This study presents a tracer technique based on the fluorescent properties of quinine to help on the visualization of shallow flows and allow a quantitative measurement of overland flow velocities. Laboratory experiments were conducted to compare the traditional dye tracer and thermal tracer techniques with this novel fluorescent (quinine) tracer by injecting a quinine solution and the other tracers into shallow flowing surface water. The leading-edge tracer velocities, estimated using videos of the experiments with the quinine tracer were compared with the velocities obtained by using thermograms and real imaging videos of the dye tracers. The results show that the quinine tracer can be used to estimate both overland and rill flow velocities, since measurements are similar to those resulting from using other commonly used tracers. The main advantage of using the quinine tracer is the higher visibility of the injected tracer under ultraviolet A (UVA) light for low luminosity conditions. In addition, smaller amounts of quinine tracer are needed than for dye tracers, which lead to smaller disturbances in the flow. It requires a simple experimental setup and is non-toxic to the environment.

When direct flow velocity measurements are not feasible, the use of tracers can be a valuable tool. In the present study, both laboratory and field experiments were conducted to evaluate the applicability of quinine as a fluorescent tracer for estimating mean sheet flow velocities in different ambient light and surface morphology conditions. Quinine excels in low-light conditions when exposed to UVA light. This tracer was compared with dye and thermal tracers, all in liquid form. In these tracing techniques the tracers were injected into the flow, after which surface velocity was estimated by tracking the leading edge of the tracer plumes and applying a correction factor to calculate the mean velocity (in a water column). The visibility of the tracers was evaluated by measuring the relative luminance and contrast ratio of the quinine and dye tracer plumes. Results show that the quinine tracer can be used to estimate sheet flow velocities over a wide variety of soil and urban surfaces; it has better visibility in comparison to the dye tracer but, in some conditions, lower visibility than the thermal tracer. Although quinine is invisible under bright ambient light conditions, this tracer technique requires low-cost experimental setup and is useful in low-light conditions (e.g., night; twilight; shielded environments).

<p>This study presents a technique based on the use of quinine as a fluorescent tracer, to estimate sheet flow velocities over various surface coverings (<em>e.g.</em>, bare; mulched; vegetated; paved) in low luminosity conditions (e.g., night; twilight; shielded environments). Quinine glows when exposed to UVA light and in the concentrations used is not harmful to the environment. Experimental work was conducted for studying sheet flows in the i) laboratory (using a soil flume), over bare and mulched surfaces, and ii) field, over vegetated and paved surfaces. Flow velocities were estimated based on the injection of a quinine solution into the water flow.  In these experiments, dye and thermal tracer techniques were used as a benchmark for assessing the performance of the quinine tracer. Optical and infrared cameras were used to record the movement of the tracers’ plumes in the flow. The surface velocity of the flow was estimated by tracking the tracers’ plumes leading-edge and calculating their travel distance over a certain time lapse. Overall, the visibility of the quinine tracer was better in comparison to the dye tracer. However, under some circumstances, lower than the visibility of the thermal tracer. Nonetheless, the results show that all three tracers yielded similar estimations of the flow velocities. Therefore, when exposed to UVA light the quinine tracer can be useful to estimate sheet flow velocities over a wide variety of soil and urban surfaces in low luminosity conditions. Despite some inherent limitations of this technique (<em>e.g.</em>, invisible under bright light conditions or heavy mulched/vegetated cover; need of a UVA lamp), its main advantage is the high visibility of the quinine fluorescent tracer under UVA light for fade light conditions (<em>e.g.</em>, night; twilight; shielded environments such as close conduits), which creates new opportunities for tracer-based surface flow velocity measurements in surface hydrology studies.</p>

This study presents techniques based on the use of fluorescent quinine as a visual tracer for surface flows, to assess surface flow velocities in channels and streams under low luminosity conditions. Fieldwork was conducted in three open channels, with different hydraulic characteristics. A quinine solution, in both liquid and solid (ice cube) forms, was applied on the water flow surface and an Unmanned Aerial System (UAS) was used to record the movement of the fluorescent quinine. The results were compared to the velocities estimated using the thermal tracer technique and flowmeter-based velocity maps. The findings show that the quinine solution, in both liquid and solid forms, can be used to estimate open-channel surface flow velocities under low luminosity conditions. While the solid form of the quinine tracer was applied in a smaller volume than the liquid tracer, its fluorescence effect persisted longer. By comparison, the liquid tracer had the advantage of continual availability and was devoid of the constraint of melting.

Fluorescent tracers have been widely used in hydrology. Recently, quinine started to be used as a fluorescent tracer for estimating the velocity of surface sheet flows over various soil surface conditions and environments. In the present work, the visibility of the fluorescent tracer (quinine) was assessed for various applications’ forms of the tracer (liquid, ice cube with quinine and soaked sponge). The brightness intensity of all tracer forms was estimated for different hydraulic conditions (hydrostatic, linear, and rotational flows) and for clear water, and water with medium and high suspended sediment loads. Results show that, when used as a flow velocity tracer, liquid quinine solution has to be applied carefully into the water and should better be used on sheet flows, shallow overland flows or shallow still waters. Its visibility in deep and muddy flows is insufficient for surface velocity estimations. The sponge soaked with quinine solution, which partially floats, is better visible in clear waters or low-medium suspended sediment loads, regardless of the water depth. However, for high turbulence and rotational flows, the soaked sponge sinks and is no longer visible. The ice cubes showed better visibility in all tested flow water depths and suspended sediment loads, although, in very shallow depths (of millimetres), ice cubes cannot be used because they might not follow adequately the fluid motion, which also applies to the sponge.