Last updated October 10, 2018

Structure and Detectability of Trends in Hydrological Measures over the Western United States

  • Examines the geographic structure of observed trends in key hydrologically relevant variables across the western United States at 1/ 88 spatial resolution during the period 1950-99
  • Identifies geographical regions, latitude bands, and elevation classes where these trends are statistically significantly different from trends associated with natural climate variations
  • Analyzes the following variables: late-winter and spring temperature, winter-total snowy days as a fraction of winter-total wet days, 1 April snow water equivalent (SWE) as a fraction of October-March (ONDJFM) precipitation total (precip(ONDJFM)), and seasonal (JFM) accumulated runoff as a fraction of water-year accumulated runoff
  • Finds that large trends (magnitudes found less than 5% of the time in the long control run) are common in the observations and occupy a substantial part (37%-42%) of the mountainous western United States
  • These trends are strongly related to the large-scale warming that appears over 89% of the domain
  • Finds that the strongest changes in the hydrologic variables, unlikely to be associated with natural variability alone, have occurred at medium elevations (750-2500 m for JFM runoff fractions and 500-3000 m for SWE/Precip(ONDJFM)) where warming has pushed temperatures from slightly below to slightly above freezing
  • Further analysis using the data on selected catchments indicates that hydroclimatic variables must have changed significantly (at 95% confidence level) over at least 45% of the total catchment area to achieve a detectable trend in measures accumulated to the catchment scale