nearly every year in Colorado. <br /> <br />Based on the Standardized Precipitation Index. 3. <br />month droughts with an index value of -1 or lower <br />(equivalent to a moderate precipitation deficit with a <br />probability of Occurrence of no more than 16% for any <br />consecutive 3-month period), occur approximately 90 <br />in 100 years at any given location (see graph below). <br /> <br />93% of time at least 5% of state (based on percent of <br />long term weather stations) is experiencing drought at <br />either a 3-, 6-, 12- or 24-month time scale. <br /> <br />What this means is that Colorado is almost always in <br />drought or near drought somewhere in the state. <br /> <br />2) Drought rarely encompasses the entire state. <br /> <br />Only about five percent of the time (or approximately <br />one year in twenty) does moderate or greater drought <br />encompass at least half of the state at the same time. <br /> <br />During recorded history, moderate drought (as defined <br />by a standardized precipitation index value of -lor <br />lower) has never covered the entire state at the same <br />time. <br /> <br />Short-tenn droughts (3-month duration) have covered <br />as much as 80 percent of the state. Longer-duration <br />droughts (2-4 years) have reached to about 70 percent <br />of the state. <br /> <br />What this means is that precipitation shortages rarely <br />occur in the entire state at the same time. Economic and <br />social impacts tied to a more "local" drought, however, can <br />in turn affect the economy of whole state. <br /> <br />3) The most common droughts are short duration (6 <br />months or less). They may be quite localized (espe- <br />cially during the growing season) or more widespread <br />(especiaHy during [he fan, wimer, and early spring). <br />Depending on where and when they occur, they may <br />have little or no impact on our lives. More widespread <br />droughts do not necessarily have a tendency to <br />become more long lasting. <br /> <br />4) Multi-year droughts occur infrequently. <br /> <br />Precipitation time series for weather stations across <br />Colorado show that periods of two or more consecu- <br />tive years with much below average precipitation (less <br />than 80% of average) have occurred a few times during <br />the 20th Century at mast Colorado stations. Below <br />average precipitation lasting three years in a row, <br />however, is quite rare. Many locations have never had <br />three consecutive very dry years, particularly over the <br />(continued on page 16) <br /> <br />Fraction of Colorado in Drought <br />3-month SPI values <br /> <br /> 1 <br />C! 0.8 <br />~ <br />- <br />0 0.6 <br />c <br />0 0.4 <br />"" <br />" <br />.. 0.2 <br />~ <br />LL <br /> 0 <br /> 0 en 0 CD ~ CD N ..... N (Xl '" (Xl ... 0> ... 0 en 0 CD c;; CD <br /> 0> 0> 0 0 ~ c;; N N '" '" ... ... "' "' CD ..... ..... (Xl (Xl 0> <br /> (Xl (Xl 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> 0> <br /> <br /> <br />Year <br /> <br />14 <br />
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