[Science Now]

UCAR logoThis newsletter is a joint project of SIRS Publishing, Inc.
and the University Corporation for Atmospheric Research

Fall 2002— VOLUME 9, NO. 1

Copyright | Note to Teachers

Understanding Drought through Science

Drought is defined by the National Drought Policy Commission as "a persistent and abnormal moisture deficiency having adverse impacts on vegetation, animals, or people." Drought, however, is not an uncommon occurrence. A drought takes place somewhere in the U.S. every year, regardless of the climate zone. In the past century, parts of the U.S. suffered the effects of multi-year droughts in 1930-1939, 1950-1956, 1961-1966, 1976-1977, mid- 1980s to mid-1990s, and again in the late 1990s. Scientists consider drought a normal feature of any climate region, even semi-desert areas. It differs from aridity in that it is temporary, lasting from a few months to several years.

Figure 1: Map of the United States indicating drought conditions as of SEptember 17, 2002. Source is http://drought.unl.edu/dm
Quantifying drought requires measuring rainfall relative to averages for season and year; assessing declines in water supplies in rivers, lakes, reservoirs, and aquifers; measuring soil
moisture content; and tracking the increase in forest fire frequency.
Several indices are used to calculate drought conditions. Among them, the Palmer Drought Severity Index (PDSI) is a popular drought-monitoring tool used by scientists and government agencies to determine when extreme weather conditions ‹abnormally wet or abnormally dry periods‹begin and end (Figure 1). The purpose of the index is to measure deviations from average amounts of rain precipitation. The severity of the drought is influenced by temperature, winds, humidity, and the drought¹s coincidence with persistent large-scale global climate patterns.

One such pattern is ENSO, the El Niño-Southern Oscillation, a cyclic pattern of sea surface temperatures and surface air pressure in the tropical Pacific Ocean. During El Niño conditions, warm waters persist along the coast of Peru. These warm surface waters affect global climate and are associated with warmer winters and higher precipitation in the southwestern United States. The other extreme of the southern oscillation, La Niña, is associated with a cooling of eastern tropical Pacific waters and is correlated with warmer and drier conditions in the southwest, the northern
plains, and southeast coast of the U.S. as well as greater variability in temperature. Those areas felt the effects of La Niña from 1998 until mid-2002. Knowledge of the relationship between prevailing "long-term" climate conditions and the daily or seasonal variation in weather is crucial to our understanding of droughts past, present, and future.

Drought History

Drought history is collected from 20th-century weather station data, tree ring analysis, and paleo-climate records. Tree ring data, gathered from living trees or trunks of dead trees, provide strong evidence for past climate conditions and have revealed some startling information. Dr. Lisa Graumlich of Montana State University and colleagues were able to reconstruct an 8000+ year record of precipitation using tree ring data from bristlecone pine in California. The Palmer Drought Severity Index (PDSI),
when applied to this data, indicates evidence of a drought in the 16th century that exceeded the Dust Bowl drought of the 1930s in duration, intensity, and total area affected. In other work conducted by Dr. Scott Stine (California State University) and researchers in the high lakes of the Sierra Nevada, radiocarbon (C14) analysis of ancient tree stumps exposed by lowering water levels provides evidence of 20+ year dry cycles or "mega-droughts" in 1000 A.D. and again in 1300 A.D. It therefore appears likely that long and unrelenting droughts are a recurring aspect of climate in North America. Obviously, given water use in the U.S. today, a drought as bad as, or worse than, that of the Dust Bowl would place extreme demands on our country¹s infrastructure.

Figure 2: Map of the US indicating Consumptive Use and Renewable Water Supply, by Water-Resources Region. Source is http://water.usgs.gov/watuse.

Impacts of Drought

Drought conditions, regardless of duration and intensity, place stress on water supplies. Indirectly, drought also affects agriculture, fire risk, recreation, energy production, water quality, air quality, and species preservation. The impacts differ in type and intensity across the U.S.

Figure 3: Aerial map of fires in Colorado on June 10, 2002. From presentation to the Drought Congressional Briefing June 14, 2002, by Timothy Killeen, Director, National Center for Atmospheric Research. (Courtesy NASA Goddard Space Flight Center.)

