Highlights

Abstract

Keywords

۱٫ Introduction

۲٫ Materials and methods

۳٫ Results

۴٫ Discussion

۵٫ Conclusion

CRediT

Declaration of competing interest

Acknowledgements

Appendix A. Supplementary data

References

Abstract

Amongst different climatic and anthropogenic drivers, water resources management can cause massive changes to the natural regime of a lake after its regulation, thereby affecting the quantity and quality of water intended for satisfying the multiple basin water requirements. Here, we investigate the multi-decadal variation of the water levels and outflows of Lake Garda, the largest in Italy, where the dam operational rules and the related basin water needs heavily altered the annual and seasonal trend of the lake regime since its regulation in 1951. Daily lake levels and outflows were first collected and digitized for the period 1888–۲۰۲۰, thus providing a unique database of 133 years that allowed a consistent comparison between natural and regulated periods. Statistical analyses highlighted a significant change of the inter-annual trend of the lake outflows, which passed from upward to downward after regulation, against a constant increasing trend of the water levels. Conversely, water levels showed a more remarkable shifts on a seasonal scale if compared to the outflows, revealing the influence of summer and winter basin water needs. Additional analyses on the inter-annual variation of the main downstream water demands regulated by the dam, i.e. the irrigation, hydropower and fluvial ecosystem requirements, outlined their relevance in changing the lake regime, influencing dam operational policies, which progressively limited the share of water released for ecosystem integrity. A comparison between the lake levels and outflows recorded for the pre-regulation and post-regulation periods of some selected European perialpine lakes finally highlighted different effects on the lake regime, drawing attention to the importance of defining the role of the dam operational policies within the current scenario of climate change and changing water demands.

۱٫ Introduction

Lakes are one of the major global sources of renewable freshwater (Gleick, 1993), and they have been massively turned into regulated systems since the second half of the XX century (World Commission on Dams, 2000; Gleick, 2003). Despite the potentially high amount of water that can be allocated through lake regulation, efficient management of lake levels and outflows is an increasingly critical issue. Major water demands, like hydroelectricity production (Zarfl et al., 2015) and crop irrigation (Oki and Kanae, 2006), long-standing water interests, e.g. navigation and touristic activities, together with climate change and the need to ensure decent environmental standards for freshwater bodies (European Commission, 2000), challenge the way we manage key lake regulation systems, such as dams, to optimally benefit the human society and the natural environment (Moore et al., 2010). Dam construction, even on previously existing natural lakes, indeed determines relevant alterations both on the downstream riverine system and on the lake itself, affecting a variety of hydrological, geochemical and biological aspects (Ward et al., 1983; Nilsson et al., 2005; Magilligan and Nislow, 2005; Poikane et al., 2020).