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Drought 2002 in Colorado: An Unprecedented Droughtor a Routine Drought?ROGER A. PIELKE, Sr.,1NOLAN DOESKEN,1ODILIA BLISS,1TARA GREEN,1CLARA CHAFFIN,1,2JOSE D. SALAS,3CONNIE A. WOODHOUSE,4JEFFREY J. LUKAS,5and KLAUS WOLTER6Abstract — The 2002 drought in Colorado was reported by the media and by public figures, and evenby a national drought-monitoring agency, as an exceptionally severe drought. In this paper we examineevidence for this claim. Our study shows that, while the impacts of water shortages were exceptionaleverywhere, the observed precipitation deficit was less than extreme over a good fraction of the state. Alikely explanation of this discrepancy is the imbalance between water supply and water demand over time.For a given level of water supply, water shortages become intensified as water demands increase over time.The sobering conclusion is that Colorado is more vulnerable to drought today than under similarprecipitation deficits in the past.Key words: Drought, precipitation, Colorado, streamflow, snowpack, paleoclimatology.1. IntroductionIn reference to the 2002 drought, the Governor of Colorado stated in his 2003State of the State address,‘‘…. scientists tell us that this is perhaps the worst drought in 350 years.’’(http://www.thedenverchannel.com/print/1913350/detail.html?use=p rint)Clearly, such an assessment of drought severity depends on how drought isdefined. Drought is characterized in a number of different ways, each with associateddefinitions of onset and recover y, duration , and related impacts. For example,1Department of Atmospheric Science, Colorado State University, Fort Collins, 80523-1371, CO U.S.A2currently at University of Idaho, Moscow, Idaho3Civil Engineering Department, Colorado State University, Fort Collins, CO 80523-1372, U.S.A.4Paleoclimatology Branch, National Climatic Data Center, NOAA, 325 Broadway, E/CC23 Boulder,CO 80305, U.S.A.5Institute of Arctic and Alpine Research, University of Colorado, Campus Box 450 Boulder, CO80309-0450, U.S.A6NOAA-CIRES Climate Diagnostics Center, University of Colorado, Campus Box 216, Boulder, CO80309-0216, U.S.APure appl. geophys. 162 (2005) 1455–14790033 – 4553/05/091455–25DOI 10.1007/s00024-005-2679-6 Birkha¨user Verlag, Basel, 2005Pure and Applied Geophysicsmeteorological drought could be measured by numbers of days below a specifiedprecipitation threshold, or departure from a baseline average; an agricultural droughtcould be measured by soil moisture deficit and impacts on crops; and a hydrologicaldrought could be measured by a period of precipitation deficit and impacts on watersupply such as streamflow and surface and subsurface water storages. Spatial andtemporal scales must also be considered in defining drought. The variety of ways todefine drought makes a simple assessment of drought severity a difficult task.In this paper, we explore the severity of the 2002 drought, defined by a variety ofmoisture-related variables including precipitation, snowpack, streamflow, reservoirstorage, and tree growth. Although the 2002 drought is considered by some to be thethird of a three-year drought, here we focus on 2002 as a single year event. Thedefinition of the year varies somewhat according to variable measured, but ingeneral, we consider it from fall 2001 though summer 2002. Its impact is gauged onthe regional to statewide level.Figure 1 shows the magnitude of the drought as determined by the U.S. DroughtMonitor (http://drought.unl.edu/dm/), where the western third of the state is in thehighest (‘‘exceptional’’) category. In this display, drought has been defined based onthe interpretation of available water deficit information by researchers at theNational Drought Mitigation Center at the University of Nebraska at Lincoln, aswell as input from a variety of experts in the field, including some of the co-authorsof this paper. ‘‘Exceptional drought’’ refers to conditions found between once everyfifty years or never before on record. This is one assessment of drought that ourpaper examines using a variety of analysis techniques.2. An Evolution of the 2002 Drought in ColoradoThe drought of 2002, with all of its devastating wildfires, profound watershortages and widespread crop losses, had its beginnings in the autumn of 1999.After a very wet spring in 1999 and a soggy August, precipitation patterns reversedand the fall of 1999 was very dry across most of Colorado. The winter of 1999–2000followed with below average snow accumulation and much above average temper-atures. The mountains of southwestern Colorado were particularly hard hit by ashortage of snow for winter recreation and summer water supply. With a very dryspring and early summer in 2000 over northeast Colorado and the South Plattewatershed, drought conditions emerged quickly. In fact, the entire western U.S. wasby then engulfed in a severe drought that resulted in the largest severe wildfire seasonin the last century for the western U.S. (http://www.nifc.gov/stats/wildlandfire-stats.html). A persistently hot summer made the situation worse, as transpirationrates were considerably higher than average over irrigated areas.The 2001 Water Year was less extreme but still tended on the dry side. Colorado’snorthern and central mountains were the driest with respect to average. While spring1456 R.A. Pielke, Sr., et al. Pure appl. geophys.,and summer precipitation was relative ly normal, hotter than average temperaturesfor the second summer in a row again resulted in high evaporation rates andcontinued depletion of soil moisture and surface water supplies. This set the stage for‘‘The drought of 2002.’’Beginning in September 2001, storm systems were few and precipitation wassparse across the Central Rockies. Much of western and southern Colo rado receivedless than half the average September precipitation and temperatures were severaldegrees C above average across the entire state. Beneficial moisture fell from twostorm systems that primarily affected the northeastern and east central counties ofColorado.October weather patterns appeared more favorable as a variety of storm systemscrossed the region. However, precipitation from passing storms was very light, andwhen the month was over precipitation totaled again less than half the average overthe majority of the state. Some areas east of the mountains received no moisture atall. Temperatures were also mild ranging from about average near


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