Uncertainty Analysis in streamflow Estimation Using Chemical Gauging Technique with Multiple Instruments in High Mountain Environments.

Measuring streamflow in high mountain environments, crucial for understanding contributions from snow and glaciers, poses logistical challenges due to the complexities of accessing study areas and the temporal and spatial variability in flow rates throughout the day, particularly during spring and summer. Consequently, the volumetric balances, essential for estimating diffuse contributions, can be disrupted by the temporal variability inherent in transient environments. In this context, this work details the fieldwork development of a synoptic streamflow measurement protocol tailored for transient high mountain environments using the chemical gauging technique. The protocol minimizes injected salt amount by measuring it at different positions with different instruments at different times of the day, ensuring correlated flow rate measurements on each instrument. Instruments include Aqua Troll 600 Multiparameter Sonde, HOBO U24-001 Conductivity Data Logger, and Sommer Tracer system. The study area focuses on the Yerba Loca stream basin in the central Andes of Chile, a protected area with an average elevation of around 3400 meters above sea level and an average slope of 28.4°. This basin is characterized by naturally occurring Acid Rock Drainage (ARD). The studied stream exhibits characteristics of high mountain runoff, with water depths not exceeding one meter, bed roughness in the order of tens of centimeters, and slopes that can reach 40%. The study area has a roughness height comparable to the flow depth, with values around unity indicating macrorough flow. The protocol's development involved conducting measurements in a test area situated approximately 2000 meters above sea level. Measurements were taken around the confluence of the stream with a lateral tributary during four field campaigns in different seasons of the year. Furthermore, the field protocol was applied in a high mountain glacierized area at around 3500 meters during the thaw season, evaluating its performance in a period of high variability within a day. As a result of these activities, more than 500 streamflow measurements were obtained and analyzed to estimate flow rates and their associated uncertainties. The uncertainties for each measurement were evaluated based on the instrument used, its distance from the injection point, and various conditions present in the stream. Additionally, recommendations are provided for conducting synoptic gauging with multiple instruments of varying precision. The authors gratefully acknowledge the financial support of the National Agency for Research and Development of Chile (ANID): ANID Project AFB230001.  Aldo Muñoz would like to thank ANID for the doctoral grant: ANID-Subdirección de Capital Humano/Doctorado Nacional/2021-21212042 and CODELCO for the funding received from Piensa Minería-Doctoral Thesis Competition.