Twentieth-Century Precipitation Variability and Directional Vegetation Change in Southeastern Arizona

The quantity and timing of precipitation in southeastern Arizona from 1898 to 1989 is examined for evidence linking changes in available moisture to observed directional vegetation changes. A small decrease in annual precipitation since 1943 is due largely to a decrease in winter precipitation. No appreciable change is evident in monthly precipitation during this century. Consecutive, non-overlapping epochs of 5, 10, 15, and 20 years identify 1904–1923 as the wettest period, and a period of significant annual precipitation variability. The driest epoch, 1945–1964, is characterized by surprisingly limited annual precipitation variability. Short-term changes in vegetation cover, species density, and species composition have occurred during this century, but there is no conclusive evidence that brief episodes of relative precipitation abundance or deficiency are responsible for long-term directional vegetation changes in southeastern Arizona.