Characterization of Annual Runoff Shape Using Curve Fitting

Abstract:

The Columbia River basin, located in the northwestern continental United States, is a snowmelt dominated system with peak flow coming in late spring. Flow is more constant throughout the rest of the year. Runoff varies among years both in magnitude and shape. This variation presents challenges when comparing annual profiles to determine trends and relationships. Characteristics of flow shape include 1) base flow magnitude, 2) peak magnitude, 3) timing of the freshet, and 4) peakedness of the freshet. Fitting annual daily unregulated flow data to an equation similar to a Gaussian Distribution allows quantitative determination of each of these characteristics. Available data sets include ensemble streamflow predictions produced by the National Weather Services’ Northwest River Forecast Center, which extend from 1949-2018, and modified stream flows produced by the Bonneville Power Administration, which extend from 1928 to 2018. The most notable trend emerging from the analysis is the date of peak runoff moving earlier by approximately 1 day every 6 years.