Hydro-climatic variability and trends in Washington State for the last 50 years

Historical records of monthly streamflow and precipitation coupled with mean, minimum, and maximum air temperatures for Washington State were used to study the variation and the trend characteristics that occurred over the last 50 years (1952-2002). Results indicate that the 1967 statewide water resource assessment needs to be updated because all of the stations used in that study exhibited a decreasing trend in annual streamflow ranging from -0.9% to -49.3%, with an arithmetic mean of -11.7% and a median value of -9.8%. Furthermore, a slightly decreasing trend in annual streamflow, although not statistically significant, was detected. The decreasing streamflow magnitude was about -1.178 mm year(-2), or 4.88 m(3) s(-1) year(-1), which caused a decrease in annual streamflow in the state of about 58.9 mm, or 244 m(3) s(-1). This magnitude was about 9.6% of the average annual streamflow for the entire state from 1952 to 2002. Contrastingly, the overall annual precipitation in the entire state increased 1.375 mm year(-2). Overall the annual means of daily mean, maximum, and minimum temperature increased by 0.122, 0.048, and 0.185 degrees C/10 years, respectively, during the study period. Thus the corresponding annual means of daily mean, maximum, and minimum temperatures increased by 0.61, 0.24, and 0.93 degrees C, respectively. All of these trends and magnitudes were found to vary considerably from station to station and month to month. The possible reasons resulting in these detected trends include, but are not limited to, human activities, climate variability and changes, and land use and land cover changes. Copyright (c) 2009 John Wiley & Sons, Ltd.