The meteorologic phenomenon of drought in the Southwest

The recent drought is one of several which have been recorded in the arid Southwest in the past century. In regions where precipitation comes chiefly from a single source, as in California and the Great Plains, prevailingly dry periods have alternated with wetter periods, each lasting 10 to 15 years. In the intervening area that includes the basins of the Colorado River and Rio Grande and numerous basins of interior drainage, a deficiency from one source may be ameliorated in some localities by precipitation from another source; in other areas all sources have failed and the drought has continued for a quarter of a century. Earlier droughts of similar duration, and some of longer period, are indicated by historic records and by studies of tree rings, lake levels, and archeologic data.