Effect of weir´s theoretical discharge coefficient on discharge measurements in small Andean streams

Abstract

Andean ecosystems provide important hydrological services for downstream communities. Due to this importance, several hydrological studies have been carried out in recent years, with emphasis on hydrological processes identification and land use change impacts.  In several studies, but also for the operation of small-scale irrigation and drinking water projects, small streams have been equipped with compound, sharp-crested weirs for discharge estimation. To transform the water level (stage) into a discharge (water rate), weir equations use theoretical discharge coefficients, which do not necessarily apply under the actual field conditions, mainly site fluviomorphology and weir construction aspects, introducing uncertainty in their measurements. Therefore, this study analyzes the effect of using theoretical coefficients instead of adjusted coefficients in field. The study was conducted on 9 micro-catchments (0.2 – 7.53 km2) located in the Zhurucay Ecohydrological Observatory in the páramo of southern Ecuador. To calibrate the coefficients, discharge curves were generated by mechanical and salt-dilution gauging methods. Results revealed that the discharge coefficients differed from their theoretical value by up to 15% for triangular (V-notch) weir section (DCvn) and by up to 41% for rectangular weir section (DCr). The DCvn affects 4 times more in low and medium discharges estimation than DCvn in high discharges. On the other hand, salt-dilution method is more precise for medium and high discharges, but at very low discharges, it overestimates discharge up to 10%. Overall, results suggest that it is essential to calibrate the discharge coefficients in the field to avoid errors in hydrological studies.