Measures to sustain seasonal minimum runoff in small catchments in the mid-latitudes: A review

In parts of the mid-latitudes, mitigation of critical low flows is becoming a major concern for science and water management institutions. Hydrological catchment models run with input data from global or regional climate models reveal decreasing minimum flow during the summer months. Small catchments (<500 km2) will be affected first. This is because even minor changes in precipitation and temperature will have a considerable impact on stream flow due to the low storage capacity and often faster reaction time of the groundwater bodies. Even a small decrease in low flow will increase the risk of water levels falling below ecologically or economically required minimum flows or even of rivers drying up. Mitigation will be necessary to prevent ecological degradation and water use conflicts. The literature on measures that increase water yield or enhance temporal water storage is reviewed. Studies focusing on measures in small catchments are scarce. For this reason, studies on larger scales and storm water retention are also included. Such measures have to be adapted to the conditions in small catchments and evaluated with regard to the aim of ensuring minimum runoff. Measures differ with respect to such aspects as the effective volume, time scale, controllability and conflict potential. Considering these criteria, an evaluation matrix is compiled which can be used to help draw up research and water management plans. Since current concepts to integrate low flow and storm water protection are inadequate, modeling tools and decision support systems that represent measures to ensure minimum runoff need to be developed and refined.