US Geological Survey


Date of this Version



From: CALIFORNIA DROUGHT, AN UPDATE April 2008, ed. Jeanine Jones. California Department of Water Resources, 2008. pp. 28-35


Copyright (c) 2008 Larry Benson.


Some droughts that occurred during the so-called Medieval Climate Anomaly (approximately AD 800-1300) appear to have been catalysts for major changes in settlement patterns of two western Native American groups - the Lovelock culture in Nevada’s Great Basin and the Anasazi people of the Four Corners area. Both groups’ subsistence bases were impacted by diminished water supplies associated with prolonged drought, leading to the dispersal of these Native Americans from their former territories.

Tree-ring-based Palmer Drought Severity Index (PDSI) reconstructions by Cook et al. (2004) indicate that over 50% of the western U.S. experienced drought conditions during the middle-12th and late-13th centuries (Fig. 1A, 2). Negative PDSI values indicate dry conditions, whereas positive values indicate wet conditions. This index was specifically designed to evaluate drought impacts on agriculture; PDSI values range from -6 (extreme drought) to +6 (extreme wet). During the middle-12th century drought, there existed a period of 23 consecutive years of negative summer PDSI that represents the single greatest North American megadrought since AD 951 (Cook et al., 2007). The AD 1150-1159 interval was the driest decade during the middle-12th century drought, having a North American average PDSI that was below -1.0.