Changes in Aridity in the Western United States

Abstract

It is often remarked that most of the western US (Figure 1) is “always in drought,” especially by visitors from wetter climates. The plants and landforms of the West, however, are more or less adapted to the region’s relatively dry but variable climates, and so important variations in the levels of drought, or aridity, characterize the landscape. During “real” droughts, broad areas of the West are subjected to drier conditions than normal, imposing—at least temporarily—arid climatic conditions on many semiarid and even humid areas. In response to these climatic conditions, the hydrologic balances between waters that run off and those that evaporate back into the atmosphere are transformed temporarily in ways that color the entire region’s water supplies and vegetation. In this article, we describe the major changes that droughts wreak on the “normal” partitioning of precipitation between evaporation and runoff, as depicted by a hydrological model that simulates historical variations of the region’s surface hydrology.