Following Droughts through a Catchment - Impacts of different Settings and human Activities

Climate change likely leads to more extreme hydrological events such as droughts and floods. Predicting the impact of hydrological extremes on groundwater resources requires a sound understanding of their propagation from the atmosphere to the groundwater. However, fluctuations of groundwater levels can also be driven by changes in land use or water management. This work aims to identify factors controlling fluctuations of groundwater levels in alluvial aquifers using the catchment of the river Mur (Austria) as an example. Using standardized time series, we analyze and evaluate drought events in groundwater, surface water and precipitation. We follow drought events through a complete basin, starting in upstream, alpine reaches, passing through series of hydro power plants, population and industry centers and ending in an agrarian lowland. Thus we compare different events and different areas, classifying them by their dominant factor. The correlation of groundwater levels with precipitation differs between the investigated subregions and is more significant in the foreland than in the upstream, alpine part of the catchment. This corresponds to a tendency towards more shallow water tables in the foreland. In contrast, the few deepest wells in the upstream part are hardly correlated with precipitation and appear to be also unaffected by river stage fluctuations. Nevertheless, the river is generally found to be a dominant driver of groundwater level fluctuations in the mostly shallow aquifers of the catchment. As a consequence, human impacts on rivers are among the most important factors when assessing shallow groundwater basins in a densely populated area. Assessing data without context therefore can lead to misjudging of events visible in the time series. E.g. the effect of the construction of a run of river power plant on a downstream well can be falsely interpreted as the onset of a long term drought. Hence, depending on the hydrology, population structure and use of the area, droughts propagate and behave very different, when assessing the same event in different areas.