Prolonged Severe Drought in Western North America: Prognosis and Lessons from the Past
by
Glen M. MacDonald
Department of Geography, UCLA, Los Angeles, CA 90095-1524 USA

Population growth in western North America, particularly the Southwest of the United States and adjacent Mexico has accelerated at a staggering pace. The population of the city of Los Angeles has grown six-fold, from about 577, 000 people to over 3.7 million since 1920. Even more striking, in the same period the population of Las Vegas grew by 200 times and the population of Phoenix grew by 47 times. This has generated a massive increase in water needs, including a 410% increase in domestic water use in the Southwest since 1950. In relatively arid areas such as the Southwest, or the western prairies of Canada, local precipitation and groundwater are insufficient to meet water demands and domestic, industrial and agricultural water needs are met through the addition of water obtained from large river systems such as the Colorado River in the United States and Mexico, and the Saskatchewan River system in Canada. Flow in these systems depends upon snowpack in the mountains of the North American cordillera.

Long-term records of variability in precipitation and river flow provide reasons for concern about water resources in western North America. It is appearing increasingly likely that water planning has been based upon overly optimistic assessments of water availability and insufficient consideration of potential drought magnitude or duration, As one example, the total allocation of water rights from the Colorado River to the states of the Colorado basin and Mexico is 16.5 million acre-feet per-year (approx. 20 megalitres per-year). Unfortunately, long-term monitoring of flow over the past century and tree-ring based estimates extending back over 500 years suggest that the actual average flow of the river is only between 13 and 15 million acre-feet (16 to 18 megalitres). In addition, tree-ring records and evidence from other sources, such as lake sediments, sand-dune activation and stream channel studies, indicate that regions such as the Colorado basin or the Saskatchewan basin have experienced severe and prolonged droughts in the past 1000 years during which time average flows were decreased by 40 to 20% for periods of decades to centuries. In some cases, such as between 900 and 1300 AD aridity extended from the western plains of Canada as far south as southern California. Current reservoir capacity on the Colorado was strained by the most recent drought (1999-2004 AD) and demonstrated that the reservoir system cannot mitigate a severe drought of more than about 5 years duration.

Paleoclimatic records not only provide evidence of past severe and sustained droughts, but also insights into their causes. Recent work has highlighted the relationships between variations in Pacific and Atlantic sea surface temperatures and prolonged droughts in western North America. In many cases, however, these records also show that the frequencies with which significant aridity episodes and droughts occur is variable-making future drought prediction difficult.

The current situation in western North America-exponential population growth of an advanced stratified society in a region threatened by chronic water shortages and potential devastating droughts is not unprecedented in the Southwest. Insights into the relationship between severe sustained droughts and societal response can be gleaned from the paleoclimatic and archaeological records of past events. The Anasazi people developed a materially advanced prehistoric culture centered in the southwestern United States between 1200 BC and 900 AD, during which time they progressed from the incipient cultivation of corn and squash to a complex society building large buildings and population centers of stone and adobe. After obtaining maximum population densities around 1000 years ago, the archaeological record shows the Anasazi suffered a series of population declines and abandonment of lands. There is also evidence of great social unrest and conflict. By the time of Spanish arrival in the 17th Century the descendents of the Anasazi were but a remnant of their former extent demographically and geographically. The remaining people were settled close to rivers that provided somewhat reliable water. Tree-ring records have long shown that the period of decline corresponds to a period of droughts between about 1100 and 1300 AD. There is evidence of efforts by the Anasazi to mitigate drought impact through small reservoir and irrigation construction. More recent work suggests that drought alone did not cause the decline of the Anasazi, but was likely coupled with unsustainable population size, depletion of other resources such as wood and severe societal unrest and warfare.

For many reasons, including current technological sophistication, the extensive network of global trade that supports modern societies and the national and international laws and governance that regulate reactions to resource supply problems, a catastrophic event such as the Anasazi decline is unlikely in the future. However, it is likely that a severe and sustained drought such as those that have occurred in the past would lead to acrimonious civil and inter-governmental litigation, significant economic reconfiguration (including substantial changes in agricultural practices), and potential declines in immigration driven population growth rates as municipalities limit construction and growth due to water concerns. Indeed, all of these features have appeared in various forms in California in response to relatively mild droughts of the past few decades.

Date received: July 14, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-36.