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Contemporary changes of the hydrological cycle over the contiguous United States: trends derived from in situ observations

TitleContemporary changes of the hydrological cycle over the contiguous United States: trends derived from in situ observations
Publication TypeManual Entry
Year of Publication2004
AuthorsGroisman, Pavel Ya, Richard W. Knight, Thomas R. Karl, David R. Easterling, Bomin Sun, and Jay H. Lawrimore
Journal of Hydrometeorology

Over the contiguous United States, precipitation, temperature, streamflow, and heavy and very heavy precipitation have increased during the twentieth century. In the east, high streamflow has increased as well. Soil wetness (as described by the Keetch–Byram Drought index) has increased over the northern and eastern regions of the United States, but in the southwestern quadrant of the country soil dryness has increased, making the region more susceptible to forest fires. In addition to these changes during the past 50 yr, increases in evaporation, near-surface humidity, total cloud cover, and low stratiform and cumulonimbus clouds have been observed. Snow cover has diminished earlier in the year in the west, and a decrease in near-surface wind speed has also occurred in many areas. Much of the increase in heavy and very heavy precipitation has occurred during the past three decades.

Citation Key49
Community Notes

Paper Takeaway Points:

  • "very heavy" rain events are defined "as an event that is observed in not more than 1 of 100 (above the 99th percentile) or in not more than 3 of 1000 daily rain events (i.e., above the 99.7th percentile) at each location."
  • "changes in very heavy precipitation vary significantly, and the magnitude of the trends is most notable in the eastern two-thirds of the country and primarily in the warm season when the most intense rainfall events typically occur"
  • a strong early spring warming has been observed in the western united states
  • Keetch–Byram Drought index
    • index uses only daily temperature and precipitation information and estimates soil moisture deficiency
    • "substantial improvement in mean summer wetness has occurred" in the Midwest
  • "during the 1948–93 period, the mean annual near-surface specific humidity under clear skies had steadily increased with a mean rate of 7.4% (100 yr)−1"
  • "With an increase in water vapor, one may expect an increasing potential for heavy precipitation with all types of weather disturbances"
  • increases in total, low, and cumulonimbus cloudiness is consistent with increases in heavy rainfall events
  • increases in stratocumulus is consistent with an increase in total precipitation
  • "Seasonal shifts were reported in surface air temperature across the United States"