A wavelet analysis of surface water – groundwater interaction in reaches affected by hydropeaking

The management of hydropower plants influences the variations in the discharge of the rivers, introducing for example weekly fluctuations. This variability can in turn influence the interaction between surface water and groundwater. In our work, we analyse an alpine area in Trentino in northern Italy, where four different river sections are considered. The four reaches (Noce, Adige before the confluence with the Noce, Adige after the confluence with the Noce, Avisio) are differently affected by reservoir operation. We hence study the influence of surface water management on the aquifer by using a transient model built with Modflow-OWHM for the hydrological years 2009/10 and 2016/17, where the year 2016/17 was characterised by particularly low amount of precipitation. The results were interpreted using continuous wavelet analysis and wavelet coherence analysis. In particular, we focused on the non-stationary weekly variations observed in the groundwater table. Such fluctuations are very pronounced near the river sections affected by reservoir operation. Moreover, the comparison of the behaviour in 2009/10 and in 2016/17 shows that the weekly variations are stronger in dry years than in wet years. Nevertheless, the proportion of the aquifer that shows a strong weekly variability remains similar. The water budget along the four river sections does not change significantly in the two hydrological years considered. However, our wavelet analysis shows that in 2016/17 a strong weekly variability is observed in surface water-groundwater exchange while in 2009/10 this dynamic is not observed.