In the Southwest and Central Plains, water is a highly valued resource because it is rarely plentiful, even in non-drought times. Consumption of water in relation to the amount of precipitation received is higher in these areas, due to agricultural demands and the needs of rapidly growing communities. The Lower Colorado River and Rio Grande River basins appear to be particularly susceptible as illustrated in Figure 2. This portion of the country is typically affected by periods of drought related to La Niña conditions, and it is currently experiencing drought. Clearly, long-term drought management is critical for these affected river basin regions. In many parts of the country, particularly in the West, drought also has a direct impact on forests (Figure 3). Lack of precipitation stresses plants as they compete for diminished water supplies. Plants under physiological stress have less resistance to insects and disease. These maladies increase tree mortality and leave behind vast stands of dead timber‹fuel for forest fires.According to Dr. Steven W. Running, the Director of the Numerical Terradynamic Simulation Group at the University of Montana, the number of acres burned in 11 Western states has dramatically increased in the past 15 years, as have the expenditures associated with fighting the fires. Costs are related to the destruction of homes and other structures, the death of livestock, and the resources needed for firefighting and suppression. The value of wildlife and human lives lost to wildland fire is incalculable. In an attempt to reduce these impacts, fire managers need to accurately assess potential fire danger in order for trained personnel and firefighting resources to be readily available in drought-affected areas. Advancement in drought prediction will only serve to improve our preparedness for potential forest fires.

Predicting Droughts

Scientific study in the cause and nature of drought is revealing that, contrary to popular belief, drought is not an anomaly but is part of the natural variability in the Earth system. Research indicates that severe multi-year drought events have occurred regularly in the climate history of North America, leading scientists to suggest that such events can occur at any time. Now is the time to plan for that eventuality.

The National Drought Mitigation Center (NDMC), located at the University of Nebraska at Lincoln, is dedicated to mitigating the impacts of drought by the development and promotion of risk management techniques. Dr. Donald A. Wilhite, NDMC Director, stresses that drought mitigation is an "investment in sustainable resource management." Mitigation involves making risk assessments of drought based on evaluating the hazard (or natural events) and vulnerability (or social factors). Planning to anticipate future droughts remains an immense challenge.

Figure 4. Map of U.S. indicating the Status of Drought Planning: May
2002. Presentation to the Drought Congressional Briefing June 19, 2002, by Donald Wilhite, Director of the National Drought Mitigation Center and the International Drought Information Center, and Associate Director, School of Natural Resource Sciences, University of Nebraska. (Courtesy National Drought Mitigation Center.)

Scientists at the National Center for Atmospheric Research, interested in long-term climate patterns, are striving to predict drought occurrence and duration using climate models. Improvements in computing power, understanding of the physical mechanisms affecting global climate patterns, and increases in the amount and quality of observational data will enhance the prognostic abilities of scientists.In the meantime, planning is vital. Most states currently have drought plans, but not all of them are comprehensive enough to include monitoring, assessments to reduce risk, and response strategies for fostering recovery (Figure 4). Increasing demands on limited water supply requires greater short- and long-term planning involving collaborative efforts of government resource managers, decision makers, and scientists to ensure the resiliency of communities to the droughts tomorrow.

 

 

 



 

 

Web-based Educational Resources about Drought

Classroom Activity:
Is Your Water Supply at Risk?

1. As a group, have students identify the sources of their water
supply using USGS water-use maps
(http://ga.water.usgs.gov/edu/mapgallery.
html), then ask them to identify the severity of drought in their region by
studying the current map of the Palmer Drought Severity Index
(http://drought.unl.edu/dm/current.html).
2. In smaller groups, have students identify the potential risks
to their water supply (such as increased water demand, drought,
pollution, etc.) and discuss ways of conserving or increasing efficiencies
of water use.
3. Return to the large group and compile a list of risk factors and a
list of action items.

 

 

Science Now is jointly published by the Walter Orr Roberts Institute at the University Corporation for Atmospheric Research and SIRS Publishing, Inc. (Social Issues Resources Series.) Science Now is published three times during the school year and is distributed to SIRS subscribers. Comments and questions should be directed to Joyce Gellhorn via Internet at jgellhorn@sprynet.com. You can also contact your SIRS representative or write to:

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Editors:
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Writer:
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