Attribution of high resolution streamflow trends in Western Austria – an approach based on climate and discharge station data

摘要

The results of streamflow trend studies are often characterized by mostlyinsignificant trends and inexplicable spatial patterns. In our study region,Western Austria, this applies especially for trends of annually averagedrunoff. However, analysing the altitudinal aspect, we found that there is atrend gradient from higher-altitude to lower-altitude stations, i.e. apattern of mostly positive annual trends at higher stations and negativeones at lower stations. At mid-altitudes, the trends are mostlyinsignificant. Here we hypothesize that the streamflow trends are caused bythe following two main processes: on the one hand, melting glaciers produceexcess runoff at higher-altitude watersheds. On the other hand, risingtemperatures potentially alter hydrological conditions in terms of lesssnowfall, higher infiltration, enhanced evapotranspiration, etc., which inturn results in decreasing streamflow trends at lower-altitude watersheds.However, these patterns are masked at mid-altitudes because the resultingpositive and negative trends balance each other. To support thesehypotheses, we attempted to attribute the detected trends to specificcauses. For this purpose, we analysed trends of filtered daily streamflowdata, as the causes for these changes might be restricted to a smallertemporal scale than the annual one. This allowed for the explicitdetermination of the exact days of year (DOYs) when certain streamflow trendsemerge, which were then linked with the corresponding DOYs of the trends andcharacteristic dates of other observed variables, e.g. the average DOY whentemperature crosses the freezing point in spring. Based on these analyses,an empirical statistical model was derived that was able to simulate dailystreamflow trends sufficiently well. Analyses of subdaily streamflow changesprovided additional insights. Finally, the present study supports manymodelling approaches in the literature which found out that the main driversof alpine streamflow changes are increased glacial melt, earlier snowmeltand lower snow accumulation in wintertime.