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Results

Only statistically significant results are reported in this section. Test results are summarized by basin at the end of this section. All statistical test results and r values can be found in Appendix B.

Total precipitation in each basin was not significantly different between the early and late periods.

Two basins had changes in antecedent precipitation indices (APIs). In the Tualatin basin, the 30 day API increased 10%, indicating more precipitation was needed to create a peak flow event. In the Johnson Creek basin, all five APIs declined, from 11% to 23%, indicating less precipitation was needed to create a peak flow event.

There were differences in the ratio of some streamflow variables to APIs, which are discussed below.

When the variable Q_p was compared to the passage of time, only the Newaukum basin had a significant correlation (0.39). The Newaukum was also the only basin with a significant change in Q_p rates, with an increase of 20% from the early to late period.

Two basins had a significant correlation between Q_p and APIs: the Newaukum and Johnson Creek (Figure 16).

An increase in the ratio between Q_p and APIs is an indication that less precipitation was needed to cause peak events. The Newaukum and Johnson Creek basins had significantly higher ratios of Q_p to several API periods (Figure 17). In the Newaukum basin, increases were about 20%. In the Johnson Creek basin, increases ranged from 21% to 34%.

Volume (Q_v)

No basins had a significant correlation between volume and the passage of time. However, all four basins had correlations between Q_v values and APIs, ranging from 0.27 to 0.54. The mean Q_v value in the Newaukum basin increased 10% from the early to late period.

There were no changes in the Q_v/API ratios in any basin.

The correlation between Q_p and Q_v is similar for all four basins, between 0.70 and 0.76.

Only the Newaukum basin had a significant correlation (0.34) between the Q_p/Q_v ratio and the passage of time.

Only the Johnson Creek basin had an increase in the Q_p/Q_v ratio between the early and late period (20%).

Each basin had some correlation between the ratio of (Q_p/Q_v)/API and the passage of time (Figure 18). Correlations were strongest for the 60, 90, and 120 day APIs.

Only the Johnson Creek basin had an increase in the ratios of Q_p/Q_v to various APIs, as much as 54% higher in the late period (Figure 19).

Basin subsets

Data for each basin were segregated into six subsets, based on season and recurrence interval (RI) (Table 14). This section summarizes some of the significant test results. Complete results are reported in Appendix B.

Table 14. Basin subsets with at least 10 events in each of the early and late periods. No basin subsets met that criterion for the 10+ year recurrence interval (RI), or the March-April season.

Throughout the data subsets, only Johnson Creek had a pattern of changed APIs. Nearly all APIs decreased in the three subsets of Johnson Creek data that were analyzed (Table 15). A decrease in an API indicates that less precipitation was needed to create a peak flow event. Decreases ranged from 14 – 31%. There were differences in the ratio of some variables to APIs in several basins, which are discussed below.

Peak flow values (Q_p) changed from the early to late period in the January-February subset in two basins. In the Newaukum basin, Q_p values increased 29%; in the Johnson Creek basin, Q_p values increased 13%. Q_p values increased in the RI 1 – 1.9 subset of the Johnson Creek basin by 10%. Q_p values increased in the RI 2-10 subset of the Newaukum basin by 20%.

Q_p/API ratios changed consistently only in the Johnson Creek basin. In each case they increased, as much as 65%. In the January-February subset, the Luckiamute basin also had changes, though the ratios decreased in that basin (Figure 20).

Volume (Q_v)

In the November-December subsets, volume values (Q_v) increased 7% in the Newaukum basin, but decreased 32% in the Johnson Creek basin.

Q_v decreased 17% in the RI 1 – 1.9 subset of the Johnson Creek basin.

Q_v increased 12% in the RI 2 – 10 subset of the Newaukum basin.

There were few changes in (Q_v/API), except in the Johnson Creek basin, where the ratio increased as much as 70% in January-February.

The Q_p/Q_v ratio increased 44% in the November-December subset of the Johnson Creek basin.

The Q_p/Q_v ratio increased 23% in the January-February subset of the Newaukum basin.

The Q_p/Q_v ratio increased 32% in the RI 1 - 1.9 subset of the Johnson Creek basin.

The (Q_p/Q_v)/API ratio increased in all subsets of the Johnson Creek data. The largest percentage change in this study was a 92% increase in the (Q_p/Q_v)/120 day API ratio in November-December (Figure 21).

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