Evapotranspiration Intensifies over the Conterminous United States

Using long-term (1948–1996) pan evaporation measurements, a 6% increase in warm-season (May–October) actual evapotranspiration (ET ) is computed over the conterminous United States between 1949 and 1996 via the complementary hypothesis. This predicted increase in ET is in agreement with the measured precipitation increase for the same period if long-term wet-surface ET is assumed to be constant. Long-term relative humidity and air temperature measurements express an increase in mean air temperature and water vapor concentration but not a statistically significant change in vapor pressure deficit. The latter implies a smaller than 6% increase in actual warm-season ET. Water-balance estimates for six watersheds, covering about 50% of the land area of the contiguous states of the United States, indicate a 3% increase in annual ET over the same period.