within a 2” screened well (DIN 4925) showing the groundwater flow in respect to the position of the scree (solid yellow area: screen bar; impermeable to flow / open yellow area: screen slots). Despite the distorsion of stream lines in the vicinity of the well screen, measurements in the radial centre of the well (yellow cross) allow to determine representative values of groundwater flow in both direction & velocity. Flew is directed from top to bottom.