The aim of the scientific work is to study the features of filtration under a defect of a polymer screen made of a geomembrane, where the movement of the filtration flow occurs with incomplete saturation of the soil with water. According to the results of experimental studies, local filtration is formed on the filtration tray under the screen defect with incomplete saturation of the soil with water, caused by the action of capillary forces and the presence of trapped air. The article presents the results of studies of a model of a polymer screen with a defect in the form of a slit with a relative width of 0.005 to 0.5. On the basis of studies in a soil flume and experimental pits with film screens, the average moisture content for sandy and loamy soil was established directly under the screen defect. According to the results of the calculations performed, the piezometric pressure along the screen slit significantly decreases with an increase in the permeability of the screen base and reaches negative values at relatively large values of the slit width. The filtration flow through the slit increases with an increase in the permeability of the base soil, and the width of the spreading zone of the filtration flow under the screen decreases. The urgency of solving this problem is due to insufficient study of the influence of moisture saturation of the base soil on filtration through a screen defect.