Science Source

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