Temporal Change in Discharge Response in Unregulated Swedish Catchments - Quantifying Potential Effects of Anthropogenic Modifications in Stream Network Properties on Flow Time Distributions

Abstract:

Fourier spectral analysis of daily discharge time-series with a duration of 55-110 years in 79 unregulated catchments revealed that the discharge power spectrum slope has gradually increased (statistically significant at the 99% confidence level) over time.

For the locations for which historical meteorological observations is available (the 41 southernmost catchments), the evaluation theoretically accounted for fluctuation in the precipitation power spectrum. The results indicate that (local) land-use changes within the catchments may have a relatively more important role (than climate change) for the temporal changes shown in the discharge power spectra.

With a basis in stream network maps from present day in two different resolutions as well as a historical map from the 1880’s, anthropogenic modifications, in terms of the flow paths within the stream networks, were identified for an agricultural catchment in southern Sweden. Through scenario modelling using a 1-D distributed routing model, the influence of common anthropogenic activities such as e.g. straightening of flowpaths, widening of stream channels to avoid damming and excavation to eliminate thresholds in the stream bottom topography, on the travel time distributions within a stream network were quantified.

The map studies showed that the average flow path length had decreased over the last century. The study also shows that all of the studied anthropogenic factors can potentially have a substantial impact on the travel times through the stream networks - by decreasing the average travel time as well as by decreasing the variance. These types of temporal changes in stream network properties leads to a diminished possibility to attenuate peakflows, and are expected to have a substantial influence on discharge hydrographs. This study verifies the hypothesis that anthropogenic impacts of stream networks can influence the hydrological response in catchments, and that land-use changes on a local scale may be relatively more important than climate change.