Section 3. Average Annual Flow

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.

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/)

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).

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.

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.

Bosch, J. M. and J. D. Hewlett. 1982. A review of catchment experiments to determine the effect of vegetation changes on water yield and evapo-transpiration. Journal of Hydrology 55:3-23.

Cuo, L., D. P. Lettenmaier, M. Alberti, and J. E. Richey. 2009. Effects of a century of land cover and climate change on the hydrology of the Puget Sound basin. Hydrological Processes 23:907-933.

Elsner, M. M., L. Cuo, N. Voisin, J. S. Deems, A. F. Hamlet, J. A. Vano, K. E. Mickelson, S.-Y. Lee, and D. P. Lettenmaier. 2009. Implications of 21st Century Climate Change for the Hydrology of Washington State. JISAO Climate Impacts Group, University of Washington, Seattle, WA.

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Jain, S., M. Hoerling, and J. Eischeid. 2005. Decreasing reliability and increasing synchroneity of western North American streamflow. Journal of Climate 18:613-618.

Luce, C. H. and Z. A. Holden. 2009. Declining annual streamflow distributions in the Pacific Northwest United States, 1948-2006. Geophysical Research Letters 36.

Mote, P. W., A. F. Hamlet, M. P. Clark, and D. P. Lettenmaier. 2005. Declining mountain snowpack in western north America. Bulletin of the American Meteorological Society 86:39-+.

Pagano, T. and D. Garen. 2005. A recent increase in western US streamflow variability and persistence. Journal of Hydrometeorology 6:173-179.

Puget Sound Partnership. 2009. Ecosystem Status and Trends. Water Quantity., Puget Sound Partnership.

Simmons, D. L. and R. J. Reynolds. 1982. Effects of urbanizatsion on base-flow of selected south-shore streams, Long Island, New York. Water Resources Bulletin 18:797-805.

United States Geological Survey. 2010a. Provisional data for Washington.

United States Geological Survey. 2010b. USGS Washington Water Science Center.