Section 3. Average Annual Flow

Background

Average annual flow rate can be affected by changes in precipitation. Analysis of historical precipitation data suggests that significant trends in historical rainfall patterns associated with climate change in the Pacific Northwest are not detectable (Hamlet et al. 2005, Mote et al. 2005, Hamlet and Lettenmaier 2007, Hamlet et al. 2007). Climate change modeling suggests that there may be only modest increases in annual precipitation by 2080 (Elsner et al. 2009). Annual rainfall has been shown to be correlated with the Pacific Decadal Oscillation and El Nino Southern Oscillation, and variations in rainfall patterns may have increased in recent years (Hamlet and Lettenmaier 2007, Luce and Holden 2009). Increases in the variability of rainfall and streamflow in the Pacific Northwest may put pressure on water supply systems, which were designed based on historical variations (Jain et al. 2005, Hamlet and Lettenmaier 2007). One analysis (Pagano and Garen 2005) suggested that low-flow years were more likely to occur in succession, potentially exacerbating water supply pressures.

Luce and Holden (2009) utilized quartile regression to investigate trends in streamflow in wet (75th percentile), dry (25th percentile), and average (50th percentile) water years in rivers in the Pacific Northwest. They concluded that the dry years were getting dryer in the Pacific Northwest, accounting for much of the increased variability in annual streamflow.

Average annual flow may also be affected by land use changes. Logging in watersheds can reduce evapo-transpiration resulting in increased annual flows (Bosch and Hewlett 1982). Results from modeling studies suggest there is an increase in annual mean streamflow due to land use change in the Puget Sound lowlands (Cuo et al. 2009). The construction of storm drains associated with urbanization may result in lower streamflows (Simmons and Reynolds 1982). Increased diversions and consumptive uses may also result in lower overall streamflows.

Status and Trends

Data from the Cedar River (below Bear Creek, near Cedar Falls) indicated a significant decrease in annual average streamflow from 1946-2009 (p=0.03; ca. 0.3% yr-1 decrease; Table 1). No other river systems showed a significant change in annual average streamflow (Table 1). The Pearson’s Correlation Coefficients for the average annual flow rate between the river systems in WRIA 3/4 indicate that there is a strong linear correlation between the annual average flow rates of the rivers evaluated (r>0.83; Table 2). There was a somewhat weaker correlation (0.68<r<0.81) between the Samish River and the rivers of the Skagit River basin,,all of which lie within WRIA 3/4.

Table 1. Average annual flow rate in cubic feet per second (CFS) and annual change in average flow rate as determined by simple linear regression (±standard error). Data from USGS Washington Water Science Center (http://wa.water.usgs.gov/)

 

 

 

AVERAGE FLOW

River

Data Years

 

Average Flow Rate

Annual Change

 

 

 

(CFS)

(∆CFS/Year)

WRIA 1 – Nooksack

 

 

 

 

Nooksack

USGS 12213100

1966-2009

 

3855

-3.7±9.0

 

 

 

 

 

WRIA 3/4 – Upper-Lower Skagit and Samish

 

 

 

 

Lower Sauk

USGS 12189500

1936-2009

 

4342

2.0±4.5

Upper Sauk

USGS 12186000

1929-2009

 

1118

0.0±1.1

Thunder

USGS 12175500

1931-2009

 

619

0.2±0.4

Newhalem

USGS 12178100

1962-2009

 

176

0.1±0.3

Samish

USGS 12201500

1945-1970 1996-2009

 

246

0.2±0.4

 

 

 

 

 

WRIA 5 - Stillaguamish

 

 

 

 

Stillaguamish

USGS 12167000

1929-2009

 

1897

2.8±1.9

 

 

 

 

 

WRIA 7 –

Snohomish

 

 

 

 

Skykomish

USGS 12134500

1929-2009

 

3957

3.5±4.1

 

 

 

 

 

WRIA 8 – Cedar/Sammamish

 

 

 

 

Cedar

USGS 12114500

1947-2009

 

161

-0.5±0.2

 

 

 

 

 

WRIA 10 – Puyallup/White

 

 

 

 

Puyallup

USGS 12092000

1957-2009

 

527

-0.4±0.6

 

 

 

 

 

WRIA 11 - Nisqually

 

 

 

 

Nisqually

USGS 12082500

1942-2009

 

772

-0.0±0.9

 

 

 

 

 

WRIA 13 - Deschutes

 

 

 

 

Lower Deschutes

USGS 12080010

1946-1963 1990-2009

 

397

0.2±0.9

Upper Deschutes

USGS 12079000

1950-2009

 

258

-0.2±0.7

 

 

 

 

 

WRIA 16 – Skokomish/Dosewalips

 

 

 

 

Duckabush

USGS 12054000

1939-2009

 

416

0.0±0.5

Table 2. Pearson's Correlation Coefficient of annual average flow rates between river systems in WRIA 3/4. All correlations are significantly different than zero (P<0.05).

 

Lower Sauk

Upper Sauk

Thunder

Cascade

Newhalem

Samish

Lower Sauk

 

0.98

0.85

0.97

0.94

0.81

Upper Sauk

 

 

0.83

0.97

0.94

0.75

Thunder

 

 

 

0.87

0.86

0.68

Cascade

 

 

 

 

0.87

0.73

Newhalem

 

 

 

 

 

0.73

Uncertainties

This analysis was derived from data within the public domain. Average annual flow data presented were calculated from average daily discharge data from USGS stations located in the Puget Sound region (United States Geological Survey 2010b). The datasets include qualification codes indicating whether data are provisional or have been approved (United States Geological Survey 2010a). We avoided using provisional data in this analysis, and we omitted data from gauging stations for which advisory notes warning against unreliable data quality had been posted.

Average daily discharge data for each water year (October 1 – September 30) were used to calculate annual average flow rates. Trends were determined by evaluating the probability that the slope of the average annual flow versus year, as determined through simple linear regression, was significantly different than zero (p<0.05).

The significance of the Pearson’s correlation coefficient was determined by calculating the probability that the correlation was different than zero based on the value of the correlation and the sample size. A significant correlation does not indicate a strong correlation.

Summary

Of the 14 locations analyzed, only one showed a significant change in overall annual flow. All other results were not significant (p>0.10). Annual Average Flow rates are informative when used in combination with other hydrologic indicators such as summer low flows and indicator of flow timing.

References

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