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Drought

"We have no good definition of drought. We may say truthfully that we scarcely know a drought when we see one.”

— Ivan R. Tannehill, Drought and Its Causes and Effects

Drought is one of many factors contributing to the complexity of frequent fire adapted forest ecosystems. Drought increases the potential for catastrophic wildfire, but drought cannot be singled out as the cause of wildfires. Other factors include fuels accumulated during many decades of unwise fire suppression, overcrowded “dog-hair” tree stands, and the long-term effects of overgrazing. These factors in combination with drought spell trouble for Western forests.

Drought occurs as a result of complex interactions of global climate cycles. Drought severity and duration is difficult to predict but we can be sure that eventually droughts will recur.

Are we in a drought, and if so, how severe is it? Some climatologists suggest that the current dry spell in the Southwest is actually a return to normal conditions following 20 years of unusually wet weather during the 1970s and 80s. Others argue that we are indeed experiencing a drought more severe than any in the past 1400 years.

According to the National Geophysical Data Center:

"The difficulty of recognizing the onset or end of a drought is compounded by the lack of any clear definition of drought. Drought can be defined by rainfall amounts, vegetation conditions, agricultural productivity, soil moisture, levels in reservoirs and stream flow, or economic impacts. In the most basic terms, a drought is simply a significant deficit in moisture availability due to lower than normal rainfall. However even this simple definition is complicated when attempts are made to compare droughts in different regions.”

The set of figures below provide data on the last major drought in the Southwest during the 1950s. Note the connection between the decrease in annual precipitation and the long-term decrease in percent forest area. The magnitude of the 1950s drought was exceeded only by the drought of the late 1500s.


(A) Changes in percent forest area between 1935 and 1975. Circles represent GIS estimates derived from mapped data; squares represent estimates for years with only partial aerial photograph coverage. The arrow indicates the time of historical observations of extensive tree mortality (22). (B) Annual precipitation at Bandelier National Monument, 1930-1980, highlighting the period of extreme drought (dashed vertical lies). (C) Annual precipitation (5-yr running average), reconstructed from dendrochronological records for the Jemez Mountains, encompassing the study site (Unpublished data from J. S. Dean and G. S. Funkhouser, personal communication). The magnitude of the 1950s drought (within the dashed vertical lines) was exceeded only by the drought of the late 1500s. Other precipitation reconstructions in the region show similar patterns (23-25). From Allen, C.D., and D.D. Breshears. 1998. Drought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences 95:14839-14842. Available online at http://www.pnas.org/cgi/content/full/95/25/14839

Regardless of how we choose to define drought, we should understand that drought is not an anomally but a recurring condition. Forest management practices that take the recurrence of drought into account will help insure long-term health for Western forests.

References

Allen, C.D., and D.D. Breshears. 1998. Drought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences 95:14839-14842. Available online at http://www.pnas.org/cgi/content/full/95/25/14839

Colorado State University. 2003. "Drought.ColoState.edu." http://drought.colostate.edu/ 4/30/03. Information for government leaders, businesses and individuals as they plan for and manage drought events in Colorado.

National Geophysical Data Center. "Drought: A Paleo Perspective -- What is Drought?" http://www.ngdc.noaa.gov/paleo/drought/drght_what.html 4/22/03.

National Oceanic and Atmospheric Administration. 2000. “North American Drought: A Paleo Perspective.” http://www.ngdc.noaa.gov/paleo/drought/drght_home.html 4/22/03.
National Oceanic and Atmospheric Administration (NOAA) web site with detailed historical, climatological and anthropological perspectives on drought.

Patterson, Mark W. 1997. “Forest Fires and Drought in the U.S. Southwest.” http://geochange.er.usgs.gov/sw/impacts/biology/fires_drought/
Explanation of research conducted on correlations between drought and wildfire. Land management issues also touched.

Other resources

McKinnon, Shaun. 2003. “Southwest's drought may last for years.” The Arizona Republic http://www.azcentral.com/specials/special26/articles/0212droughtadd12.html 4/22/03. On-line news story with drought history and predictions.

National Drought Mitigation Center. http://drought.unl.edu/ 5/14/03. NDMC stresses drought preparation and risk management rather than crisis management.

Shaw, Robinson. 2000. “Southwest warned to plan for dry decade.” Environmental News Network. http://www.enn.com/news/enn-stories/2000/02/02212000/pdo_10139.asp 4/22/03. On-line news story describes how of the Pacific Decadal Oscillation (PDO) effects drought in the Southwest.

Stade, Kirsten and Nicole Rosmarino. 2002. “Administrative Procedures Act Petition to Develop Regionwide Drought Policy.” http://www.fguardians.org/docs/drought-petition.htm 4/22/03. Forest Guardians request Regional forestry officials to adopt drought policies where none are in place.

U.S. Drought Monitor. 2003. http://www.drought.unl.edu/dm/monitor.html 5/13/03.
Weekly updates and maps on regional drought conditions across the U.S.

Western Regional Headquarters - National Oceanic and Atmospheric Administration (NOAA). “Is there Evidence Of Long-term Drought In The Southwest?” http://www.wrh.noaa.gov/Flagstaff/science/drought2.htm 4/22/03. A group of researchers who study tree rings have found evidence of a "mega-drought" in the 16th century.

C.L.

Last edited May 21, 2003

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