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 t he conterminous United States between 1949 and 1996 via the complementary h ypothesis. This predicted increase in ET is in agreement with the measured precipitation increase for the same period if long-term wet-surface ET is a ssumed to be constant. Long-term relative humidity and air temperature meas urements express an increase in mean air temperature and water vapor concen tration but not a statistically significant change in vapor pressure defici t. 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.