Testing the use of a fluorescent quinine-based tracer for estimating velocities of sheet flow under simulated rainfall: laboratory experiments

The estimation of sheet flow velocities is crucial to understanding and modelling the dynamics of surface flow processes. When direct flow velocity measurements are not feasible, the use of velocity tracers can be a valuable tool. Recent studies have shown that fluorescent quinine-based tracer can be used to estimate sheet flow surface velocities over various soil and urban surfaces under low luminosity conditions, when exposed to ultraviolet light. In this study, a quinine solution was used to test the applicability of this tracer to estimating the velocity of sheet flow disturbed by rainfall with different intensities. For this purpose, a series of laboratory experiments using a soil flume and a rainfall simulator were conducted to study flows under simulated rainfall. Several hydraulic conditions were explored. The rainfall simulator included a downward-oriented full-cone nozzle from Spraying Systems Co. The nozzle was positioned at an average height of 2.5 m from the geometric centre of the flume’s soil surface, with a spray angle of 90°. The working pressure on the nozzles was kept approximately constant at 50 kPa, producing rainfall at a maximum intensity of 150 mm h^-1 just below the nozzle, with average drop sizes of approximately 1.7 mm. Flow velocities were estimated by injecting a quinine solution into the sheet flow. By tracking the leading-edge of the tracer plume and calculating the travel distance of the tracer’s leading edge over a certain time lapse, the surface velocity of the flow was evaluated. The results show that for high rainfall intensities, the disturbance of the water surface by the rainfall drops affected the visibility of the tracer and, thus, the ability to accurately estimate flow velocities using this tracer technique.