The system of drainage channels in polders is influenced by precipitation that flows on the surface and underground water that flows subsurface into the channels. Excess water from the canal is pumped into the recipient in a cyclic mode of operation, whereby the water level in the canals is lowered and groundwater infiltration is enabled. Available data are water levels in the channel network, tidal oscillations and channel geometry. The hypothesis of this work is that the correlation between water level and tidal oscillations can determine the dependence between the hydraulic gradient and the inflow of underground water into the canals. The aim of the work is to analyze the water regime in the drainage channel for the entire range of water levels and to calculate the inflow of water into the channel network at different sea levels. The research is conducted on the drainage system of the Vidrice polder located in the area of the Neretva river valley. As part of the implementation of the DELTASAL project, 4 water measuring stations were installed in the polder, which continuously measure the water level along the main channel and on one lateral channel. The water level data will be filtered to remove noise and analyzed with statistical methods. The analysis will be focused on the summer period in order to assess the contribution of tidal oscillations at a time when there is no influence of precipitation or surface runoff on the water regime. The processed data will be used for the correlation between the flow and the slope of the water surface on the characteristic profiles of the channel network.