Critical Assessment of Discharge Measurement Approaches in Small Streams: A Comparison of Traditional Methods and the Novel Thermal Imaging Method

Discharge evaluations in spring-fed headwater streams are crucial for understanding hydrological processes and improving water resource management. Small streams, however, pose challenges due to low flow and turbulent conditions that limit the reliable application of traditional methods such as impeller devices, electromagnetic sensors and acoustic doppler profilers.

This study tested a variety of discharge measurement methods in two catchments with differing hydrological and physical attributes. The tested methods included impeller and electromagnetic current meters, volumetric gauging, a floating method, chemical and optical tracers and an innovative thermal imaging technique. The thermal imaging method involved introducing hot water into the stream and observing its heat dispersion using a thermal imaging camera.

Results highlighted the strengths and limitations of each approach under varying conditions. At sites with very low discharge of 0.1 to 0.3 L s^-1 or highly turbulent flows of 15 to 22 L s^-1, discrepancies between methods reached up to ±45%. In contrast, measurements at sites with moderate discharge of 2 to 6 L s^-1 and smoother riverbeds, showed error margins mostly below 10%. The novel thermal imaging approach proved to be reliable, easy to use, minimally invasive, and particularly effective for small or hydrologically complex spring systems.