Abstract
Abstract. The results of streamflow trend studies are often characterized by mostly insignificant trends and inexplicable spatial patterns. In our study region, Western Austria, this applies especially for trends of annually averaged runoff. However, analysing the altitudinal aspect, we found that there is a trend gradient from higher-altitude to lower-altitude stations, i.e. a pattern of mostly positive annual trends at higher stations and negative ones at lower stations. At mid-altitudes, the trends are mostly insignificant. Here we hypothesize that the streamflow trends are caused by the following two main processes: on the one hand, melting glaciers produce excess runoff at higher-altitude watersheds. On the other hand, rising temperatures potentially alter hydrological conditions in terms of less snowfall, higher infiltration, enhanced evapotranspiration, etc., which in turn results in decreasing streamflow trends at lower-altitude watersheds. However, these patterns are masked at mid-altitudes because the resulting positive and negative trends balance each other. To support these hypotheses, we attempted to attribute the detected trends to specific causes. For this purpose, we analysed trends of filtered daily streamflow data, as the causes for these changes might be restricted to a smaller temporal scale than the annual one. This allowed for the explicit determination of the exact days of year (DOYs) when certain streamflow trends emerge, which were then linked with the corresponding DOYs of the trends and characteristic dates of other observed variables, e.g. the average DOY when temperature crosses the freezing point in spring. Based on these analyses, an empirical statistical model was derived that was able to simulate daily streamflow trends sufficiently well. Analyses of subdaily streamflow changes provided additional insights. Finally, the present study supports many modelling approaches in the literature which found out that the main drivers of alpine streamflow changes are increased glacial melt, earlier snowmelt and lower snow accumulation in wintertime.
Highlights
Climate change alters the hydrological conditions in many regions (Parry et al, 2007)
We derived trends of filtered daily streamflow, temperature and snow depth, to support our hypotheses. These seasonal trends were further applied in the attribution approaches: (1) a combination of characteristic dates and trends, (2) a multiple regression model for streamflow trends and (3) hourly trends
The positive annual streamflow trends at higher-altitude glaciated watersheds might be a sign that glaciers in Western Austria are still in the phase, where overall streamflow still rises due to increasing glacial melt
Summary
Climate change alters the hydrological conditions in many regions (Parry et al, 2007). Watersheds in mountain regions are more sensitive than those in lowlands (Barnett et al, 2005; Viviroli et al, 2011) This is mostly due to the strong connection between mountain hydroclimatology and temperature increase, which is at least twice as strong in mountainous areas compared to the global average (Brunetti et al, 2009): on the one hand, increasing temperatures result in diminishing glaciers, earlier snowmelt and less precipitation falling in the form of snow; on the other hand, the local climate is changed by interdependencies such as the snow– albedo feedback (Hall et al, 2008). Magnusson et al, 2010; Tecklenburg et al, 2012; Vormoor et al, 2015) Another way of understanding climate change impacts on local hydrology is to analyse trends in observed streamflow data (e.g. Stahl et al, 2010; Dai et al, 2009). The aim of finding clear changing patterns is often hindered by strong noise in the data, as well as the fact that signals are usually small. Viviroli et al (2011) note in their review paper on climate change and mountain water resources that trend studies in alpine regions often report “inconclusive or misleading findings”
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