Measures to adapt to climate change and mitigate the impact of droughts in alluvial aquifers and rivers

As a result of climate change, periods of drought are likely to be longer and more frequent. Many small and medium-sized rivers are already drying up in summer and becoming temporary watercourses with all the limiting factors such as oxygen deficiency, excessively high temperatures and their effects on aquatic fauna in particular. One way to counteract the drying up of impacted rivers is to artificially recharge the alluvial aquifers that feed the rivers via a geoengineering method called Managed Aquifer Recharge (MAR). There are various infiltration-based methods employed in MAR, including groundwater recharge via natural and technical infiltration basins or particularly in built-up areas with injection wells that recharge the water directly into the aquifer. The enhanced exfiltration of groundwater as a result from MAR to targeted rivers is called Managed Surface Water Recharge (MSWR). By artificially raising the groundwater level using MAR from nearby rivers/basins to such an extent that the groundwater level is subsequently hydraulically higher than the river level, so that the groundwater can flow into the rivers affected by drought. To sustainably achieve MSWR, MAR should take place when surface water is abundant during medium and high river discharge periods such that groundwater later can exfiltrate into rivers during low water periods. Another positive effect of MSWR lies in the fact that low water periods and drought often occur during hotter summer months. While MAR is often optimal during the cooler snowmelt periods or rain intensive transitional seasons, resulting in comparatively lower temperature of water entering into the ground. MSWR is thus often automatically accompanied by an ecohydrologically relevant cooling effect achieved via the enhanced exfiltration of comparatively cooler groundwater during hotter summer periods. Thus, in addition to an increase in groundwater exfiltration into surface waters during drought periods, MSWR also has the benefit of ecological enhancement in rivers with respect to water temperature and quality. Here, we present first results of our current research into practical MAR-MSWR, which we have conducted at different sites in urban and rural environments in pre-Alpine Switzerland. We present process-based results for different spatial, temporal and operational scales including both local (river-reach) as well as regional (river-basin) perspectives.