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Significant changes have occurred in two of the four Pacific Northwest basins in this study, and increased urbanization is the most likely cause.
The null hypotheses for this study were that there have been no changes in streamflow characteristics, including:
· Peak flows (Qp),
· Partial event volume (Qv),
· Ratio of peak flow to partial volume (Qp/Qv), or
· Ratio of these values to antecedent precipitation indices (Qp/API, Qv/API, (Qp/Qv)/API).
A change in at least one of these variables was expected in the urbanizing basins because of disruptions to the natural drainage system.
For the reference basin, the Luckiamute, none of the null hypotheses were rejected. These findings were generally true for data subsets by season and by recurrence interval as well.
For the Tualatin basin, none of the null hypotheses were rejected. These findings were generally true for data subsets, with one notable exception. In the January-February subset, (Qp/Qv), (Qp/15 day API), and (Qv/15 day API) all decreased, suggesting that the 7% - 13% decreases need further study.
For the Newaukum basin, several of the null hypotheses were rejected: Qp increased 21%; Qv increased 10%; Qp/all APIs increased about 20%. These findings were generally true for data subsets as well.
For the Johnson Creek basin, several of the null hypotheses were rejected: Qp/Qv increased 20%; Qp/API for periods of 30, 60, 90 and 120 days increased 21% - 34%; and (Qp/Qv)/API for all periods increased 32% - 54%
These finding were generally true for data subsets as well, although the magnitude of change varied. The largest change in the study was an increase of 92% in the (Qp/Qv)/120 day API in the November-December subset.
These results suggest that urbanization has changed basin stormflow characteristics in urbanizing basins smaller than 400 km2 in the Pacific Northwest. Because the two smallest urbanizing basins had increases in flows, but the largest basin did not, basin scale appears to play a significant role in attenuating increased flows from tributary streams due to urbanization.
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