Precipitation Increase Over the US in the 20th Century

T. R. Karl and R. W. Knight (1998) report finding a 10% increase in precipitation over the contiguous United States between 1910 and 1996. Examining the data in more detail, they find that the number of days per year with precipitation has increased by 6 over a 100 year period. A second observation is that about half of the 10% increase is due to an increase in both the frequency and intensity of heavy precipitation events. The proportion of total annual precipitation derived from heavy and extreme events has increased relative to amounts from more moderate events.

Segal et al (1998) investigate whether this increase might be due to changes in irrigation over this period. They find that the expansion of irrigation in the US in the 20th century likely has increased precipitation slightly but probably no more than about 1.7%.

Reference

Karl, T. R., and R. W. Knight, 1998: Secular trends of precipitation amount, frequency, and intensity in the United States. Bull. Amer. Meteorol. Soc. 79, 231-241.

Segal, M., Z. Pan, R. W. Turner, and E. S. Takle, 1998: On the potential impact of irrigated areas in North America on summer rainfall caused by large-scale systems. J. Appl. Meteorol. 37 325-334.

Eugene S. Takle
1 March 1998