q 2004 American Meteorological Society Increasing Evapotranspiration from the Conterminous United States

Recent research suggests that evapotranspiration (ET) rates have changed over the past 50 years; however, some studies conclude ET has increased, and others conclude that it has decreased. These studies were indirect, using long-term observations of air temperature, cloud cover, and pan evaporation as indices of potential and actual ET. This study considers the hydrological cycle more directly and uses published precipitation and stream discharge data for several large basins across the conterminous United States to show that ET rates have increased over the past 50 years. These results suggest that alternative explanations should be considered for environmental changes that previously have been interpreted in terms of decreasing large-scale ET rates. 1. Evapotranspiration and the hydrologic budget Controversy continues over whether potential evap-oration rates are increasing (e.g., Manabe 1997; Szilagyi et al. 2002) or decreasing (e.g., Peterson et al. 1995; Roderick and Farquhar 2002) and over trends in actual evaporation (Ohmura and Wild 2002). Evaporation or evapotranspiration (ET) trends can be identified for