Critical assessment of discharge measurements in challenging headwater streams: conventional vs. hot-water injection thermal imaging

Varying flow and morphology challenge precise discharge evaluations in spring-fed headwater streams. This study compared the site-specific suitability of conventional methods (dye, salt tracers, floating spheres, current meters, and bucket-and-stopwatch gauging) with an innovative hot-water-injection thermal imaging technique across four hydro-morphologically challenging sites. Statistical analyses showed thermal imaging is a reliable option in small turbulent or non-turbulent sites under the condition of complete mixing (width <1.2 m, depth <0.2 m, 0.9–64 L s−1), offering chemical-free applications and fast deployment. Limitations included incomplete mixing at large sites (>2 m, >0.3 m, >136 L s−1) and reduced thermal contrasts (thermal masking) under warm conditions. Salt and dye tracers performed comparably. Floating spheres overestimated discharge in shallow or high flows, impellers underestimated under turbulence and required depths >0.05 m, and gauging underestimated discharge in shallow, turbulent sites. These findings highlight that careful site-specific method selection remains essential to minimize errors in hydro-morphologically challenging headwater streams.