Author(s): Michael Tritthart; Daniel Wildt; Gabriele Weigelhofer
Keywords: Drought; Intermittent streams; Hydrodynamic model; Climate change
Abstract: Many smaller headwater streams have shifted from a perennial flow regime to an intermittent one during the past decades due to climate change, even in temperate climate zones. However, these streams fulfil an important role in the ecosystem due to their self-purifying capacity, which often is paramount to providing safe drinking water to populations nearby. In order to understand the potential changes following the expected higher frequency of droughts in the future, we set up a modelling framework that combines knowledge obtained from field measurements and laboratory studies to make an estimation at the reach scale. Sediment samples were taken at three reaches of different intermittent streams in Austria in two subsequent years and were analyzed regarding microbial activity and gas emissions in the lab. In parallel, using our in-house hydrodynamic code RSim-2D we set up two-dimensional hydrodynamic models for each of the streams in order to obtain the wetted areas for a number of low-flow discharges. The field samples were analyzed in different drying stages, and oven-dried to less than 5% moisture content, which is considered representative for sediment under severe, unshaded drought conditions. Microbial activities were found to be mostly uncorrelated with the water content of the sample, except for the very dry stage, for which a statistically significant decrease could be observed. The modelling framework developed during the study combines the wetted areas at different simulated flow rates with the measured gas emissions as proxy of microbial activity and thus the self-purifying capacity. Depending on data availability, it can also consider wet-dry durations. In the streams investigated, we could indeed identify reduced microbial activity for lower discharges. This effect increased when assuming a reduction of the shading provided by the vegetation cover of the streambed. Our recommendation for the river management is therefore to ensure the availability of a vegetation cover for sustaining the self-purifying capacity of intermittent streams under a warming climate in the future.