Changing runoff patterns during the 20th century due to changing surface water volume

Fourier and wavelet spectral analyses are used to reveal dominant trends and coherence of more than century-long time series of precipitation and discharge in several watersheds in Sweden, two of which were subjected to hydropower and intense agriculture. During the 20th century there is a gradual, significant drift of the dominating discharge periodicity in the agricultural watershed, despite the fact that no trends were observed in variance or mean of the discharge. The steepness of the Fourier spectrum of runoff increases gradually, which suggests a more predictable runoff pattern (more apart from white-noise). There is only a moderate change of the cross-correlation of precipitation and runoff, suggesting that climate change can have only a relatively small impact on the changing runoff pattern. In the hydropower regulated watershed there is an opposite sudden change of the discharge spectrum when regulation commenced in the 1920s. The changes in runoff spectra in these two watersheds can be shown to be theoretically consistent (all though opposite magnitude) and are explained by the anthropogenic change in surface water volume, hence, kinematic wave celerity and water transit times. Slope of spectral density for May to October periods each year for the five investigated rivers in Sweden.