Estimating Hydraulic Diffusivity and Streambed Resistance Using Surface Water-Groundwater Interactions under Hydropeaking Conditions

Generating hydropower alters the streamflow dynamics of rivers, especially in the river reaches downstream of  hydroelectric plants. This causes hydropeaking, i.e. sudden and frequent fluctuations in river discharge. If a river is hydraulically connected with the adjacent aquifer, these fluctuations can be observed in the groundwater, depending on the distance from the river. In this study, we used this interaction to estimate the hydraulic diffusivity of the aquifer and the streambed resistance of two rivers located in the Adige valley (Northern Italy) by applying an analytical solution. We compared our model results with the observations of several piezometers located near the Noce and Adige rivers. While seasonal streamflow variability is significantly reduced and short term (sub-daily and weekly) fluctuations are increased in the Noce river, hydropeaking is less pronounced in the Adige. We compared the optimal model results of six different time windows, including low flow, medium and high flow events. Our results allowed for the estimation of the hydraulic diffusivity and revealed high spatial and temporal variability in the streambed resistance. The use of an analytical solution enables a rapid estimation of theses parameters, which can assist in calibrating numerical groundwater models. However, we highlight the importance of satisfying the necessary conditions required to apply this analytical approach; we demonstrate that while these conditions may be met under normal flow, they are not necessarily maintained under extreme conditions (i.e., floods and droughts).