BOARD01080 - Laserfiche WebLink

Home Browse Search

Ko1orado Wain Resources Research Institute. moumains. and oLher are~ with more reliable precipi- talion, accumulated deficits only reach about 1.2 yean ofa\crage precipitation. For example, if Breckenridge averages 22 inches of precipitation per year, a very severe muhi-year-drought might result in an accumu- lal~ deficit of more than 22 inches over a few years time before above average precipitation reduces those defici~. --- ..... '" .. . .' j. 1 I . . jl,J,.U. .~ , I. i " . - 7) Timing of drought - When does it start? When docs il end? Shon duration droughts can l1egin and end in any ~eason. ~tajor droughts with duralions of one year or longer tend 10 begin in and end in the ~ason that is locally the ....et season. This varies from place to place in lhe state. For example: .\llljor droughls in Ihe mmmtain.s tl'nd to begin or end during thl' wintu or sprin~ months. In I'xtrl'me south"'l'stun Colorado, significant droughts ha~'e both begun and ended in the fall. The mou dramalic the uasonal pruipitation cycle is. the more difficult il is to end a major droughl during the time of year that is the climatological dry uason for tOOl area. For nample. o\'u eastern Colorado. the months of December. January and February art' rypically dry: The three months combined produce, on Ql'uor:l', less than IfYk of the amllla/ prl'cipilation. Rarl'/)' does I'nough pruipitalion fall dun'ng this time of .war to significantly alleviate longu.term moisture deficits. Ho.....erer. a wuk or two of ~'ery' "'et .....eather during 1mI' spring f'(llt of the mOllntains can bring a major drought to an end. For example, the spring oj 1995 and the lasr fe..... days of :lpn'/ /999 d~li\'l'rl'd enough precipitation to compensat~ jor large deficits. There are several examples of droughts ending abruptly .....ilh the appearance of .....idespread excess precipitation. For example, the seven: drought of the 1950s was followed by the wettest year in Colorado's history in 1957. Then: are OIher examples of droughts that have ended more subtly. however. 8) Does a dry winlerfon:tella wel summer? lbere is endless folklore concerning drought. Even before hEI Nino" found its way inloclimatologicaljargon. people have talked ways to predict drought. Does the climate of one season foretell tilt- next? ~Iuch folklore would suggest thaI. Our analyses, however. did not bear thai out. We looked al a number of combinations. What happens after a very dry wimer In the mountains? What happens after a \'ery dry autumn at lower elevations (such as fall 1999 in Colorado)? Our analyses sho.....cd that sometimes dry wimers in the mountains .....ere follo.....ed by wet summers{like 1999), but other years they weren't. Sometimes dry springs along Lhe From Range were followed by hot dry summers (like 195~), bUI olher years they weren't. Further analysis would hi: required to detennine how and if precipitation during one season helps fon:telllhe next here in Colorado. Less than 1091- of the variance in ~ummerprecipitation is explained any.....here in Colorado by the variations in winter preCIpitation. 9) Are there drought cycles? People don't ask climatologists whethl-r there arc drought cycles In Colorado: most people arc positive lhallhere are. Some say there isa 3 year cycle, while others claim 7. The sunspot cycle of II years has caught some people's allenlion, while many slrongly belie\'e that a ::!2 }'ear drought cycle (double sunspot cycle, controls Colorado's drought pallerns. We examined our rainfall ft.'Cords in Colorado in search of drought cycles. There is some evidence of a two to three year cycle over pc:mions of southern and eaSlern Colorado. The dry periods in the 1890s. 1930s, 1950s and again in the 1970s have con\'inced some obseners thai the double sunspot cycle really does affecl drought pallems in Colorado. Thai theory docsn't explain why the 19105 were so wet. why pans of the 1960s were very dry. and why we have been wet for the belter pan of 18 years in a ro...... bul many slilll1elieve it. As for a seven oreleven-year cycle. there isn't much supponing evidence for thal. It is true that dry f"=riods are followed by wet. and wet followl.-d by dry? That makes a cycle, doesn"t it'? The problem is that those cycles just aren't \"Cfy reliable. As such. they don't help us much if at all in predicling ....hat.....i11 happen next year or the year after thai. E\'en thro....ing in the iITegularcycle of the EI :"Jino Southern Oscillation, ....e are stillldt with a great deal of unexplained variability in our precipitation. 17